Classification of animals: Protozoa, Porifera and Coelenterata

This article includes Basics classification like binomial nomenclature, Taxa hierarchic, Five kingdoms of Robert H. Whittaker, Levels of Organization, and Classificationa and features of Protozoa, Porifera and Coelenterata

1. Taxonomy: T. Y. B.Sc. Zoology, Sem. V; Course-USZO501; Unit-II: University of Mumbai 2017-18

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I. Linnaeus hierarchy
 Carolus Linnaeus is the father of taxonomy. Any group in Linnaean classification is referred as
“taxon”. Linnaeus arranged these taxa into hierarchical manner from biggest group (Kingdom) to
smallest one (species).
 He divided living organisms into some groups dependent on the broad relationships and similarities.
If any two given organisms can be grouped under the same lower rank or taxon, it implies that the
two organisms are structurally, functionally, embryologically similar and that they have had a
comparable evolutionary history.
 Within the living world as a whole, the biggest taxonomic rank is Kingdom. He further divided them
into more closely related smaller groups with fewer and more closely related characteristics. The
lower the rank of a group, the more similar are the organisms grouped in it.
 Using comparable criteria of similarities and relationships the larger taxon could be classified into
smaller ones e.g. ‘Phylum’ into few ‘Classes’ and a ‘Class’ into several ‘Orders’ and so on up to
species level.
Kingdom : Animalia
Phylum : Chordata
Class : Mammalia
Order : Primates
Family : Hominidae
Genus : Homo
Species : sapiens
 Kingdom: The largest as well as the first category is the Kingdom. The five kingdoms are Plantae
which consists of plants, Animalia which comprises of animals, Protista which comprises protisits,
Fungi which consists of mold, mushrooms, yeast etc and last Monera which consists of three types
of bacteria.
 Phylum: There can be many different Phyla within the Kingdom. The Phyla are smaller groups
responsible for the recognition of animals with closer characteristics. Different Phyla are Annelida,
Arthropoda, Mollusca and Chordata etc. It is customary to call ‘Division’ in plant classification in
place of ‘Phylum’ in animal classification.
 Class: Phyla further can be divided into more small groups referred as the Class namely Phylum-
Chordata is divided into Classes Amphibia, Reptilia, Aves, Mammalia, and Chondriechthys .

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 Order: After Class, the next category is the Order. Each Class comprises of one or more Orders
namely Class-Mammalia is comprised of Orders Primates, Chiropetra, Rodentia, Artiodactyla,
Proboscidea etc. and many more.
 Families: The next category is Families. Orders can be further categorized as the Families like
Order-Primates divided into Families like Lemuridae (lemures), Loridae (loris), Cebidae (monkeys),
Hylobatidae (gibbons), Hominidae (apes), etc.

 Genus: After Families comes the next category the Genus, namely Family-Hominidae is divided into
Genera Pongo (oranguttans), Gorilla, Pan (chimpanzees) and Homo (humans) etc.

 Species: The lowest and the last category of Animal Taxonomy Hierarchy in Species which is result
of further breaking down of the Genus. For example Genus Homo includes several human species
like habilis, antecessor, erectus, ergaster, helmei, sapiens etc. Genus name is always placed in front
of the Species name.
Homo habilis
Homo antecessor
Homo ergaster
Homo helmie
Homo erectus
Homo neanderthalensis
Homo sapiens

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 Species is generally the lowest taxonomic rank representing organisms that are very much closely
related to one another. A species is defined as a group of closely related, structurally and functionally
similar organisms which can breed among themselves, producing fertile offspring and which are
reproductively isolated from such other groups.
 Example of classification of Human species:
 Kingdom–Animalia……Phylum–Chordata…….Class–Mammalia…..Order– Primate……..Family –
Hominidae…..Genus – Homo……Species- sapiens.

II. Binomial nomenclature
Binomial or binary nomenclature is a system of two part scientific names. The person who popularized
this system for use was Swedish botanist and physician Carolus Linnaeus (1707–1778).
The formal introduction of this system of naming species was effectively used by him in his work Species
Plantarum in 1753. It consists of two parts of the name comprised of first part as genus name and a
latter as species. Genus name starts with capital letter and species name never starts with capital letter.
Usually the binomial names are typed in italics, in case hand written then the names is underlined
separately. Fro example Homo sapiens is typed as Homo sapiens and if hand written it is expressed as
Homo sapiens.
The binomial nomenclature is now governed by various internationally agreed codes of rules such as the
International Code of Zoological Nomenclature (ICZN) and the International Code of Nomenclature for
algae, fungi, and plants (ICN)
Salient feature of binomial nomenclature
 Widespread: worldwide use instead of different local names
 Clarity: avoids the same term used for different species. Example: "robin" used for different birds in
America compared to Europe
 Uniqueness: One name for a species
 Stability: the rules favour stability
 Economy: said to be easier to remember than the previous system
Prior to the adoption of the modern binomial system of naming species, a scientific name consisted of a
generic name combined with a specific name that was from one to several words long. Together they
formed a system of polynomial nomenclature. These several words of species name included the species
name proper and a little description about the species e.g. Plantago foliis ovato-lanceolatus
pubescentibus, spica cylindrica, scapo tereti ("Plantain with pubescent ovate-lanceolate leaves, a
cylindric spike and a terete scape"), which we know today as Plantago media.

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III. Five Kingdoms
R. H. Whittakaer (1969) classified the living organisms into five kingdoms. Viz. Monera, Protista,
Plantae, Fungi and Animalia based on their following characters-
Monera: Group of Archaebacteria and eubacteria. Prokaryotes, unicellular, may absorb or
photosynthesize food, motile or non-motile, exhibit asexual reproduction.
Protista: Group of unicellular organisms. Prokaryotes, unicellular or multicellular, may absorb, feed or
photosynthesize food, motile or non-motile, exhibit sexual or asexual reproduction
Plantae: Group of Plants. Eukaryotic, multicellular, cells with cell walls, photosynthesize hence
autotrophic, non-motile, exhibit sexual reproduction.
Fungi: Group of fungi. Eukaryotic, multicellular, showing saprophytic habit (absorb food), non-motile
and exhibit sexual reproduction.
Animalia: Group of Animals. Eukaryotic, multicellular, heterotrophic ingest food, motile, exhibit sexual
reproduction.

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LEVELS OF ORGANIZATION

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(Protos = primitive; zoon = animal)
Unit 2.2: Phylum Protozoa (5 lectures)
2.2.1: General characters and classification
2.2.2: Locomotion in Protozoa - amoeboid, flagellar, ciliary, gliding
2.2.3: Reproduction in Protozoa - asexual and sexual
2.2.4: Morphology, life cycle, pathogenicity and control measures: Plasmodium, Entamoeba
Unit 2.2.1: General characteristics and classification
Introduction:
Goldfuss (1818) coined the term Protozoa but he included many non-protozoan forms under protozoa.
Von Siebold (1845) recognized the unicellular nature of the protozoa and established the term Protozoa
in its true sense.
Protozoans are primitive, aquatic, microscopic, eukaryotic, and unicellular organisms exhibiting cell
level of organization and protoplasmic level division of labour. They also exhibit mobility and irritability.
There are about 50,000 species belong to Phylum Protozoa, out of which, 30,000 still exist and
remaining 20,000 are extinct.
Habit and Habitat
Protozoans are aquatic and occur in marine and fresh waters; some live in moist soil and some as
parasites. They are solitary or colonial, free living, commensals, mutualistic or parasitic. They exhibit
irritability as they are sensitive to stimuli like touch (thigmotaxis), temperature (thermotaxis),
chemicals (chemotaxis), and light (phototaxis). Protozoans also use pheromone signaling using
serotonin, β-endorphin, acetylcholine and cyclic-AMP.
Morphology
Protozoans measure from 10µm to 52µm although many are big enough to be recognized in a dissecting
microscope or even with a magnifying glass (up to 2mm in size i.e. Amoeba proteus). Body single celled,
without cell wall, made of outer cell membrane, and inner protoplasm containing cytoplasm, nucleus,
and other organelles. Organelles are protected by internal skeleton made of microtubules called as
pellicle or epiplasm and in some the exoskeleton in form of test or shell. Organelles include chloroplasts,
food vacuole, contractile vacuole, pigments and granules etc. some have many nuclei. Although body is
single celled, many tend to form clusters or colonies e.g. Volvox colony. Shape is disc shaped, spherical,
oval, conical or cylindrical and exhibit asymmetry or radial symmetry.

