Introduction to non chordates

biology

1. INTRODUCTION TO NON CHORDATES
WHAT ARE INVERTEBRATES?
Animals are broadly classified into two groups: invertebrates and vertebrates. The animals which
lack backbone or vertebral column are called invertebrates. These constitute the main bulk of animal
kingdom. Out of about 1.25 million animal species 95% constitute invertebrates and only 5%
constitute vertebrates. We know that worms, butterflies, sponges, snails, crabs, starfishes etc. vary
greatly from one another in their morphology, physiological and behaviour but even than these have
been included in invertebrates. This has been possible only due to the fact that these do not possess
a backbone.
INVERTEBRATA VERSUS NON-CHORDATA
Animals are distinguished into two major categories: chordates and the non-chordates. This division
is based on the presence or absence of a single character the notochord. Phylum Chordata includes
those animals which possess notochord during any period of their life. Sub-phylum vertebrata
includes those animals in which notochord are partly or wholly taken over by a segmented vertebral
column.
But animals lacking notochord are called non-chordates and those lacking a vertebral column are
called invertebrates.
INVERTEBRATE PHYLA AND NUMERICAL STRENGTH
These are about 1.25 million species of animals in the world. These are divided into about 32 phyla
out of which 30 are invertebrate phyla which are characterized by:
1. Anatomical features of the members of each phylum constructed on same basic plan.
2. Functional inter relationship and integration.
3. All members of a group have common ancestory.
These are about 1.25 million species of living animals in the world. Of these about 50000 species
belong to the chordates. The remaining about 1.2 million species constitute the invertebrates. Out of
these nearly one million are Arthropods, 80,000 molluscs, 50,000 protozoans, 5,000 sponges, 11,000
coelenterates, 15,000 flat worms, 12,000 nematodes, 8,700 annelids and 6,000 echinoderms.

2. The number of extinct species is around seven times the number of living species.
Table 1 listing the 30 invertebrate phyla with approximate number of species in each phylum is given
below:
S.no Phylum No. of Species S.no Phylum No. of Species
1 Protozoa 50,000 16 Sipunculida 275
2 Mesozoa 50 17 Mollusca 80,000
3 Porifera 10,000 18 Echiurida 60
4 Coelenterata 11,000 19 Annelida 8,700
5 Ctenophora 90 20 Tardirgada 180
6 Platyhelminthes 15,000 21 Onychophora 65
7 Nemertinea 750 22 Pentastomida 70
8 Acanthocephala 300 23 Arthropoda 900,000
9 Entoprocta 60 24 Phoronida 15
10 Rotifera 1,500 25 Ectoprocta(bryozoa) 4,000
11 Gastrotricha 175 26 Brachiopoda 260
12 Kinorhyncha 100 27 Echinodermata 6,000
13 Nematoda 10,000 28 Chaetognatha 50
14 Nematomorpha 250 29 Pogonophora 80
15 Priapulida 8 30 Hemichordata 80

MOJOR AND MINOR PHYLA
On the basis of number of species and individuals and their participation in ecological communities
the invertebrates are divided into two categories: major phyla and minor phyla.
DIFFERENCES BETWEEN MAJOR PHYLA AND MINOR PHYLA
Characters Major Phyla Minor Phyla
1. No. of species
2. Participation in ecological
communities
3. No. of phyla
4. Examples
More no. of species.
Great Participation.

Eleven.
Protozoa, Porifera,
Less no. of species.
Less participation.

Nineteen.
Mesozoa, Ctenophora,

3. coelenterates, Platyhelminthes,
rotifera, nematodes, annelida,
arthropoda, mollusca,
Ectoprocta and Echinodermata.
Nematomorpha, Onychophora,
Phoronida, Brachiopoda,
Pogonophora etc


LOWER AND HIGHER INVERTEBRATES
On the basis of their size and body organization invertebrate phyla are divided into two categories:
Lower invertebrates and higher invertebrates.
Differences between lower and higher invertebrate:
Characters Lower Invertebrates Higher Invertebrates
1. Size Generally small sized. Generally large sized.
2. Body
organisation
Simple. Complex.
3. Symmetry Radial, biradial, asymmetrical. Bilateral symmetrical.
4. Germ layers Diploblastic. Triploblastic.
5. Coelom Acoelomates or pseudocoelomates. True coelomates.
6. Gut Generally incomplete and non-
muscular.
Always complete and muscular.
7. Circulatory
system
Not well developed, blood vascular
system absent.
Well developed.
8. Examples Protozoa, Porifera, Coelenterata,
Platyhelminthes and Nematoda.
Annelida, Arthropoda, mollusca,
Echinodermata.

