Herdmania Total Teaching Hours For Semester : 45

Herdmania.pdf

1. Herdmania pallida - A Sea Squirt

2. •Sub-phylum Urochordata (Gr., uros = tail; chordae = cord) — Nerve cord; notochord - confined
only to tail - Later lost
•Marine animals - Found in all seas and depths (shallow to abyssal depth of over 5 km)
External Morphology
Shape, Size and Colouration:
•Roughly oblong (rectangular) in outline
•Narrower at its attached end than at its free end
•At its free end - Provided with two external openings — the branchial and atrial apertures
•The average size of the adult — 9.5 cm long and 7 cm broad

3. •The general colour of the body in a fresh specimen — Pink
•The presence of bright red patches, formed of terminal knobs (ampullae) in the blood vessels of
the test, is a characteristic feature of Herdmania
•The test is soft and leathery
•Body Divisions: Divided into two parts,
Body proper and foot
•Entire animal is covered by the test.

4. 1. Body Proper
•It is the distal free portion of the body
•The branchial aperture marks the anterior end of the animal; consequently the opposite end
attached to the substratum is the posterior end
•The side on which the atrial aperture is placed marks the dorsal side of the body, which is very
limited
•The side opposite to that on which the atrial opening is placed and which is partly attached to
the substratum is ventral side which is extensive.
•The branchial and atrial apertures are situated on short
protuberances of the body called branchial and atrial siphons
respectively
•When fully extended, the atrial siphon is longer than the branchial

5. •In a large-sized animal the atrial siphon measures about 1.5 cm, while the branchial siphon is
only about 1 cm in length.
•The average diameter of the branchial aperture, when fully expanded, is about 2 cm and that of
the atrial aperture about 1.2 cm
•At the base of branchial siphon, there is a ring of long branchial tentacles, while at the base of
the atrial siphon there is a ring of slightly serrated folds which constitute the atrial tentacles.
•The test of the siphons is very elastic and can contract to close the apertures at the slightest
disturbance of sea water.
•

6. 2. Foot
•The foot - Varies in character according to the nature of the substratum which the animal
inhabits
•If the substratum is of fine sand, the foot has an oval shape and the test is quite hard in
consistency
•But if the substratum consists of coarse and the broken shell-pieces, the foot is irregular in
outline and more or less soft in consistency
Test or Tunic
•The external covering - Leathery, translucent test or tunic composed of tunicin, a substance
similar to cellulose of plants.
•It is about 4-8 mm thick. It is ectodermal in origin.

7. •The test has,
•(i) Matrix - Gelatinous & made up of polysaccharide (tunicin)
•(ii) Cells - Mesodermal in origin & migrated into test - Small amoeboid cells, Granular cells,
Round vacuolated cells, Nerve cells with several processes, Squamous epithelial cells etc
•(iii) Interlacing fibrils - Form fine network in test & resemble with smooth muscle fibres
•(iv) Calcareous spicules - Has large number of spicules - All bear several rings of minute spines-
all pointing in the same direction
•(v) Blood vessels - Form network in the test & end in bulb-like
dilations called Vascular ampullae.
•Vascular ampullae & vessels - Transport blood & accessary repi. organ

8. Mantle or Body Wall
•Inside the test is the body wall or mantle
•The mantle secretes the test
•The mantle is present beneath the test and is attached at the branchial and atrial apertures where
it forms branchial and atrial siphons
•The mantle encloses a large atrial cavity or atrium, containing water
•Histologically the mantle — An outer layer of ectoderm, middle layer of mesoderm, and an
inner layer of endoderm forming the outer lining of atrial cavity

9. Digestive System of Herdmania
•Digestive system includes the alimentary canal and digestive glands
•Lying inside the mantle is a large bag-like pharynx or branchial sac
•The alimentary canal is coiled, beginning from the mouth and ending in anus
Alimentary canal
1. Mouth
•The mouth or branchial aperture lies at the top of branchial siphon, which is designated as the
anterior end of the body
•It is guarded by four lips (lobes) of the test
•The mouth leads into a branchial siphon

10. 2. Branchial Siphon:
•The mouth leads into a branchial siphon
•It is a narrow and tubular cavity lined by ectoderm and is called the stomodaeum or buccal
cavity
•At the base of the stomodaeum is a ring of branchial tentacles which act as a sieve allowing only
minute food particles to go in
3. Pharynx
•It occupies the major part of the body cavity
and is differentiated into two - Prebranchial zone
and Branchial sac.
•

11. (i) Prebranchial Zone:
•It is the smaller anterior region having smooth walls without folds, cilia and stigmata or gill-slits
(ii) Branchial sac or pharynx :
•It is the posterior larger part of the pharynx
•The pharynx almost fills the atrial cavity.
•The wall of the pharynx is perforated by several rows of stigmata arranged in transverse rows,
through these the cavity of the pharynx communicates with the atrial cavity
•Edges of stigmata with cilia - Drive water current from pharynx to atrial cavity
•Stigmata are not gill-clefts, but are formed by sub-
divisions of a few original gill clefts
•Blood vessels between stigmata & wall of pharynx

12. 4. Oesophagus
•It is a short bent tube having 4 ciliated grooves in its lining which direct the food into the
stomach.
5. Stomach
•The stomach is wider than the oesophagus
•Walls are thin and have a tubular, branching
pyloric gland
•Stomach opens into a thin-walled intestine
6. Intestine
•The intestine joins a short ciliated rectum opening by an anus into the dorsal part of the atrial
cavity known as cloaca.
•The cloaca leads dorsally into the atrial siphon, opening outside through atriopore or atrial
aperture.