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Nutrition
Nutrition is holozoic (heterotrophic), holophytic (autotrophic) and/or saprophytic (parasitic). Holozoic
feeding is achieved through phagocytosis. Ciliates develop mouth-like cytostome and anus-like cytopyge
for ingestion and egestion of food respectively. Cytoplasmic streaming is maintained known as ‘Cyclosis’
to distribute nutrients throughout the body. In some, food vacuoles perform the cyclosis. Holophytic
nutrition is performed by photosynthetic pigments like chlorophylls and carotenoids.
Respiration
The respiration is performed through direct surface diffusion. The concentration gradients cause the
exchange of gases through cell membrane.
Excretion and osmoregulation
They are ammonotelic and excrete ammoniacle matters through direct diffusion through the surface.
The contractile vacuoles play important role in maintaining the osmotic pressure of the body by
expelling excessive water (exosmosis) from the body which enters through endosmosis, especially in
fresh water forms.
Reproduction
Reproduction is performed asexually by fission, fragmentation and budding whereas sexually through
syngamy, autogamy, conjugation, hemixis, endomixis and cytogamy. Fission occurs in three types’ i.
binary fission (longitudinal as in Euglena or transverse as in Paramecium); ii. multiple fission and iii.
sporulation.
Classification
The classification of Phylum Protozoa is based on locomotory organelles. It is divided into four classes
as follows-
1. Class – Rhzopoda (Sarcodina) locomotory organelles are pseudopodia; amoeboid forms are
dominant; some have hard shells or test; pseudopodia are further modified as lobopodia, axopodia,
actinopodia, filopodia, reticulopodia etc. they are generally creeping forms. Reproduce mostly
through binary fission and syngamy.

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Fig. Different types of Pseudopodia: A. Lobopodia of Amoeba
B. Filopodia of Euglypha C. Reticulopodia of Chlamydophrys
D. Axopodia of Actinophrys.
The shells are developed in foraminiferans are of great ecological and economic significance. While
some others like Arcella, Diflugia, Euglypha and Clathrulina contain outer shell or test.

Globigerina

Lagena

Polystomella



Spiroloculina

Textularia

Discorbina
A. Arcella; B. Diflugia and
C. Euglypha D. Clathrulina
Fig. Foraminiferan shells
2. Class – Flagellata (Mastigophora) locomotory organelles are flagella. Body has definite shape due
to pellicle. They are solitary or colonial. Flagella may be single (monoflagellate), double
(dinoflagellate), four (quadriflagellate) or numerous (multiflagellate) viz. Euglena, Chlamydomonas,
Trichomonas and Trichonympha respectively. Reproduce through longitudinal binary fission or
syngamy.

Euglena Chlamydomonas Trichomonas Trichonympha

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Mastigophores are Autotrophic or holozoic. Autotrophs contain photosynthetic pigments hence they are
sometimes included as algae.
3. Class – Sporozoa no locomotory organelles since they are parasitic in habit. They are saprophytic
and can feebly move through gliding mechanism. They reproduce by asexual and sexual phases of
life cycle e.g. Plasmodium, Monocystis.
4. Class – Ciliata (Ciliophora) locomotory organelles are cilia. These are evolved protozoans. They
have two nuclei viz. micronucleus to control reproduction and meganucleus for growth and
metabolism. These are sedentary or free living; solitary or gregarious.
Unit 2.2.2: Locomotion in Protozoa
Protozoans have pseudopodia, flagella or cilia as locomotory organelles. Parasitic forms lack with any
locomotory organelles. Protozoa can move by using flagella and cilia that will beat and propel them.
Protozoa can also move by extending cytoplasmic extensions known as pseudopodia, or some have no
locomotory organelles and can glide in the available medium as evident in endoparasitic forms like
Monocystis. Various types of locomotion in protozoans are discussed as follows:
I. Amoeboid movements: Locomotion in rhizopods is achieved by formation of temporary fingerlike
processes called as pseudopodia (lobopodia) known as amoeboid movements. An amoeboid
movement has been explained in various manners or theories like sol-gel theory, folding and rolling
theory, walking movement.
A. Sol-gel theory
According to Hyman (1917), Pantin (1926) and Mast (1931) pseudopodia are produced or withdrawn
due to cyclic viscosity change in the cytoplasm (i.e. from sol to gel and vice versa) the liquefied
plasmasol from posterior end is pushed to anterior where the hyaline cap strikes the object causing
weakening of plasmagel. This forces the pseudopodia anteriorwards and the liquefied plasmasol
flows from anterior to posterior during which it again becomes gelated.


Amoeboid movement after ‘sol-gel’ theory by Mast
Sol-gel theory has been accepted universally and is explained in various ways as follows—

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i. Molecular folding unfolding theory
According to Goldacre and Lorch (1950), the contraction of the plasmagel tube cannot supply enough
force for moving the animal. Hence they have explained the phenomena of solation and gelation on
the molecular basis. They state that all proteins gelate when their molecules unfold and they solate
when their molecules fold. In the fluid endoplasm the protein molecules lie folded compactly, these
molecules unfold at the tip of the advancing pseudopodia to form a layer of straightened and
attached molecules.

Amoeboid movement after folding and unfolding theory by Goldacre and Lorch

ii. Front contraction theory or Fountain zone theory:
Ectoplasm is divided into outer thin hyaline ectoplasm and inner thick gel ectoplasm while the
endoplasm has an axial core of gel surrounded by a thin zone of sol (Allen, 1962). Axial core of gel
pushes cytoplasm at anterior end forming a fountain of ectoplasmic gel which develops hyaline cap.
The hyaline cap adheres to the substratum. The plasma gel returns back to convert into sol at
posterior. The adhered hyaline cap is then pulls the amoeba forward.


Fountain zone contraction amoeboid movement
iii. Rear contraction and Hydraulic theory (Proposed by Rinaldi and Jahn in 1963):

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By observation and analysis of the motion pictures of granule movements in advancing pseudopodia
they supported the concept of Mast (1923), but criticised the view of Allen (1962). According to them
the contraction in gel at the posterior end gives rise to hydraulic pressure in sol.
This pressure is not equally distributed. It is highest at the posterior end, lowest at the anterior end
and moderate in the middle. Gel at the posterior end always changes to sol due to contraction of the
plasmagel and gel at the anterior end becomes thinner. Sol from the posterior end flows forward,
breaks the gel of the anterior end and forms a new pseudopodium and brings about the forward
movement of Amoeba.

iv. Contractile jacket and osmotic theory
The cytoplasm of Amoeba contains both actin and myosin protein molecules and their interaction
helps in the formation of pseudopodia. During conversion from endoplasm to ectoplasm a mesh of
actin filaments develops by the polymerization of actin molecules which form the rigid gelatinous
condition of ectoplasm.
When the chemical signals bind to the membrane receptors, the ectoplasm transforms into
endoplasm at the rear end by depolymerization. Then the myosin subunits bind to the actin
molecules which transform the actin mesh into a contractile jacket that forces the fluid interior
endoplasm forward.
The breakdown of actin network by depolymerization increases the osmotic pressure which draws
water from the endoplasm to the periphery of the protoplasm of Amoeba that creates the extension
of the cell forming a pseudopodium (Fig. 10.62). At present most of the zoologists such as Ruppert
and Barnes (1994), Pechenik (2000) and others support this view.

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B. Gel-sol theory
The pseudopodium is formed anteriorly by swelling of cytoplasm due to thinning effect of acid and
absorption of water. The ectoplasmic plasmagel starts flowing backwards exerting pressure on
endoplasm and pushing it from back to front. The endoplasmic plasmasol thus starts flowing
through the ‘ectoplasmic plasmagel tube’ from back to front thus the locomotion occurs. This was
observed by Pantin (1923) in Amoeba limax.

Gel-sol mechanism in Amoeba limax
C. Rolling theory
Amoeboid verrucosa develops single pseudopodium hence it is called as unipodal species. It moves
by simple rolling mechanism.


Amoeboid verrucosa showing rolling movement

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D. Walking movement
According to Dellinger (1906) the walking locomotion Amoeba proteus walks on pseudopodia. When
pseudopodia strikes an object it sticks to is causing contraction in endoplasm thus pulling whole
body mass close to the pseudopodium.

