In this article we will discuss about Ascidia:- 1. Habit and Habitat of Ascidia 2. External Structures of Ascidia 3. Body Wall 4. Coelom 5. Locomotion 6. Digestive and Respiratory Systems 7. Circulatory System 8. Excretory System 9. Nervous System 10. Reproductive System 11. Development and Life-History 12. Zoological Importance of Tadpole Larva 13. Classification.
Contents:
- Habit and Habitat of Ascidia
- External Structures of Ascidia
- Body Wall of Ascidia
- Coelom of Ascidia
- Locomotion in Ascidia
- Digestive and Respiratory Systems of Ascidia
- Circulatory System of Ascidia
- Excretory System of Ascidia
- Nervous System of Ascidia
- Reproductive System of Ascidia
- Development and Life-History of Ascidia
- Zoological Importance of Tadpole Larva in Ascidia
- Classification of Ascidia
1. Habit and Habitat of Ascidia:
Ascidia is exclusively marine and occurs at all depths of the sea. It is sessile in adult stage, but the larva is a free-swimming form. The adults remain attached to the bottom of the sea or other substrata by its base. Ascidia is a microphagous animal, i.e., it feeds on microorganisms. The food materials are obtained by ciliary action.
2. External Structures of Ascidia:
Ascidia, in adult stage, are a solitary animal having a bag-like body (Fig. 3.1 A). The surface is smooth and naked. The body is covered by a thick, slightly cartilaginous and translucent tunic or test. Because of the presence of tunic, the urochordates are called Tunicata (tunicates).
The tunic is composed of a carbohydrate, and some amount of it is a cellulose-like compound called tunicin. The outer surface of the test or tunic forms a thin, transparent cuticle (mantle) that is composed of protein entirely. Feeble musculature is seen throughout the mantle, even around siphons.
The free end of the body bears two apertures — one at the top is oral or branchial siphon containing the mouth and other is placed at the side and is called atrial funnel or atrial siphon. The atriopore is situated in the middle of the atrial funnel (Fig. 3.1 A).
The oral funnel is usually eight lobed while the atrial funnel is six lobed in Ascidia. In living condition, stream of water passes through the oral funnel and goes out by the atrial funnel. If the animal is disturbed, the body wall contracts instantaneously.
As a result, the water contained inside the body is expelled out like jets, so the common name ‘sea squirt’ has been attributed to the specimen. The attached end of the body is called stolon or base. The attachment site of the test is expanded into a margin in some species.
3. Body Wall of Ascidia:
The body, as already stated, is covered over by tunic. The structure and function of the ascidian test has discussed in detail by S. M. Das (1936b). The test of Ascidia is referred to a cuticle. It is derived initially from the ectoderm. The tunic contains cells which are actually the derivatives of the mesoderm. These cells migrate through the overlying ectoderm and remain scattered in the cuticle.
A number of branching tubes, each terminating in a tiny bulb-like dilatation, are present. Blood circulates through these tubes. Underneath the tunic, there lies the true body wall composed of living tissues called mantle. The mantle is covered by single-layered epidermis. The epidermis is composed of cuboidal or squamous cells.
The mantle bears an underlying layer of connective tissues and muscle fibres which extend in various directions. The longitudinal muscle fibres- are numerous. Except at the regions of oral and atrial siphons the muscle fibres are oriented in an irregular network.
They cross one another in all directions. Majority of the muscle fibres are either longitudinally or transversely disposed. Non-metameric organisation is a notable feature in Ascidia. This means the mesoderm remains un-segmented.
4. Coelom of Ascidia:
Due to extensive development of the atrium, the coelom is greatly reduced and exists in few doubtful derivatives, like pericardial cavity, gonadal cavities, etc. Reduction of coelom is regarded to be a degenerative acquisition. The atrium is formed by involution of the body surface and it is not a coelom.
5. Locomotion in Ascidia:
Ascidia, in adult stage, are a fixed form so locomotion in true sense is absent. The movement is restricted to the contraction of the body by muscle fibres and the closure of the funnels. But the tadpole larva swims actively in water. The mechanism of swimming in the tadpole is exactly similar to that already described in Branchiostoma.
6. Digestive and Respiratory Systems of Ascidia:
The digestive system of Ascidia contains alimentary canal and digestive glands. The alimentary canal forms a double loop on the left side of the branchial sac and starts from the mouth ends into the anus.