13. Digestive Glands
•1. Liver:- Bilobed (larger left lobe), dark brown, formed of large number of fine tubules - Unite
together form 11 or 12 hepatic ducts - Opens into stomach
•2. Pyloric gland - At posterior part of stomach, Opens into intestine - Digestive enzymes (pepsin
- Digest protein), removes blood toxin & stores glycogens
Food
•Microorganisms such as protozoans, pieces of decaying animals, zooplanktons and algae

14. Digestion
•The liver secretes a yellowish-brown digestive fluid into the stomach
•It has many enzymes
•Amylase which splits carbohydrates into maltose
•Protease which breaks down proteins, and a weak lipase which probably acts on fats.
•The liver also stores starch as reserve food material.
•The pyloric gland is an accessory digestive organ - It is probably pancreatic in nature
•Food is digested mainly in the stomach and absorption occurs in the intestine.

15. Respiratory system
•Sea water current brings Dissolved Oxygen to Pharynx (simple diffusion)
•Pharyngeal wall is thin & rich blood supply in blood vessels
•Exchange of O2 and CO2
•Exchange of gases - In atrium where Vascular trabeculae (Hollow strands contains blood
vessels) are present
•Test has Vascular ampullae - Act as accessory respiratory organ - Exchange of gases through the
surface of the test

16. Circulatory System
•System — Heart and pericardium, blood vessels and blood
Heart and Pericardium
•Heart - A thin-walled, tubular, contractile heart with striated muscles - Inside the pericardium
•Pericardium - Lying below the right gonad is a tubular, elongated with thick non-contractile
wall.
•It is about 7 cm long and 3 mm wide. It is closed at both the ends and contains a colourless
pericardial fluid
•Circulation - By Peristaltic movement

17. Blood Vessels:
•No true blood vessels in most ascidians
•Blood vessels are well developed in Herdmania
Blood
•The blood is slightly reddish or transparent
•It has colourless plasma, many pigmented and non-pigmented corpuscles (may or may not be
nucleated) & few colourless amoeboid nucleated leucocytes
•They may have no pigment, while others may contain orange, brown or yellowish-green
pigment
•The green pigment is vanadium which is claimed to be respiratory pigment, but its oxygen
absorbing power is so low that it cannot be respiratory

18. Nervous System
•In the adult — A brain or nerve ganglion which is about 4 cm long
•It lies embedded in the mantle mid-dorsally between the two siphons
•The brain has bipolar and multipolar nerve cells and gives off three nerves towards the branchial
siphon and two towards the atrial siphon
•Brain - Represents the degenerate remains of the anterior part of well developed N.S. found in
larvae

19. Receptors
•There are no special sense organs.
•Many cells receive a nerve, especially in the test and the siphons.
•1. Red pigmented cells situated on the margins of siphons and vascular ampullae are
photoreceptors being sensitive to light
•2. Sensory cells of the test, margins of siphons and tentacles are tangoreceptors being sensory to
touch & pressure
•3. The cells on the margins of siphons are rheoreceptors - Sensitive to water currents & respond
•4. Cells lining the siphons are thermoreceptors - Sensitive to changes of temperature
•5. The tentacles and dorsal tubercle are the chemoreceptors

20. Excretory organ
•Neural gland (Supra neural gland) - Excretory organ - Located in the mantle, above the brain, and is
oval-shaped and brown in colour - The gland is 4 mm in diameter and has branching tubules that open
into a non-ciliated canal
•Nephrocyte cells of blood - Collects waste particles (mainly Xanthine & Urates) — Enter lumen of
Neural gland — Pass out into pre-branchial zone of pharynx
•Xanthine - A purine base that is a product of purine metabolism. The body converts xanthine to uric
acid through the enzyme xanthine oxidase
•Urates - Salts of uric acid
Reproductive Organs
•The animal is hermaphrodite or bisexual and protogynous, i.e., ovary matures first, thus, self-
fertilisation is avoided
•Fertilisation is external
•Since the animal is protogynous, i.e., ova matures earlier than the sperms, only cross fertilisation occurs

21. Affinities and Systematic Position of Herdmania
•The systematic position of Urochordata had been remained controversial
•Lamarck (1816) first called them Tunicata
•Cuvier (1817) classified them along with Mollusca
•But Lamarck placed them between Radiata and Vermes (worms)
•Milne Edwards (1843) established the class Molluscoidea in which he included Bryozoa and
Tunicata, to which Huxley (1853) also added Brachiopoda
•Kowalevsky (1886) put an end to the controversy when he described the development of a
simple ascidian and established their chordate nature

22. •The adult ascidian derived through retrogressive metamorphosis of tadpole larva has lost most
of its chordate characters, yet it shows its chordate affinities in the following points:
•1. The branchial tentacles resemble the velar tentacles of Amphioxus
•2. Endostyle is homologous with thyroid gland of vertebrates
•3. The pharynx of tunicates is like that of Cephalochordata with numerous stigmata and
glandular ciliated endostyle
•4. Ciliary filter feeding mechanism and respiratory system in urochordates and cephalochordates
are similar (Simple diffusion)
•5. Origin and position of neural gland suggest its homology with the pituitary gland of
vertebrates
•Even more than the adult, the tadpole larva shows its chordate characteristics in its nervous
system, stigmata and notochord