Amoeba proteus showing walking movement
E. Adhesion theory
The path is followed towards the surface tension is high thus following a indefinite track. The
protoplasm flows like a drop in a path of greater adhesion upon a surface. The pseudopodia
grow due to adhesive property of the surface. However this explanation is not sufficient since
lack to explain how pseudopodia are develop sometimes without any contact with the surface.
F. Surface tension theory
Cytoplasm changes continuously from gel (plasmagel) at posterior end to sol (plasmasol) as it
migrates to anterior and ultimately it turns to posteriorwards to form gel again. This causes
contraction of plasmagel at posterior which produces a current of cytoplasm to anterior which
pushes the plasmasol at anterior extending pseudopodium ahead.
II. Flagellar movements
Flagella are locomotory organelle and characteristics of Mastigophores. They are few in number,
longer made of nine+two microfibrils and believed to be controlled by basal bodies called
blepharoplasts. However, movement of flagellum is related to the contraction of its all fibrils. The
energy for the contraction of these fibrils is derived from ATPs formed in the mitochondria of
blepharoplasts.

Euglena: T.S. of flagellum

Action of flagellum. A—Recovery stroke, successive stages
from 1 to 7; B—effective stroke, from 8 to 13; C—horizontal
move and D—backward move
Vickerman and Cox (1967) have suggested that the flagellum makes direct contribution to locomotion.
However, several theories have been put forth to explain the mechanism of flagellar movement.

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Butschli observed that the flagellum undergoes a series of lateral movements and in doing so, a
pressure is exerted on the water at right angles to its surface. For quite a long time it was generally
presumed that the flagellum is directed forwards during flagellar movement but now it is generally
agreed that the flagellum is straight and turgid in effective stroke (A) and dropped backwards in the
recovery stroke (B).
This pressure creates two forces one directed parallel (D), and the other at right angles (C), to the main
axis of the body. The parallel force will drive the animal forward and the force acting at right angles
would rotate the animal on its own axis (as below). The waves proceed along the flagellum in a spiral
manner and cause the body of Euglena to rotate once in a second. Thus, in its locomotion, it traces a
spiral path about a straight line and moves forward. The rate of movement is 3 mm per minute.

Euglena: Successive stage in the flagellar movement
Gray (1928) suggested that a series of waves pass from one end of the flagellum to the other. These
waves create two types of forces, one in the direction of the movement and the other in the circular
direction with the main axis of the body. The former will drive the animal forward and the latter would
rotate the animal.
Euglena sometimes shows a very peculiar slow wriggling movements. A peristaltic wave of contraction
and expansion passes over the entire body from the anterior to the posterior end and the animal moves
forward. The body becomes shorter and wider first at the anterior end, then in the middle and later at
the posterior end.
This type of movement is called euglenoid movement by which slow and limited movement occurs.
Euglenoid movements are g brought about by the contractions of cytoplasm or by the contractions of
myonemes present in the cytoplasm below the pellicle.

Euglena: Successive stage in the euglenoid movement

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Recently Lowndes (1941-43) has pointed out that the flagellum is directed backwards during locomotion.
According to Lowndes, a series of spiral waves pass successively from the base to the tip of the
backwardly directed flagellum at about 12 per second with increasing velocity and amplitude.

Euglena: A-Position 1-4, when moving towards direction I of
light; 5 in shock reaction and 6-8 moving towards direction II.
B-Avoiding reaction on trial and error basis.
It has been found that the region in front of the eyespot is more sensitive to light. Euglena positions
itself parallel to light rays. The animal adjusts its position to the direction of light moving either towards
or away from it. Also Euglena has been observed of giving response to various stimuli like thermal,
chemical or mechanical stimuli. During this Euglena moves in a stationary circlet and finally escapes
towards the safe direction.
III. Ciliary movements
Structure of cilia: Cilia are short, fine thread like extensions of the cell membrane. The length of the
cilia ranges from many microns to many hundred microns and the diameter varies in from 0.1 to 0.5
micrometer. A cilium consists of series of continuous fibers which run longitudinally through the entire
cilium. There are nine peripheral fibers, each made of two sub-fibers joined together to form a doublet
and two central fibers. The 9 + 2 fibers are made of microtubule. The fibrils are covered by the
extension of cell membrane.

T. S. of cilium (diagramatic)

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Mechanism of movement: In 1955 Bradford suggested that movement of cilia is due to sliding of
double fibrils in two groups one after the other.
(a) Effective stroke: Five out of nine double fibrils contract or slide simultaneously. As a result cilium
bents or shortens. It is called effective stroke.
(b) Recovery stroke: Four out of nine double fibrils contract and cilium becomes straight. It is called
recovery stroke. As a result of this Paramecium swims against water.
Energy: The energy for ciliary movement is provided by ATP. The enzyme ATP-ase, splits ATP and stops
ciliary action.
Co-ordination: The cilia are coordinated. Thousands of cilia beat together to move the animal in a
direction.
Cilia of the same transverse row beat together and those of the same longitudinal row beat one after the
other from the anterior to the posterior end.

Ciliary movement in Paramecium

Effective and recovery strokes of Ciliary
movements,


Metachronous movement of cilia and direction of movement in Paramecium
This coordinated movement of cilia is called metachronal rhythm, which is due to the infra-ciliary
system; this causes swimming forward by the animal. But when the body cilia are beating obliquely
backwards, then at the same time the longer cilia of the oral groove beat more vigorously which causes
the anterior end to swerve to the left.
The action of cilia of body and oral groove makes the animal to rotate on its long axis. This rotation is
always to the left (except in P. calkinsi which rotates in a right hand spiral).

Spiral path of Paramecium and the direction of movement

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IV. Gliding movement
Monocystis is sluggish in locomotion. Monocystis moves by wriggling or gliding movement brought
about by the rhythmic contraction and relaxation of myonemes which are arranged in lines below the
pellicle. These slow movements are called gregarine movements which are like euglenoid movement of
Euglena. Nutrition: Monocystis absorbs the nourishment from the fluid of the seminal vesicles through
the general body surface.

Diagrammatic representation of Monocystis
2.2.3: Reproduction in Protozoa - asexual and sexual
Asexual reproduction in Protozoa
Most protists (protozoans) can continue to live, multiplying asexually for prolonged periods and may
undergo sexual reproduction only at irregular intervals. The mode of reproduction in which there is no
union of gametes is known as asexual reproduction. In such a case, only one animal can produce new
individuals. Protozoa usually reproduces asexually by binary fission and multiple fission.
Binary fission: It is the process where the division of nucleus along with the cytoplasmic division takes
place which is called karyokinesis and cytokinesis respectively. The nucleus divides into two through
mitosis (karyokinesis). The cytoplasm divides into two equal halves by a constriction. The daughter
individuals can reconstruct the lacking structures which it does not obtain from the parent. And the two
daughter cells are finally separated (cytokinesis). Hence, the newly formed organism has 1 new copy of
DNA. Binary fission is one of the most common methods of reproduction in protozoan. In different
group of protozoan the division takes place differently that involves various plane of symmetry. In
Amoeba the division is irregular whereas in Euglena it’s longitudinal. Likewise, in paramecium; it is
transverse.

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Irregular binary fission: Binary fission is irregular in some protozoans which do not have definite
shape. There is no definite plane of division is occurred in Irregular fission. Example Amoeba

Binary fission in Amoeba
Longitudinal binary fission: The animal splits into two along the long axis of the body. Examples is
Euglena.

Binary fission in Euglena
Transverse binary fission: Example is Paramecium. The animal divides transversely into two. The
micronucleus divides into two by a simplified form of mitosis. The macronucleus divides into two by
amitosis. Asymmetrical structures like gullet, peristome of Paramecium cannot be equally shared by both
the daughter individuals.

Binary fission in Paramecium
Oblique binary fission: It is common in dinoflagellates. In this case the division takes place oblique to the
body axis. Example is Ceratium

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Multiple fission: It is the division of nucleus into several daughter cells. Firstly, parent cell divide its
nucleus into many daughter nuclei. The cytoplasm fragments and a small bit of it surrounds each
daughter nucleus and, thus, many minute animals are formed. It is generally seen in sarcodines and
mastigophores. A word that is frequently heard in case of multiple division is schizogony. It is the
process of asexual multiple division in protozoan. During unfavourable conditions Amoeba forms cyst
around cell membrane before it undergoes multiple fission. It can remain dormant in cyst form until the
favourable conditions occur. Under favourable circumstances the cyst bursts and these small animals
come out and grow to the adult stage. This kind of multiple fission is referred as sporulation.

Multiple fission in Amoeba
Plasmotomy: In this case, only the cytoplasm divide and not the nucleus. It is because the parent
already has multinuclei and hence after the division of cytoplasm, multinucleate daughter is formed.
Example includes: Opalina
Budding: It is also one of the most common method of division in protozoan. In this process, a small
outgrowth is first seen in the parent organism which is known as bud. This outgrowth slowly gets
separated from the parent having an individual nuclei. There can be only one bud or multiple buds can
be located. Example includes suctorian protozoan.