GENERAL CHARACTERISTICS OF INVERTEBRATES
1. Habitat. The invertebrates occupy a great variety of habitats and have adapted themselves in
different mode of life. All the 30 phyla must probably originate in the sea.
Protozoans and cosmopolitan in their distribution and may be fresh water (amoeba) or marine
(elphidium) or parasitic (entamoeba histolytica) or symbiotic (trichonympha) or commensal
(opalina).

4. Most of sponges (except spongilla) and coelenterates (except hydra) are marines in their habitat.
Helminthes also vary greatly in their habitats and may be free living (planarians) or ectoparasitic
(gyrodactylus fluke) or endoparasitic (blood fluke).
Annelids occur in different habitats and are mostly aquatic (fresh water common leech and marine
polychaetes) while a few annelids are terrestrial and found in moist soil (earthworm).
Arthropods form the most diverse and biological successful group of animals. They live in all kind of
habitats. Among arthropods only crustacea is largely marines while spiders, centipedes, millipedes,
and insects are most terrestrial. Insects are the only invertebrates capable of flies.
Molluscs are mostly marines while a few snails (pila) and clams (unio) are fresh water and a few
slugs (limax) are terrestrial.
Echinoderms are exclusively marines.
2. Size. The invertebrates exhibit a great variation in size. They range from microscopic protozoans to
large sized cephalopods. Smallest protozoan is Plasmodium of human RBC. Giant squid (Architeuthis)
is a large sized cephalopod measuring about 16.5 meters in length while largest invertebrate is
Diphyllobothrium tapeworm.
3. Symmetry. All symmetries are represented by the invertebrates. Protozoans show bilateral as well
as radial symmetry while some are asymmetrical. Sponges are either asymmetrical or radially
symmetrical. Coelenterates are radially symmetrical. Sea anemones and ctenophores have biradial
symmetry. The members of remaining phyla are mostly bilaterally symmetrical. Spherical or
universal symmetry is represented in some spherical protozoans like heliozoa and radiolaria.
Echinoderms have pentamerous radial symmetry.
The animals having radial symmetry are placed in radiata while the animals from the flatworms
onward and having primarily bilateral symmetry are placed in bilateria.
The radial symmetry is supposed to be primitive than the bilateral symmetry. The radial symmetry is
further of following types:
(a) Tetramerous radial symmetry in which body parts are in 4 or multiple of 4 eg. Medusa of obelia
and all scyphozoans (jelly fish).
(b) Biradial symmetry in which body can be divided into two similar parts by only one or two vertical
planes eg. Sea anemones

5. (c) Pentamerous radial symmetry in which body parts are arranged in five or multiple of five eg.
Echinoderm
(d) Hexamerous radial symmetry in which body parts are arranged in six or multiple of six eg. Sea
anemone
4. Level of organisation. Invertebrates exhibit all the three levels of organisation eg.
(i) Cellular organisation. The animal body is formed of many cells but the cells show no coordination
to form tissues eg. Sponges
(ii) Tissue organisation. The animal body is formed of many cells which coordinate for the specific
function and form four types of tissues eg. Epithelial tissue, connective tissue, muscular tissue and
nervous tissues examples Coelenterates like hydra.
(iii) Organ system organisation. The body is multicellular and cells coordinate to form tissues, organs,
system eg, from flatworm onward.
The protozoans show protoplasmic grade of organisation.
5. Germ layers. Protozoans do not have any germ layer. Other invertebrates are divided in to two
categories on the basis of germ layer.
(i) Diploblastic animals. Whole body is formed from two germ layers (outer ectoderm and inner
endoderm) of gastrula eg. Sponges and coelenterates
(ii) Triploblastic animals. Whole body is formed from three germ layers (outer ectoderm, middle
mesoderm and inner endoderm) of gastrula eg. From flatworm onward
6. Body plan. Invertebrates show all the three basic body plans:
(i) Cell aggregate plan. In this the body is just an aggregate of cells with no coordination between
them and little differentiation eg. Sponge
(ii) Blind sac plan. In this multicellular animal has organ system organisation but there is incomplete
alimentary canal in which single opening acts as both mouth and anus (for regeneration eg.
Flatworms) it is also found in coelenterates (hydra).
(iii) Tube within a tube plan. In these multicellular animals has organ system organisation and is with
complete alimentary canal having two separate openings mouth for ingestion at the anterior end
and anus or cloacal aperture for egestion at the posterior end. It is again of two types:

6. (a) Protostomous. In this blastopores of archenteron of gastrula larva forms mouth and is formed
earlier than anus examples Roundworms, annelids, molluscs and arthropods.
(b) Deuterostomous. In this blastopore of gastrula forms anus so anus is formed earlier than mouth
examples are in echinoderms and chordates.
(Fig. 1 Types of body plan. A cell aggregate plan B blind sac plan C tube within a tube plan)
7. Coelom (Body cavity). It is the space between body wall and gut wall. So the coelom separates the
muscles of gut and body wall. On the basis of nature of coelom animals are divided into the
following categories:
(i) Acoelomates
(ii) Pseudocoelomates
(iii) Eucoelomates
(iv)Haemocoelomates
(i) Acoelomates. Among invertebrates up to flatworm there is no coelom and they are called
acoelomates.
(ii) Pseudocoelomates. In Aschelminthes (Ascaris) coelom is present but is not lined by peritoneal
layer is called pseudocoel or falsecoelom so the round worms are pseudocoelomates it is developed
from blastocoel.
(iii) Eucoelomates. From annelids to mammals body cavity is lined by peritoneal layers. These are
parietal peritoneum on outer side and visceral peritoneum on inner side. It is called true coelomates
or Eucoelomates. It is filled with coelomic fluid.
(iv)Haemocoelomates. In arthropods and molluscs true coelom is reduced and body cavity is a
pseudocoel with blood and called haemocoel and animals are called Haemocoelomates.
On the basis of origin coelom is of two types:
(a) Schizocoel. When coelom is formed by splitting of mesoderm examples are annelids, arthropods
and molluscs. The animals having schizocoelic coelom are grouped in schizicoela.
(b) Enterocoel. When coelom is formed from the enteron of gastrula examples are Echinoderms and
chordates. Such animals are grouped in enterocoela.

7. 8. Cephalization. It is the differentiation of head at the anterior end of body due to concentration of
brain and sense organs. It is absent in protozoans, sponges and coelenterates while it is present from
flatworms onward. It is so as anterior end of body is first to come in contact with environment.
9. Metamerism. The repetition of internal body parts is called Metamerism. Among the
invertebrates it is found in annelids, arthropods and molluscs only. In annelids (earthworm) body is
externally divided into segments called metameres or somites by ring like grooves called annuli
while body cavity is internally divided by septa. External division corresponds with internal division
and is called metameric segmentation.
In tapeworms, new segments called proglotids, are formed from the neck by the process of
strobilation just behind the anterior end of body and is called pseudometamerism.
10. Digestive tract. The digestive tract is altogether absent in Mesozoa and parazoa while most of
eumetazoa have a digestive tract so also called enterozoa. Digestive tract when present is of two
types:
(i) Incomplete gut. In this gut has only mouth both ingestion and egestion examples are
Coelenterates and flatworms.
(ii) Complete gut. In this gut have mouth for ingestion and anus/cloacal aperture for egestion
examples are from roundworms onward.
Digestion may be intracellular in the protozoans and sponges while it is intercellular or extracellular
in most of eumetazoans. Coelenterates show partly intercellular digestion.
11. Respiratory organs. Different invertebrates have different respiratory organs for the exchange of
environmental O2 and body CO2 to oxides glucose to produce energy rich ATP molecules.
Respiratory organs and modes of respiration in invertebrates:
Mode of respiration Respiratory organ Eg
1. Body surface respiration Body surface Protozoans, Sponges, coelenterates and
many worms.
2. Cutaneus respiration Skin Most of annelids (eg. Earthworms)
3. Branchial respiration Gills Crustaceans (eg, prawn) and some
annelids (eg. Amphitrite)
4. Tracheal respiration Dermal branchiae
Ectodermal tubes called
Echinoderms.
Insects, millipedes and centipedes.