The following parts form the alimentary canal:
Mouth:
The mouth is situated at the centre of the velum placed in the posterior end of the oral funnel (Fig. 3.1 B). The velum which partially separates the pharynx from the mouth is provided with velar tentacles.
These finger-like sensory tentacles help to examine the water which enters through the oral or branchial siphon and exclude the larger particles before water flows into the pharynx or branchial chamber. The opening and closure of the mouth are regulated by muscle bands.
Buccal cavity or stomodaeum:
The mouth leads into a short, wide oral passage called the buccal cavity or stomodaeum.
Pharynx or branchial chamber:
The stomodaeum opens into a greatly enlarged sac-like pharynx or branchial chamber (Fig. 3.1 B). The pharynx extends nearly up to the base of the body and remains attached with the mantle along the ventral side. Except this region the whole of the pharynx is enclosed by a special cavity, called the atrium (Fig. 3.1 B). The atrium opens to the exterior through an atriopore.
The pharyngeal wall and the body wall connected by a number of bands of vascular mesodermal tissue, called trabeculae. The thin and delicate wall of the pharynx is pierced by numerous vertically disposed apertures, called stigmata. Through these apertures the pharynx communicates to the atrium. The stigmata are developed by the subdivision of a few (presumably three) gill-slits.
Each gill-slit is divided by the developing tongue-bars which become connected by transverse connecting bars, called synapticulae. The stigmata bear series of papillae containing muscles.
By this way numerous stigmata are produced out of a few gill-slits. The inner lining of the atrium is derived from ectoderm and the pharynx was believed to be developed from embryonic endoderm, but recently it has demonstrated that pharynx develops from ectoderm-like atrium.
The lateral sides of the stigmata are lined with numerous strong cilia. The beating of cilia produces a current that enters through the oral aperture and passes from the pharynx to the atrium through the stigmata. The pharynx of tunicates is used both for feeding and for respiration.
Good body (1974) considers that pharynx serves the dual roles of a food-collecting apparatus and a site for gaseous exchange.
An endostyle (Fig. 3.1 C) is a ciliated glandular groove in the whole length of the floor of pharynx or branchial sac, the secretion helps in getting food, homologous to thyroid gland of vertebrates.
The morphology and histo-chemistry of endostyle in urochordates (tunicates) have been worked out by a number of workers, especially Barrington (1957), Olsson (1963), Godeaux et al. (1968) and Thorpe et al., (1972) may be well-mentioned.
The structure of endostyle of different species in urochordates may be more or less similar. The floor of the branchial sac or pharynx possesses a pair of mid-ventral longitudinal folds separated by a groove. This specialised thickened groove which resembles a keyhole is the endostyle of Ascidia and other members of tunicates.
In light and electron microscopic studies, 7 distinct cellular zones have been identified in each side except the basal zone.
Zone 1 is unpaired, situated at the base of the groove and contains tall and narrower cells with basal nucleus. These cells bear long flagella, extended up to the tip of the groove and a number of mitochondria at the apical region of the cells. These cells also bear a few PAS (Periodic Acid Schiff) positive secretory cells.
The flagella seem to be used to spread secretions throughout the endostylar groove Barrington (1957) and Thorpe et al., (1972) consider that this zone secretes mucus but Olsson (1963), Godeaux and Firket (1968) claim that this does not secrete mucus but glycogen.
Zones 2 and 4 are called as ventral, median and dorsal glandular tracts respectively. These zones are devoid of cilia and are composed of a small number of large cells, arranged like a fan.
These cells are aggregated compactly towards the endostylar groove region but remain widely spread condition towards the basal region. Cells possess basal nuclei, prominent Golgi cisternae, much folded endoplasmic reticulum, and PAS positive secretory granules.
According to the function of these zones, the workers are not unanimous among themselves. Most workers like Levi and Porte (1964), Godeaux and Firket (1968) and Thorpe et al., (1972) have agreed that these zones secrete protein, not mucous, but Olsson (1963) claims of the secretion of mucopolysaccharide.
Zone 3 is usually referred to as ventral ciliated zone but works on different species by different authors claim that this region is a highly organised composing of flagellated cells. The cells are tall, with a basal nucleus, a good number of mitochondria in each cell and a few PAS positive secretory granules. The function of this zone is to mix the secretions, secreted from the zones 2 and 4.