Gamogony:

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Adults act as gametes with little physiological changes. An example is Monocystis.

Gamogony in Monocystis
Scizogony or agamogony occurs in asexual stages:
The resulting individuals are known as agametes or merozoites. Example is Plasmodium.

Scizogony in Plasmodium
Sexual reproduction in Protozoa:
The modes of reproduction in which two gametes unite to form a new individual is known as sexual
reproduction. The two units (male and female gametes) from two separate individuals unite by fusion of
their cytoplasm, followed by the union of their nuclei.
Most protists (protozoa) can continue to live, multiplying asexually for prolonged periods and may
undergo sexual reproduction only at irregular intervals. However, there are many protozoans in which
sexual reproduction is of regular occurrence. Sexual reproduction involves meiotic division reducing the
chromosomes to haploid number. In majority, reduction division occurs shortly before syngamy. This is
called gametic meiosis, in which gametes become haploid.
But in some protozoans reduction division occurs in one of the subsequent divisions after formation of
zygote. This is termed as zygotic meiosis, in which only zygote is diploid but rest of the life cycle is
haploid. Thus in this case genes of both the parents are present in the daughter cell. Of different types of
sexual reproduction in protozoans syngamy, conjugation, automyxis are important.
Syngamy:

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Syngamy is the complete and permanent union or fusion of two specialised protozoan individuals or
gametes resulting in the formation of a fertilized cell or zygote or oospore. The nuclei of the gametes
fuse to form the zygote nucleus or synkaryon. The zygotes develop into adult, either directly or through
encystment and fission of various types. Depending upon the degree of differentiation of the fusing
gametes syngamy may be of the following types:
a. Autogamy:
The gametes derived from the same parent cell fuse. Examples: Actinophrys, Actinosphaerium,
Paramoecium aurelia, etc.
b. Hologamy:
The two ordinary mature protozoan individuals do not form gametes but themselves behave as gametes
and fuse to give rise to zygote. Example: Copromonas.

Hologamy in Copromonas
c. Paedogamy:
The fusion occurs between two nuclei coming from two different cells of a parent. A single organism
encysts and then divides into two or more gametocytes. The nuclei of these gametocytes undergo
meiosis and the gametes thus produced unite in pairs forming the zygotes. Examples: Actinosphaerium,
Actinophrys, etc.

Paedogamy in Actinophrys sp.: A. Encysted individual divides; B. Each daughter undergoes
maturation division; C. Left daughter individual acts as male, right as female; D. Zygote
d. Merogamy and Sporogony:
In merogamy the uniting sexual individuals are smaller than the ordinary vegetative individuals, called
merogametes. Sporogony is similar to merogamy except sexual individuals are surrounded by a
resistant covering called as cyst and merogametes are termed as spores. Motile spores are known as
swarmer’s or swarmospores. Flagellospores or Spores bearing flagella and Pseudopodiospores or
amoebospores bearing pseudopodia.

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Sporogony in Monocystis
e. Isogamy:
When two fusing gametes are similar in size and shape but differ in behaviour, they are called
isogametes and their union, isogamy. The isogametes are produced by multiple or repeated binary
fission. Isogamy has been reported in Elphidium, Phytomonadina Chlamydomonas, Copromonas and
Gregarinida Monocystis.

f. Heterogamy or Anisogamy:
The two fusing gametes differ in size, shape and behaviour. The gametes are termed as heterogametes
or anisogametes and their fusion is known as heterogamy or anisogamy. The formation of
morphologically different gametes, is the first indication of sex differentiation in Protozoa.
The smaller gametes, the microgametes, or male gametes, are active, motile, generally flagellated and
more numerous. They are produced by multiple or repeated fissions.
The fusion of two microgametes is called Micro-gamy. Example: Foraminifera, Arcella, etc. The larger
gametes, macrogametes, are immotile, voluminous, and referred to as female gametes. The fusion of
two macrogametes is called Macro-gamy. Examples: Plasmodium, Eimeria, Volvox, etc.
The syngamy brings about a combination of two different lines of hereditary characters. It increases the
external differences in offspring’s. It also renews the vigour which is lost due to repeated binary fissions.
The fusion of two nuclei initiates the development of eggs.

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II. Conjugation:
The conjugation is the temporary union of two mating types of individuals of the same species to
facilitate exchange of nuclear materials. They retain their distinct individuality and separate out after
nuclear exchange. The pairing gametes are known as conjugants. The conjugants may be either
isogamous (Paramecium) or anisogamous (Vorticella).
In conjugation (i) reorganization of a fresh meganucleus occurs to accelerate the metabolic activities, (ii)
rejuvenation and revival of lost vigour, (iii) new nuclear combinations and new hereditary combinations
arise.

Conjugation in Vorticella

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Conjugation is a mode of sexual reproduction,for the exchange of nuclear material, which is very
characteristic in Vorticella.
Vorticella divides by binary fission into two unequal cells; the larger cell is the imacroconjugant, while
the smaller cell is called a micro-conjugant (anisogamous conjugation).
The microconjugant detaches and swims about. While swimming, when a micro-conjugant approaches
a macro-conjugant of other Vorticella, then it gets attached to the lower end of that macro-conjugant
near its stalk.
The macronuclei of both the conjugants degenerate and finally absorbed in the cytoplasm.
The micronucleus of macroconjugant divides to form four daughter micronuclei and the
micronucleus of microconjugant divides to form eight daughter micronuclei. All the micronuclei of
the conjugants degenerate except one micronucleus of each conjugant.
In macroconjugant,the micronucleus becomes female pronucleus, while in microconjugant it
becomes the male pronucleus. The male and female pronucleusi are haploid. Themale pronucleus
from the microconjugant migrates into the macroconjugant. The microconjugant disintegrates
The male and female pronuclei fuse together to form zygote nucleus or synkaryon.
In macroconjugant the synkaryon divides to form eight daughter nuclei, seven of these become
macronuclei and one micronucleus.
These seven macronuclei and one micronucleus undergo a series of three successive fissions to form
ultimately seven daughter vorticellas.
III. Automixis:
Automixis is the fusion of two gametic nuclei originating by the division of the single nucleus of an
individual. Example Paramecium.
Automixis may be of the following types:
a. Autogamy:
The fusing nuclei come from the same cell as in Paramecium. All the steps in nuclear changes are similar
to conjugation but the union occurs between the pronuclei of the same individual. Autogamy has been
reported in P. aurelia (W.F. Diller, 1936) which has one macronucleus and 2 micronuclei. In this, the
macronucleus degenerates while the two diploid micronuclei divide by meiosis producing 8 haploid
daughter cells. Seven of the micronuclei degenerate and the remaining one micronucleus divide
mitotically producing two gamete nuclei which fuse to form a diploid nucleus or synkaryon. The
synkaryon divides twice to yield 4 nuclei of which two becomes macronuclei and two micronuclei. The
micronuclei divide twice along with cell body giving rise to two daughter cells with one macronucleus
and two micronuclei each.

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b. Cytogamy:
The beginning of the process i.e., nuclear division resembles that of conjugation but the later stages
resemble that of autogamy. In this, two individuals come together and fuse temporarily by their oral
surfaces but there is no nuclear exchange. Synkaryon is formed by the union of two haploid gametes
within each cell as in autogamy. The whole process gets completed in about 13 hours as reported in P.
caudatum (R. Wichterman 1940). Cytogamy is said to be intermediate between conjugation and
autogamy.
c. Endomixis:
It is a process of nuclear reorganization within a single individual. It was reported for the first time in P.
aurelia by Woodruff and Erdmann (1914). The vegetative macronucleus degenerates while the
micronuclei divide twice producing 8 micronuclei. Six of the micronuclei degenerate leaving two
micronuclei. The cell divides producing two daughter cells, each receiving one micronucleus. The
micronucleus within each cell divide twice mitotically producing four nuclei each. Out of the four nuclei,
two becomes macronuclei and two remains as micronuclei. The two micronuclei within each cell divide
along with the cell resulting in two daughter cells each with one macronucleus and two micronuclei.

d. Hemixis:

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Diller (1936) reported the hemixis in Paramecium aurelia. Hemixis is primarily a process of macro-
nuclear fragmentation and division without any unusual micro-nuclear activity. Diller classified hemixis
into four types, namely A, B, C, and D in P. aurelia but he also encountered all types in mass cultures of
P. caudatum and P. multimicronucleatum.