8. trachea
5. Pulmonary respiration Lungs Terrestrial gastropods called slugs.
6. Book gill respiration Book gills Limulus
7. Book lungs respiration Book lungs Arachnids

Most of the invertebrates are aerobes but some endoparasites are anaerobes.
12. Circulatory system. It is concerned with the transportation of materials. Sponges and
coelenterates show water circulation maintained by the beating of flagella of choanocytes and
gastrodermal cells respectively. Flatworms show parenchymal circulation while the nematodes show
pseudocoelomic fluid circulation. But the blood vascular system is absent upto the nematodes.
Annelids are first animals to have blood vascular system. It is of two types:
(i) Open circulatory system. In this type blood comes out of blood vessels in spaces called sinuses. It
is found in arthropods most of molluscs and leeches. In this, blood flows at slow speed and pressure.
There is direct exchange of materials between blood and body.
(ii) Closed circulatory system. In this type blood remains inside the blood vessels. It is found in most
of annelids and all vertebrates. In this blood flows at faster speed and high pressure due to force
provide by the contraction of heart. Exchange of materials occurs through tissue fluid. It can be
regulated according to the needs of body.
Arthropods and molluscs have body cavity which is filled with blood depends upon pigments present
in blood. Examples
In annelids, blood is red due to an iron containing haemoerythrin present in plasma.
In serpulids, spirobids, and sabelid fan worms, blood plasma has a green pigment called
chlorocruorin.
In molluscs blood is blue coloured due to copper containing haemocyanin in plasma.
In insects blood is colourless and is called haemolymph.
13. Excretory organs. It expels the nitrogenous wastes out of body to maintain homeostasis inside
the body. Invertebrates have a variety of organs for excretion.
Excretory organs and mode of excretion in the invertebrates:

9. Mode of excretion Examples
Body surface excretion Sponges, coelenterates and echinoderms.
Protonephridia (female cells) Flatworms.
Nephridia Annelids
h-shaped and formed of two intracellular tubes Roundworms
Antennary or coxal glands Crustaceans (prawn)
Malphigian tubules Insects, centipedes and millipedes
Coxal glands Scorpion and spiders.
Kidneys Pulmonate gastropods

These excretory organs perform the function of excretion and osmoregulation. On the basis of
nature of nitrogenous wastes excretion is of three types:
(i) Ammonotelism. When a main nitrogenous waste is ammonia and the animals possessing
Ammonotelism are called ammonotelic animals. It is found in aquatic animals like protozoans,
sponges, coelenterates, crustaceans, flat worms and echinoderms.
(ii) Ureotelism. When main nitrogenous waste is urea and such animals are called ureotelic animals.
It is found in land forms of annelids and terrestrial molluscs.
(iii) Uricotelism. In this main nitrogenous waste is uric acid and such animals are called urecotelic
animals. It is found in insects, millipedes and centipedes.
Molluscs like limmaea, unio etc are aminotelic and expel amino acids.
14. Nervous system. It controls and coordinates the body functions to maintain homeostasis inside
the body. It is absent from the protozoans and the sponges. It is of different type in different groups
of animals:
Animal group Types of nervous system
1. Coelenterates Diffuse type (nerve fibres not differentiated into dendrons and
axons), plexiform (forming network) and synaptic. It is formed of
a ring of nervous tissue encircling the body.
2. Flatworms and roundworms Ladder type (found of a nervering and many solid nerve cords