Zones 5 and 8 are two different ciliated tracts. Of the two, the zone 8 is comparatively larger ciliated, topmost tract. The cells of these two zones are cuboidal with cilia. The cells possess basal nucleus and a good number of mitochondria. The cilia of these zones are related to the lateral transport of endostylar secretions.
Zone 6 is a broad band of cells with a very few cilia. The secretion from this zone is protein, but Thorpe et. al., (1972) claim that the cells are rich in tyrosine and a sufficient amount of tryptophan. Due to presence of a few cilia it is presumed that these cilia are related to lateral transport of the secretions.
Zone 7 is a narrow band of ciliated cells. The region composes columnar cells which contain rough endoplasmic reticulum and a few secretory granules. The region is the site for binding of free iodine with the subsequent formation of 3-mono-iodo-tryosine (MIT) and 3, 5-diiodotyrosine (DIT) together with a number of other active iodinated compounds.
Function:
The exact chemical nature of the secretion from the endostyle is uncertain but it may be a mucoprotein which is formed by iodinated protein and mucus. The complex iodinated protein which forms a sheet, serves as filtration membrane inside over the pharynx.
The food particles are trapped in this sheet which moves upwards and then pushes back into the oesophagus by the action of cilia of dorsal lamina or numerous hanging curved bodies, called languets. The endostyle and the dorsal lamina are connected by the peripharyngeal ciliary bands. The formation of 3-mono-iodo-tyrosine in urochordates takes place by the binding of iodine with amino acid tyrosine.
The urochordates (tunicates) mainly get iodine from the oxidation of iodides taken from the environment where they live. The organic binding of iodine in the urochordate endostyle was first shown experimentally by Barrington and Thorpe (1965). Young (1981) proposes that there is little or devoid of triiodothyronine (T3) and thyroxine (T4) in urochordates (tunicates). 
The relationship of endostyle with the thyroid gland of vertebrates is advocated by many workers. The thyroid gland incorporates iodine. Experimental studies with the isotopes of iodine show those certain cells on the glandular tracts of endostyle incorporate iodine. In ascidians (e.g., Ascidia) organically bound iodine has been found in the cuticle of test, and in the endostyle.
Oesophagus:
The pharynx or branchial sac opens into a short narrow and thick-walled tube, called oesophagus (Fig. 3.1 B).
Stomach:
The oesophagus is connected with an expanded tube called stomach. The stomach has a fusiform shape (Fig. 3.1 B) and its folded inner wall contains digestive glands and it is the site for extracellular digestion.
Intestine:
The stomach joins a U-shaped thin-walled tube, called intestine. It ascends and opens into the atrial funnel by the anus (Fig. 3.1 B). The wall of the alimentary canal is non-ciliated.
The wall of the intestine is thickened internally into a pad, called typhlosole which increases the absorptive surface of the intestine. There is no sharp demarcation between the stomach and the intestine and anus is smooth edged in Ascidia.
Digestive Glands:
The inner wall of stomach contains digestive glands. These glands secrete digestive enzymes like protease, lipase, amylase and invertase. The gastric secretion has a strong carbohydrate-splitting enzyme and small amounts of lipase and weaker proteolytic enzymes containing tryptic type.
There is no liver. A pyloric gland composed of numerous small sacs or ampullae remains in contact with the outer walls of the anterior intestine.
Ductules arising from these ampullae unite to form a duct. This duct opens into the rear end of the stomach. The physiological role of pyloric gland is not fully ascertained. Both digestive and excretory functions have been suggested by many workers. According to Young (1981) — “The secretion of the pyloric gland does not contain enzymes but may help to break up the food cord that passes through the intestine”.
Food:
Ascidia is a microphagous animal, i.e., it feeds on planktonic microorganisms like algae, diatoms, bacterians, etc.
Food collection:
The beatings of cilia in the stigmata set up a water current which enters into the pharynx and goes out through the atriopore via the stigmata. The food particles suspended in the water current are entangled by the mucus secreted by the glandular tracts of endostyle. The mucus-entangled food particles move upwards and are then pushed back into the oesophagus by the ciliary action of the dorsal lamina.