Hemixis in P. aurelia
Other modes of reproduction:
1. Plasmogamy:
Two or more individuals may fuse by their cytoplasm to form a plasmodium and separate out
unchanged with their distinct nuclei. This sexual phenomenon is known as Plasmogamy and occurs in
certain Rhizopoda and Mycetozoa.
2. Regeneration:
The regeneration and replacement of lost parts among free-living and few parasitic protists is
widespread. A proper proportion of cytoplasm and nucleus can regenerate into an entire individual.
3. Parthenogenesis:
The gametes which fail to fertilize start their development parthenogenetically. Examples: Actinophrys,
Chlamydomonas, etc.

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2.2.4: Morphology, life cycle, pathogenicity and control measures:
Plasmodium
Plasmodium, a genus of parasitic protozoans of the sporozoan subclass Coccidia that are the causative
organisms of malaria. Plasmodium, which infects red blood cells in mammals (including humans), birds,
and reptiles, occurs worldwide, especially in tropical and temperate zones.
Ultra structure of trophozoite:
The ultra-structure of Plasmodium vivax has been revealed by the electron microscope. the Plasmodium
trophozoite in a red blood corpuscle possesses a double membrane, the plasma lemma closely applied to
the cytoplasm. The cytoplasm of Plasmodium vivax contains small dense particles probably containing
ribonucleoproteins.
The endoplasmic reticulum is not well developed and appears as vesicles of variable shapes. The vesicles
are either smooth surfaced or rough surfaced and are loosely scattered in the cytoplasm. The
mitochondria possess double membrane and show peripheral cristae and a structure less central region.
The trophozoite has several mitochondria. The Golgi apparatus is composed of small vesicles arranged
in rows. A double layered concentric body is also found in the cytoplasm attached with the plasma
lemma of Plasmodium Vivax. One or two double membrane vacuoles with structure less matrix, also
occur in the cytoplasm. The function of these vacuoles is unknown. The nucleus is large and its
nucleoplasm is composed of granular and fine fibrillar material. The nuclear membrane is double, to
which RNA particles are attached. The nucleolus lies centrally in the nucleus. Pinocytosis vacuoles are
common in the cytoplasm and serve as food vacuoles. The food vaculoes may also contain hemozoin
depending upon the species of Plasmodium.

Plasmodium trophozoite stage in R.B.C.
Plasmodium species that infect humans

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Until recently, there were four plasmodium species that were considered responsible for malaria disease
in humans: P. vivax, P. falciparum, P. ovale and P. malariae. In 2008, P. knowlesi, a species that used to
infect exclusively apes of the genous Macaque, was recognised by WHO as the fifth plasmodium species
that infect humans is P. knowlesi.
There are several species that have been isolated from chimpanzees, including P. reichenowi and P.
gaboni. P. falciparum, P. gaboni, and other species have been isolated from gorillas. Examples of
parasites found in reptiles include P. mexicanum and P. floridense, and those in birds include P.
relictum and P. juxtanucleare.
Life cycle of Plasmodium
The malaria parasite has a complex, multistage life cycle occurring within two living beings, the vector
mosquitoes and the vertebrate hosts i.e. human. The two first stages take place exclusively into the
human body, while the third one starts in the human body and is completed into the mosquito
organism. The life cycle is almost the same for all the five species that infect humans and follows three
stages:
Transmission routes
The main mode of transmission of the disease is by bites from infected Anopheles mosquitoes that have
previously had a blood meal from an individual with parasitemia. When an infected female anopheles
mosquito bites a person and injects infected with sporozoites saliva into the blood circulation. That is
the first life stage of plasmodium. Less common routes of transmission are via infected blood
transfusion, transplantation, infected needles, and from a mother to her fetus during pregnancy. Other
insects and some mites may also transmit forms of malaria to animals.
Cycle in human body
 Infected female anopheles mosquito bites a person and sporozoites enter into the blood
circulation of human.
 The sporozoites are rapidly taken up by the liver cells.
 In all species of Plasmodium, these parasites develop to form schizonts (the multinucleate stage
of the cell during asexual reproduction), from which several thousand merozoites develop. Red
blood cells are the ‘centre stage’ for the asexual development of the malaria parasite.
 In Plasmodium vivax and Plasmodium ovale only, a proportion of the liver-stage parasites
(known as hypnozoites) remain dormant in the hepatocytes. In this stage the parasite can remain
dormant for months or several years. These two species of parasite can therefore initiate a cycle
of asexual reproduction causing clinical symptoms in the absence of a new mosquito bite,
giving P. vivax infection the name relapsing malaria.
 When the liver cells rupture, the merozoites are released into the bloodstream where they
rapidly invade the red blood cells. These blood-stage parasites replicate asexually – rapidly

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attaining a high parasite burden and destroying each red blood cell they infect, leading to the
clinical symptoms of malaria.
 The trigger is as yet unknown, but a small percentage of merozoites, differentiate into male and
female gametocytes
Cycle in mosquito body
The trigger is as yet unknown, but a small percentage of merozoites, differentiate into male and female
gametocytes, which are taken up by the mosquito in her blood meal. It is these gametocytes that cause
the cycle of transmission to continue back to the mosquito. Male and female gametocytes fuse within the
mosquito forming diploid zygotes, which in turn become ookinetes.
These ookinetes migrate to the midgut of the insect, pass through the gut wall and form the oocysts.
Meiotic division of the oocysts occur and sporozoites are formed, which then migrate to the salivary
glands of the female Anopheles mosquito ready to continue the cycle of transmission back to man.

Plasmodium life cycle
Pathogenicity
When the liver cells rupture, the merozoites are released into the bloodstream where they rapidly invade
the red blood cells. These blood-stage parasites replicate asexually – rapidly attaining a high parasite
burden and destroying each red blood cell they infect, leading to the clinical symptoms of malaria.
Within the red cells, repeated cycles of parasitic development occur with precise periodicity, and at the
end of each cycle, hundreds of fresh daughter parasites are released that invade more number of red

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cells. The merozoites released from the liver recognize, attach, and enter the red blood cells (RBCs) by
multiple receptor–ligand interactions in as little as 60 seconds. This quick disappearance from the
circulation into the red cells minimises the exposure of the antigens on the surface of the parasite,
thereby protecting these parasite forms from the host immune response. The process of attachment,
invasion, and establishment of the merozoite into the red cell is made possible by the specialized apical
secretory organelles of the merozoite, called the micronemes, rhoptries, and dense granules. The initial
interaction between the parasite and the red cell stimulates a rapid “wave” of deformation across the red
cell membrane, leading to the formation of a stable parasite–host cell junction. Following this, the
parasite pushes its way through the erythrocyte bilayer with the help of the actin–myosin motor,
proteins of the thrombospondin-related anonymous protein family (TRAP) and aldolase, and creates a
parasitophorous vacuole to seal itself from the host-cell cytoplasm, thus creating a hospitable
environment for its development within the red cell. At this stage, the parasite appears as an
intracellular “ring” known as signet ring. The growing ring stages overcomes the immunity by red cells
by several mechanisms (by restriction of the nutrient to the abundant hemoglobin, by dramatic
expansion of the surface area through the formation of a tubovesicular network, and by export of a
range of remodeling and virulence factors into the red cell). Hemoglobin from the red cell, the principal
nutrient for the growing parasite, is ingested into a food vacuole and degraded. The parasite depends on
anaerobic glycolysis for energy, utilizing enzymes such as pLDH, plasmodium aldolase etc. As the
parasite grows and multiplies within the red cell.
The duration of each above desribed phase is different for each of the plasmodia as shown in Table 1 that
follows.
Plasmodium species
P. vivax P. ovale P. malariae P. falciparum
Pro-erythrocytic phase (days) 6-8 9 14-16 5-7
Erythrocytic cycle (hours) 48 50 72 48
Incubation period (days) 12-17 or even 6-12 months 16-18 or more 18-40 or more 9-14
Sporogony (days) 8-10 12-14 14-16 9-10
Control measures:
W.H.O. Ministerial Conference held in October, 1992 at Amsterdam evolved a Global Strategy for
Malaria Control as follows—
Insecticide spraying
Since the discovery of DDT in the 1940s, many other insecticides (e.g. malathion. fenitrothion) are
available for spraying. The spraying of indoor surfaces of houses with these insecticides is still the most
effective measure to kill the adult mosquito. Problem is Drug resistance.
Individual Protection