10. interconnected by connectives.
3. Annelids Central nervous system (cns) formed of a circumpharyngeal
nerve ring and ventral solid and ganglionated nerve cord.
4. Arthropods CNS formed of a circum oesophageal nerve ring and a double
ventral solid and ganglionated nerve cord.
5. Molluscs Formed of a few ganglia interconnected by some commissures
and connectives.
6. Echinoderms Two nerve rings (oral and aboral) and radial nerves.
15. Sense organs. Though different invertebrates have different types of sense organs but in general,
these are of much simpler type:
Animal group Types of sense organs
1. Protozoans Protoplasm with irritability; stigma or eye spot in the flagellates.
2. Sponges No distinct sensory cells.
3. Coelenterates Distinct sensory cells in both epidermis and gastrodermis. These may be
organized to form ocelli (photoreceptor), olfactory pits and statocysts
(equilibrium).
4. Flatworms Eye in planaria; statocysts in convoluta; but are reduced in parasitic forms.
5. Roundworms Simple type and include papillae, amphids (gustatory) and phasmids
(chemoreceptors).
6. Annelids Simple eyes (in polychaetes and leeches), tentacles (tangoreceptors) and
buccal receptors (gustatory).
7. Arthropods Compound eyes with ommatidia (insects and crustaceans); also simple
eyes; statocysts (prawn); antennae (tactile and olfactory); sensillae
(insects).
8. Molluscs Osphradium (chemoreceptor), statocyst, complex eye (in cephalopods).

11. 9. Echinoderms Poorly developed as tangoreceptors, chemoreceptors and photoreceptors
etc.
16. Reproductive system and development. In invertebrates, the mode of reproduction varies from
simple asexual binary fusion (in protozoans like amoeba, paramecium) and external (sponges and
hydra) or internal budding (spongilla) to most complicated sexual reproduction. Some invertebrates
undergo parthenogenesis (unfertilized egg developing into adult organism) examples are aphids,
rotifers, drones of honey bee while some invertebrates like gall fly, miaster undergo paedogenesis
eggs produced by immature individuals developing into larvae.
Sexually reproducing invertebrates have reproductive organs called gonads (testes and ovaries). On
the basis of nature of gonads, individuals are unisexual or dioecious (sexes are separate eg Ascaris,
cockroach etc); or bisexual or monoecious or hermaphrodite (eg Taenia, Fasciola, earthworm, leech
etc). In unisexual invertebrates, the sexes may be morphologically similar (eg all the echinoderms) or
may show sexual dimorphism (eg ascaris, cockroach and prawn etc.).
Fertilization involves the fusion of sperm and ovum to form the zygote. Fertilization may be external
(all echinoderms) or internal (most of insects and cephalopods). Unisexual invertebrates always
show cross fertilization. Bisexual invertebrate may be undergoing self-fertilization (Fasciola Taenia)
or cross fertilization due to protandrous condition (earthworm and leech); or protogynous condition
(Scypha). Zygote formed may be mosaic having pre-determined fate so complete egg should be
present to form complete organism examples annelids and molluscs or equipotential (egg can
compensate the loss of a part of it example echinoderms). Egg is either microlecithal and
homolecithal (eg sea urchins) or macrolecithal and centrolecithal (eg insects).
Cleavage is either holoblastic or complete (eg echinoderm); or meroblastic or partial (eg insects).
Cleavage is radial (eg sponges, coelenterates and some echinoderms like starfish) or spiral (eg
Helminthes, annelids and molluscs). On the basis of breeding behaviour, invertebrates may be
oviparous (eg all echinoderms) or viviparous (eg scorpion). Only a few invertebrates are viviparous.
Development in the invertebrates may be direct (eg earthworm, leech etc.) or indirect (young differs
from adult in morphology and physiology and is called larva). Different types of invertebrates have
different larva forms:

12. Animal group Larval form
1. Sponges Parenchymula in leucosolenia; Amphiblastula in scypha.
2. Coelenterates Planula larva in all coelenterates; scyphistoma and ephyra in jelly fish.
3. Annelids Direct development in oligochates and hirudines.
Trochophore larva in polychaetes.
4. Arthropods Zoaea metazoaea, nauplius, cypris, Mysis, phyllosoma, etc. in crustaceans;
wrigller larva in mosquitoes; maggot larva in houseflies; naiad larva in
dragonflies; caterpillar larva in butterflies and moths; grub larva in beetles.
5. Molluscs Trochophore larva in chiton, glochidium larva in unio, velliger larva in
scaphods.
6. Echinoderms Bipinnaria larva in starfishes, pluteus larva in brittle stars; echinoplutes larva
in sea urchins. Auricularia larva in sea cucumber.
Larva undergoes morphological, physiological and behaviour changes called metamorphosis to
generally form a more advanced adult called progressive metamorphosis. But in sacculina larva
attaches its host, the carp and undergoes degenerative changes to form a highly degenerate adult
called retrogressive metamorphosis.
17. Body temperature. All the invertebrates are cold blooded or poikilothermal animals.
18. Skeleton. Any hard structure of body which gives shape, support and protection to body is called
skeleton. Different invertebrate phyla have different type of skeleton:
Animal group Skeleton structure
1. Protozoa Lorica of cellulose plates in dinoflagellates, test or shell in arcella, diffugia,
polystomella, globigerina, etc.
2. Sponges Endoskeleton of calcareous spicules or siliceous spicules or spongein
fibres.