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Man–vector contact can be reduced by other preventive measures such as the use of repellents,
protective clothing, bed nets (preferably impregnated with safe, long lasting repellent insecticides),
mosquito coils, screening of houses, etc. The methods of personal protection are of great value when
properly employed. However, they have rarely been used on a large scale because of the costs involved.
Control of mosquito breeding grounds
Techniques to reduce mosquito breeding sites which include drainage or filling, deepening or flushing
management of water levels, changing the salt content of water and intermittent irrigation are among
the classical methods of malaria control to which attention is being paid again.
Malarial vaccines
No definitive malarial vaccine has been approved the world over against malaria. A number of vaccines
of potential value in controlling malaria are currently under development. A synthetic “Cocktail” vaccine
for Plasmodium falciparum, called Spf66 and developed by Dr. M. Pararroyo in Colombia, has been
tested extensively in South America and more recently in Africa and South East Asia.
Man’s Role in Malaria Control: Man is the most important link in the malaria control chain. He can be
made to understand the problem and he can help in breaking the chain at multiple points. Therefore
great emphasis should be laid on educating the people about malaria and its control, so that common
people can effectively contribute in controlling this disease. This includes education of doctors about the
need for early diagnosis and prompt treatment of malaria.
1. Early diagnosis and treatment – treat early to reduce parasite load, hence spread; prevent deaths
2. Treat completely to prevent spread and relapse
3. Ensure compliance with complete treatment
4. Personal Protection-prevent malaria by using bed nets, insecticide sprays etc., and by
chemoprophylaxis.
5. Seek his help in mosquito control
6. Chemoprophylaxis: Travelers to endemic areas and high risk individuals living in endemic areas
(pregnant, elderly, patients with end organ failure) should be started on chemoprophylaxis against
malaria. This involves taking antimalarial drugs every week (some drugs may have to be taken
everyday) so as to suppress malaria.
Entamoeba histolytica
Amebiasis is caused by Entamoeba histolytica (see the image below), a protozoan that is found
worldwide. The highest prevalence of amebiasis is in developing countries where barriers between
human feces and food and water supplies are inadequate. It is estimated that up to 15% of the world's
population is infected by the pathogen Entamoeba histolytica and E. dispar. Every year, over 100,000
people die of the disease amoebiasis, making it the second most common parasitic cause of death, after
malaria. Entamoeba is a parasite and lives in the mucous and sub-mucous layers of the large intestine of

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man. It may occur in the liver and lungs. Rarely does it invade brain, spleen etc. causing ulcers. The
disease caused by the parasite is known as “Amoebic dysentery.”
Morphology
Trophozoite of E. histolytica is also known as the trophic or magna form. It is the most active, motile,
and feeding form which is pathogenic to man. It lives in the mucosa and submucous layer of the large
intestine of man. It usually measures 20 to 30 microns in diameter and, more or less, resembles the
common amoeba in all structural details. Its body is covered by plasmalemma and cytoplasm is
differentiate into ectoplasm and endoplasm. Endoplasm contains a single spherical nucleus and food
vacuoles . Nucleus has a peripheral crown of chromatin blocks and a centrally located nucleus. Cysts
vary in diameter from 10-20 micro. The cysts are spherical. The cyst wall is double and the cytoplasm
usually bears four nuclei. The cytoplasm is clear and often contains black rod-like chromatoid bar or
bodies. Presence of these bodies is the characteristic of E. histolytica cyst. They occur singly or in
multiples of two.

Life cycle of Entamoeba histolytica
The new hosts are usually infected by drinking water contaminated with the feces of people
shedding Entamoeba histolytica cysts. Consuming foods that might have been contaminated by infected
water, prepared by people with poor personal hygiene or prepared on contaminated surfaces can also
spread the disease, although this mode is far less common. Tropical countries with poor sanitation are
often hit hardest by outbreaks.
The lifecycle of Entamoeba histolytica is pretty typical for a protozoan parasite. Cysts, which are the
round, dormant, resistant and infectious stage, are acquired by the fecal-oral route, usually by drinking
contaminated water. Once in the intestine, excystation occurs, with trophozoites emerging from the
cyst. Trophozoites are the motile, feeding stage of Entamoeba histolytica.
They are amorphous cells containing prominent round vacuoles that move via oozing pseudopod motion
through the length of the intestine, until they reach the large intestine. Once there, they multiply by
binary fission and begin to form new cysts. These cysts are excreted in the feces, can survive outside the
body for several weeks and are capable of infecting new hosts.

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Pathogenicity
Excystation then occurs in the terminal ileum or colon, resulting in trophozoites (invasive form). The
trophozoites can penetrate and invade the colonic mucosal barrier, leading to tissue destruction,
secretory bloody diarrhea, and colitis resembling inflammatory bowel disease. In addition, the
trophozoites can spread hematogenously via the portal circulation to the liver or even to more distant
organs.
E histolytica is capable of causing a spectrum of illnesses such as—
Asymptomatic infection, Symptomatic noninvasive infection, Acute proctocolitis (dysentery), Fulminant
colitis with perforation, Toxic megacolon, Chronic nondysenteric colitis, Ameboma, Perianal ulceration,
Extraintestinal conditions resulting from E histolytica infection include the following—
Liver abscess, Pleuropulmonary disease, Peritonitis, Pericarditis, Brain abscess, Genitourinary disease.
Preventive measures:
Amoebic dysentery is a food and waterborne disease that can be found worldwide. Effective prevention
and control rely on (a) maintaining good environmental sanitation, especially in controlling quality of
drinking water; (b) prompt investigation of cases and implementation of control measures to prevent
spread of the disease; (c) health education to the general public and food trade on observance of good
personal, environmental and food hygiene. The existing prevention and control measures are examined
and some potential areas are discussed for further improvement.
(porus=pore and ferre=to bear)
Unit 2.3: Phylum Porifera (3 lectures)

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2.3.1: General organization and classification
Poriferans, commonly known as sponges are pore bearing animal hence the name phylum Porifera.
They have no tissues like nerve, muscles and internal organs. So they are referred as most simple
animals on the globe. Hence they are often placed in Branch Parazoa of Animal Kingdom. They are
exclusively aquatic (98% are marine), sedentary, solitary or colonial and exhibit mostly radial symmetry
or asymmetry. They are Multicellular organisms having cellular grade of organization without true
tissues; body, comprised roughly of two germ layers (diploblastic= ectoderm and endoderm); enclosing
a cavity, spongocoel; endoskeleton made of spicules and spongine fibres; and numerous pores (ostia)
leading to internal canal system to maintain water current leading to spongocoel which opens to the
exterior with a single large apical opening called osculum. The two germ layers, an ectoderm and
endoderm are attached to each other by a middle cementing matrix called as mesenchyme or Mesohyl.
Mesenchyme contains nearly seven types of amoeboid cells performing different physiological functions
of connective tissue viz. archaeocytes (undifferentiated cells highly totipotent and regenerative),
porocytes (pore), thesocytes (storage), scleroblasts (secrete spicules), collencytes (branched food
supply), chromocytes (color) and myocytes (contractile cells). Ectoderm, known as pinacoderm is made
of flat, polygonal cells called as pinacocytes innervated by porocytes to form ostia. Endoderm in made of
choanocytes hence called as choanoderm. Choanocyte has a single nucleus, granular cytoplasm and a
collar with centrally placed flagellum. Flagella of choanocytes maintain water current in canal system by
beating synchronously.

Sponges have no definite organs and systems. No mouth and gut. All are filter feeders. Digestion is
intracellular. Muscles and nerve cells are absent. All sponges possess power of regeneration. The cells of
sponges show a high degree of independence. Reproduction, both asexual and sexual types. Asexual
reproduction takes place by the fragmentation or through the production of gemmules or buds. Sexual
reproduction by sperms and ova. Mostly hermaphrodite. Fertilization internal. Cleavage is holoblastic

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and radial. Two types of flagellated larval forms are seen in sponges- coeloblastula larva and
Parenchymula larva.

Phylum Porifera is classified into three classes based on spicules and canal system.

Class- Calcarea:
i) Skeleton; made up of calcareous spicules. ii) Radially symmetrical. iii) Exclusively marine shallow
water dwellers. iv) They are commonly referred as chalk sponges. v) Canal system is asconoid or
syconoid type.
Example: Scypha or Sycon

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Scypha or Sycon is widely distributed, a small,
solitary or colonial, shallow water marine sponge
found attached to rocks, shells etc. Body is slender
vase shaped measuring 20-25 mm in length and 5-
6 mm in diameter. Each individual in the colony
grows to about 3 inches in height and are attached
to each other by a horizontal tube base. The body
surface is perforated with numerous ostia. The free
end bears a large osculum, encircled by giant
monoaxon spicules called oscular fringe. Canal
system is syconoid type.
Example: Leucosolenia

Leucosolenia is colonial, small, simple finger like
sponge consists of a few vertical tubes united below
by stolon; found in shallow (coastal) marine waters
below low tide mark of tropics. Each tube of the
colony may reach up to 25 mm in height and also
produces a number of buds. The surface of body is
perforated by numerous pores called Ostia or
incurrent pores. At the anterior end of its body a
single large osculum is present which opens into a
large central body cavity called as spongocoel. Each
main tube opens to the exterior by an aperture
called osculum at the top.
Leucosolenia colony
The cavity of the tube is known as spongocoel or paragastric cavity. Canal system is simplest
asconoid type. Leucosolenia has phylogenetic significance because sycon and leucon sponges pass
through ascon stage during their development.
Class Demospongia:
i) Body, asymmetrical. Ii) Skeleton consists of siliceous spicules or sponging fibres or a combination of
both or absent. Iii) Canal system is leuconoid type. Spongocoel is entirely absent. iv) Canal system is
leuconoid type. v) Class Demospongiae includes approximately 4,750 species in 10 orders including
more than 90 % of all known species. Their geographic distribution in the marine environment is from
the intertidal to the abyssal zone; some species belonging to a family inhabit, freshwater. vi)
Demospongiae skeletons are composed of spongin fibers or siliceous spicules. vii) Members of the
Demospongiae are asymmetrical and range in size from a few millimeters to over 2 meters in largest
dimension. viii) They occur in various shapes e.g. thin encrustations, lumps, finger-like growths, or urn

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shapes. Pigment granules in amoebocytes often make members of this class brightly colored, including
bright yellow, orange, red, purple, or green.
Example: Spongilla
It is best known fresh water sponges. It is a colonial sponge. Exhibits
various shades of green colour due to presence of Zoochlorellae a green
alga in the tissues. Body wall consists of very thin dermal membrane
provided with dermal pores or ostia and several oscula. Skeleton is of
spongin fibres with interspersed silicious monaxon spicules. Asexual
reproduction is by gemmules. Sexual reproduction is by way of unusual
free swimming larva which is characteristic of Spongilla.

Class- Hexactinellida:
i) Body cylindrical. ii) The skeleton is composed of hexactinate siliceous
spicules. iii) Canal system is of leucon type. iv) Solitary exclusive marine
forms found in deep seas. iv) Commonly referred as glass sponges. Canal
system is either syconoid or leuconoid type.
Example: Hyalonema:
Popularly known as glass rope sponge. The spicules are often fused to form a
lattice like skelton giving the sponge a glass like appearance. The spicules of
root tuft continue through the sponge body as an axis or columella and
projecting above as gastral cone. The root spicules are compact, stalk like
elongated twisted and giving appearance of a rope. The middle part of the
columella commonly has symbiotic polyps attached to it.
2.3.2: Skeleton in sponges
Spicules in Sponges
1. The body wall of sponges is supported by spicules or spongin fibres or both.
2. The spicules are secreted by special mesenchymal cells called as scleroblasts.
3. Spicules may be calcareous or siliceous i.e. made up of calcium carbonate or silica.
4. The spicule has an axis of organic material around which is deposited calcium carbonate or silica.
5. The spicules are of two types- megascleres or supporting spicules and microscleres or small and
non-supporting spicules.
6. The spicules are further classified according to the number of their axes and rays. Words
designating the number of axes end in axons, those referring to the number of rays end in actine or
actinal.
Classification of megascleres on the basis of main axes and rays into:-

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The megascleres are the larger skeletal spicules that constitute the chief supporting framework of the
sponge. There are five general types of megasclere spicules, viz., monaxons, tetraxons, triaxons,
polyaxons and spheres.
(i) Monaxons: These are formed by growth in one or both directions along a single axis, which may
be straight or curved. When growth has occurred in one direction only, the spicule is called
monactinal monaxon. On other hand if growth occurs on either end of the spicule is known as
diactinal monaxon. Styles are typically rounded (strongylote) at one end and pointed (oxeote) at
the other. Styles in which the broad end is knobbed are called tylostyles; those curved with
thorny processes are acanthostyles.
(ii) Tetraxons:
Tetraxon spicules are also called tetractines and quadriradiates. They consist typically of four
rays, not in the same plane, radiating from a common point. The four rays of the tetraxon
spicule may be more or less equal in which case the spicule is called a calthrops.
Generally one ray, rhabdome, is elongated bearing a crown of three smaller rays; such spicules
are termed triaenes. By loss of one smaller ray results into a diaene. If the elongated ray bears a
disc at both ends, it is called amphidisc. Loss of elongated ray results into a triradiate or triactinal
spicule, called a triod characteristic of calcareous sponges.
(iii) Triaxons:
The triaxon or hexactinal spicule consists fundamentally of three axes crossing at right angles,
producing six rays extending at right angles from a central point. The triaxon spicules are
characteristic of class Hexactinellida.
(iv) Polyaxons: These spicules in which several equal rays radiate from a central point.
(v) Spheres: These are rounded bodies in which growth is concentric around a centre.
Classification of Microscleres:
The microscleres are the smaller flesh spicules that occur strewn throughout the mesenchyme.
However, they do not form the supporting framework. The microspheres are of two types, viz., spires
and asters.
(i) Spires:
Spires are curved in one plane or spirally twisted and exhibits many shapes. The most common types
are the C-shaped forms, called sigmas; the bow-shaped ones, or toxas and the chelas with recurved
hooks, plates or flukes at each end.
(ii) Asters:
Asters include types with small centres and long rays and large centres and small rays. Among the small
centred forms are oxyasters with pointed rays, strongylaster with rounded ends and tylasters with

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knobbed rays. Large-centred forms include spherasters with definite rays and sterrasters with rays
reduced to small projections from the spherical surface.
Short spiny microscleric monaxons are known as streptasters, of which the principal sorts are the
spirally twisted spirasters.

2.3.3: Canal system in sponges
Body of all sponges is the perforated by large number of apertures through which water enters inside
body and flows through a system of criss-crossing canals collectively forming the canal system which
is a characteristic feature of poriferans. The entire physiological activities of the animal depend on the
water current and the exchanges between the body and the exterior arc maintained through the water
current. The food and oxygen are brought through this current while excreta and reproductive bodies
are excluded through this current. Following types of canal systems are found in sponges:

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In Leucosolenia, canal system is seen in its simplest type. In other forms, it may attain a high degree of
complexity.

Asconoid type

Asconoid canal system in Leucosolenia
Leucosolenia has a simplest canal system called
as asconoid type:
a) Dermal Ostia: Externally numerous ostia open
into their incurrent canals which lead to spongocoel.
b) Incurrent canal: Ostia and incurrent canals are
formed by a special amoebocyte called as porocyte.
c) Spongocoel: Spongocoel is lined internally by a
single layer of choanocytes. Spongocoel opens to the
exterior by a single opening called osculum.
Course of water current

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Syconoid canal system in Scypha
Syconoid type
a) Dermal ostla : On the body surface of the Scypha
dermal ostia are present. They open into incurrent
canals.
b) In current canals : These are narrow canals. These
are covered inside by epithelial cells called as
porocytes. They open to the next chambers (radial
canals) through numerous narrow prosopyles.
c) Radial canal: The body wall is folded internally to
form radial canals. As per name these radial canals are
arranged radially in the body wall. Radial canals are
lined internally by choanocytes. Prosopyles open into
these Radial canals.
d) Spongocoel: All the radial canals open to their
corresponding excurrent canals by an opening called
apopyle. Excurrent canals open to a central spongocoel.
Spongocoel finally opens to exterior through a common
opening, the osculum.



Rhagon type Canal system of Spongilla larva


The rhagon type is a little complex than
the complex syconoid type and is found
in the larval stage of Demospongiae.
Between the epidermis and the
chambers lies a considerably thickened
mesenchyme which is traversed by
incurrent canals and subdermal spaces.
The body is conical with osculum
situated at the summit. The spongocoel
is bordered by oval flagellated
chambers, opening into it by wide
apopyles

Leuconoid type

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Spongilla is a pond sponge and has diplodal-
leuconoid type of canal system.
Subdermal space: The surface is perforated
by dermal pores. The dermal pores lead into
subdermal spaces or incurrent canals, which
are found in mesenchyme. The subdermal
space and incurrent canals lead into the small
rounded flagellated chamber through an
opening, termed as prosopyle.
Flagellate chambers: In the leuconoid canal
system the choanocyte lining of radial canals
evaginates into many small chambers to give
rise to a cluster of small flagellated chambers.
These flagellate chambers are often inter-
connected by narrow canals forming a network
of canal system.
Excurrent canals: The flagellated chambers
open by apopyles into excurrent canals.
Excurrent canals unit to form a larger
excurrent canal (reduced spongocoel). The
largest excurrent canal leads to osculum. Few
gradations of leuconoid type of canal system
are seen in sponges.