13. 3. Coelenterates Perisarc in obelia, sertularia, etc. calcareous coenosteum in millipora;
calcareous and branched corallum in madrepora.
4. Flatworms Cuticular spinnules in flukes.
5. Nematodes Histologically complex cuticle.
6. Annelids No mineralized skeleton.
7. Arthropods Sclerotized exoskeleton of chitinous plates called sclerites.
8. Molluscs Calcareous shell of one or two or more valves formed of conchiolin.
9. Echinoderms Spines of calcareous dermal ossicles.
Table-comparison (Differences) between invertebrates and vertebrates.
Features Invertebrates (non- chordates) Vertebrates (chordates)
1. Symmetry Radial, biradial or lacking. Bilateral.
2. Metamerism True or pseudometamerism or
lacking.
True Metamerism.
3. Post-anal tail Lacking. Usually present projecting beyond
anus.
4. Grade of organization Protoplasmic to organ system. Organ system.
5. Germ layers 2(diploblastic), 3(triploblastic) or
lacking.
3(triploblastic).
6. Coelom Acoelomate, pseudocoelomates or
truly coelomate.
Truly coelomate.
7. Limbs derivation From same segment. From several segments.
8. Notochord Notochord or backbone lacking. Present at some stage or replaced
by a backbone made of ring like

14. vertebrae.
9. Gut position Dorsal to nerve cord. Ventral to nerve cord.
10. Pharyngeal gill slits Absent. Present at some stage of life.
11. Anus Opens on the last segment or
absent.
Differentiated and opens before the
last segments.
12. Blood vascular
system
Open, closed or absent. Closed and much developed.
13. Heart Dorsal, lateral or absent. Ventrally placed.
14. Dorsal blood vessel Blood flows anteriorly. Blood flows posteriorly.
15. Hepatic portal
system
Absent. Present.
16. Haemoglobin In plasma or absent. In red blood corpuscles.
17. Respiration Through body surface, gills or
tracheae.
Through gills or lungs.
18. Nervous system Solid. Hollow.
19. Brain Above pharynx or absent. Dorsal to pharynx in head.
20. Nerve chord Double, ventral, usually bearing
ganglia.
Single, dorsal, without ganglia.
21. Segmental Nerve
roots
Dorsal and ventral roots not
separate.
Dorsal and ventral roots separate.
22. Reproduction Asexual reproduction
predominant.
Sexual reproduction predominant.
23. Regeneration power Usually good. Usually poor.
24. Body temperature Cold blooded. Cold or worm blooded.

15. Phylogenetic tree of animals
The natural classification being based on structural resemblances and evolutionary relationship has
enabled the zoologists to prepare a family tree of the animals. This tree indicates a probable
if not exact course of animal evolution.
Chordata Arthropoda
Hemichordata

Echinodermata Annelida

Mollusca

Coelomata
Nematoda
Coelenterates Platyhelminthes
(Cnidaria)
Bilateria
Porifera
Ancestral metazoa

Choanoflagellate Protozoa (Protista) Plants
Early Protoplasm

Fig: proposed phylogenetic tree of animal phyla. Only major phyla are shown.

Outline classification of animals

16. The kingdom animalia divided into three sub-kingdoms: Mesozoa, parazoa and eumetazoa. Phylum
protozoa are now placed in kingdom Protista.
(i) Sub-kingdom I Mesozoa
(1) Minute, worm-like parasites of marine invertebrate animals.
(2) No tissue; body composing of an external layer of ciliated cell, cellular organisation.
(3) Digestive cells, surrounding one or more reproductive cells. Nutrition is absorptive and ingestive.
(4) Body symmetry bilateral or none.
Mesozoa Examples are- Dicyema, Rhopalura.
(ii) Sub-kingdom II Parazoa