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a) Eurypylous type:It is a
simplest and most primitive
leucon type of canal system.In this
type, the flagellated chambers
communicate directly by broad
aperture, the apopyles, with
excurrent canals.Ex.Plakina.
b) Aphodal type:In this type, the
apopyle is drawn out as a narrow
canal, called aphodus.This
connects the flagellated chamber
with excurrent canal.Ex.Geodia.
c) Diplodal type:In some
sponges, besides aphodus, another
narrow tube, called prosodus, is
present between the incurrent
canal and flagellated chamber.The
pattern is called the diplodal type,
Ex Spongilla Oscarella.

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(Coelom=Body cavity; enteron=digestive)
Unit 2.4: Phylum Cnidaria (2 lectures)
2.4.1: General characters and classification
Cnidarians also referred as coelenterates are multicellular, diploblastic and radially symmetrical aquatic
animals exhibiting organ-tissue level of organization. Body wall is made-up of external ectoderm and
internal endoderm between which lies a transparent matrix called mesogloea. Epidermis has modified
venom cells called cnidoblasts containing nematocysts which are offensive and defensive in function.
Mouth is present; but anus is absent. Body cavity is modified archenteron of gastrula. It is digestive and
circulatory in function hence referred as coelenteron or gastro-vascular cavity. Digestion is extracellular
as well as intracellular.

They occur in asexual sedentary polyp and sexual free swimming medusa forms. Sexual forms contain
gonads in their gastro-vascular cavity. Some exhibit polymorphism. In polymorphism the individuals are
referred as zooids.

These organisms exhibit tissue level organization and develop muscle cells, nerve cells, sensory cells,
statocysts and mucus cells. Unlike sponges these can therefore make movements effectively in water
with balancing and coordination of body. Nervous system is diffuse-type, formed or nerve-nets.

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Respiratory, excretory and circulatory system are absent. Cnidarians often secrete outer protective,
horny exoskeleton called as perisarc by underneath living surface called as coenosarc. Some secrete
external calcareous exoskeleton called coral. Fertilization is external and cleavage is holoblastic,
development is indirect and there are one or two larval forms viz. planula and ephyra. The life cycle
exhibits alternation of sexual and asexual generation.

Class1: Hydrozoa
(Hydra; water; zoon: animal)
 Habitat: mostly marine, few are fresh water
 Habit: some are solitary and some are colonial
 Asexual Polyps is asexual and dominant form. Tentacles are solid.
 Medusa possesses true velum and is known as hydromedusa.
 Mesogloea is simple and acellular.
 There are different kinds of individual forming a colony hence known as polymorphic colony.
 Examples: Hydra, Obelia, Physalia physalis (portuguese man of war), Tubularia
Class 2: Scyphozoa or Scyphomedusae
 Habitat: exclusively marine
 Habit: solitary, freely swimming
 Medusa is dominant and it is Large bell or umbrella shaped and is sexual form. Four horseshoe
shaped gonads develop into stomach pouches. Sexes separate.
 Polyps is short lived or absent
 Mesogloea is usually cellular and thick giving jelly like look.
 Examples: Aurelia aurita (Jelly fishe), Rhizostoma
Class 3: Anthozoa or Actinozoa

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(Anthos: flower; zoios: animal “flower like animals”)
 Habitat: exclusively marine
 Habit: Solitary or colonial
 Medusa stage is absent. Polyp is sexual form.
 Tentacles are hollow and are arranged in many circles around the mouth.
 Mesogloea contains fibrous connective tissue and amoeboid cells.
 Mouth leads to ciliary gullet like siphonoglyphs which is eversible while feeding.
 Gastrovascular cavity is divided by vertical partitions called as mesenteries.
 Exoskeleton is mostly calcareous and develops into coralites which unite to form body called coral.
Coral grow together to form huge coral reefs.
 Examples: Metridium (sea anemone), Telesto, Tubipora, Xenia
2.4.2: Obelia - Polymorphism, life cycle and alternation of generations
Obelia
1. It is marine sedentary colonial form growing to about few cm in height.
2. A colony made up of horizontal branches called as hydrorhiza and vertical branches as hydrocaulus.
3. These branches have external nonliving transparent covering called as perisarc and the internal
cellular living part coenosarc. Coenosarc contains gastrovascular cavity.
4. The colony consists of two types of individuals namely, (a) Feeding zooids called as gastrozoids
which are polyp like and (b) the club shaped gonozoids where budding occurs. The bud develops into
medusa.
Polymorphism in Obelia:

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Obelia forms a trimorphic colony of three kinds of individuals exhibiting polymorphism. The colony
contains Gastrozoid or hydranth, gonozoids or blastostyles whereas the buds present in gonozoid are
developing medusa. Gastrozoids are digestive in function. It opens at apex by a mouth which is
surrounded by tentacles. Tentacles are used to capture the food. Gastrozoids supply digested food to
gonozoids and other parts of the body. The gonozoids are club shaped and are reproductive individual.
Gonozoid has a fertile central stalk developed from coenosarc called as blastostyle. Blastostyle undergoes
asexual reproduction by budding. These buds later develop into sexual form called as medusa.
Life History of Obelia:
Fertilisation:
Fertilisation usually takes place in open sea water where the gametes are set free. Sometimes, the
sperms are carried into the female medusae with water currents and there they fertilize the eggs in situ.
However, the parent medusae die soon after liberating their respective gametes.

Development:
The zygote undergoes complete or holoblastic and equal cleavage to form a single-layered blastula with a
blastocoele. Some cells migrate into blastocoele, eventually filling it completely to form a solid gastrula
known as stereo gastrula. Its outer cell layer becomes the ectoderm and inner cell mass the endoderm.
The gastrula elongates and. its outer layer of ectoderm cells becomes ciliated, and now it is called
planula. Soon, a cavity called enteron develops in the solid endodermal cell mass by the process of
delamination and the planula becomes a two-layered larva having an outer ciliated ectodermal cells and
an inner layer of endodermal cells.

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The planula after a short free-swimming existence settles on some solid object by its broader end. The
free end forms a manubrium with a mouth and a circlet of tentacles. Thus, a simple polyp or hydrula is
formed which grows a hydrorhiza from its base, from which an Obelia colony is formed by budding.

The free swimming planula stage in the life history of Obelia, helps in the dispersal of the species. The
life history may be represented as male and female gametes → zygote → planula larva → hydrula →
colony → sexual medusae → gametes → zygote and so on.
Alternation of generations
It is clearly evident from the life history of Obelia that there is an alternation of polypoid and medusoid
generations.
The polypoid generation is asexual and produces polyps and blastostyles by asexual budding. The
blastostyle also produces medusae by asexual budding. The medusae do not produce medusae but they
give rise to gametes, which after fertilisation develop into a polypoid colony from which medusae are
produced again by budding.
Thus, an asexual polypoid generation alternates with a sexual medusoid generation. This phenomenon
is known as alternation of generations, till recently. The term alternation of generations means that the
individual exists in two distinct forms, which alternate each other regularly in the life history.
One individual possesses the power to reproduce the other by asexual reproduction, which again by
sexual reproduction gives rise to the next generation. Therefore, a true alternation of generations is
always between a diploid asexual and haploid sexual generations, as is exhibited by fern plant.
But, in Obelia the condition is somewhat different and, therefore, objections were raised to use the term
alternation of generations for it. Because, in Obelia, there are no true two generations to alternate each
other. The medusae are modified zooids capable of free swimming existence and moreover they are not
produced directly from a zygote but are budded off from the blastostyle.
The gonads found in medusa are not formed in it but actually they are formed in the ectoderm of
blastostyle which later on migrate into the medusa and become situated on its radial canals. Thus, it is
rather difficult to distinguish between sexual and asexual generations. Hence the term metagenesis is
used to replace the term alternation of generations in Obelia.
Thus, in the life history of Obelia, there is a regular alternation between fixed polypoid and free-
swimming medusoid phases, both of them being diploid. Such an alternation of generations between
two diploid phases is known as metagenesis.