Digestion:
From the oesophagus, the food particles pass on into the stomach where they are subjected to the action of the digestive ferments. The digested food is absorbed in the intestine and the undigested products are expelled out through the anus and atriopore.
Respiration:
The extensive ciliation in the pharyngeal cavity and the cilia bounding the stigmata set up a constant flow of water current passing inwards through the mouth. During the inflow of water current, the water—containing fresh oxygen in dissolved state—passes into the atrium via the stigmata. This process is supplemented by the periodic muscular contractions of the body wall.
From the atrium the water goes out to the exterior through the atriopore. The walls of the stigmata are highly vascular where exchange of gases takes place. After oxidation, the resultant carbon dioxide passes out along with the water from the atrium.
7. Circulatory System of Ascidia:
The circulatory system consists of blood, a heart, blood vessels and sinuses. The blood contains a colourless plasma and a few corpuscles. Ascidia lacks capillaries. As a result the blood and body tissue fluid become intermingled. The fluid contains lymphocytes, phagocytic macrophages, vacuolated compartment cells and many coloured and colourless Cells.
Some ascidians possess vanadocytes (cells having green vanadium-containing pigment) in the plasma. The pigment is presumed to be respiratory pigment and these special cells may help in respiration.
Some of the corpuscles are phagocytic and the others contain pigments. Red blood corpuscles are not encountered. Ascidia lacks the power of regulation of osmotic pressure. As a consequence, the blood is isotonic with the sea water.
The heart is a fusiform sac-like structure, with a single layer of myoepithelial cells lining its inner wall. It is surrounded by pericardium. The pericardial cavity is the only remnant of the coelom (Fig. 3.2). The heart is situated below the pharynx.
The heart communicates with a system of blood spaces, called the haemocoel. Some of the larger spaces have an endothelial lining and thus form the blood vessels. Below the endostyle the heart gives a ventral vessel to supply the various parts of the pharynx. From the opposite end of the heart another vessel, called visceral vessel, emerges out to supply blood to the viscera (Fig. 3.2).
Mechanism of circulation:
The direction of flow of blood through the heart is periodically changed due to the reversal of peristalsis of heart muscles. Such a phenomenon is very rare and is not ordinarily observed in the circulatory system of any animal.
8. Excretory System of Ascidia:
There is no definite excretory organ in Ascidia. The nephrocytes present in the blood are found to accumulate excretory products in the cytoplasm. These cells contain particles of urates and xanthine. It is recorded that about 95% of the nitrogenous wastes is excreted in the form of ammonia. Julin has suggested that the neural gland functions as the excretory organ.
A peculiar hollow neural gland is found embedded in the ventral side of the nerve ganglion. The gland opens into the pharynx near the velum through a ciliated funnel (Fig. 3.3B). This gland originates mainly from the larval ectoderm and partly from the pharynx. This double origin is quite significant. The gland is regarded to be a homologous structure with the hypophysis of vertebrates.
9. Nervous System of Ascidia:
The whole of the nervous system is reduced to a single elongated solid nerve ganglion called cerebral ganglion or brain. It is situated dorsal to the neural gland and between the atrial and oral funnels (Fig. 3.1 B). This ganglion gives off nerves, specially to the atrial and oral funnels and velum and thus helps to regulate the flow of water current.
Sense Organs of Ascidia:
The special sense organs, in the strict sense, are absent. The pigment spots or ocelli that are present in the adults around the funnels are regarded to be photo-receptors in a lowly developed condition. But in a fully- grown tadpole larva the photoreceptive organ is highly developed.
10. Reproductive System of Ascidia:
Ascidia is a hermaphroditic animal. The gonads are sac-like structures situated in close association with the intestine. The gonads are mesodermal in origin. The ovary and testis possess elongated gonoducts (see Fig. 3.1 B).
The gonoducts open into the atrium. The sex cells are discharged into the atrium and from there these are expelled to the exterior. Fertilization is external. The sperms and eggs attain maturity at different times, thus self-fertilization is prevented.
But in Ascidia mentula, self-fertilization takes place. Besides sexual reproduction, multiplication by budding is not of rare occurrence. Ascidia is noted for its great potency of regeneration.
11. Development and Life-History of Ascidia:
The marvel of life is nowhere more wonderfully displayed than in the developmental history of an individual from the zygote, the resultant product of fusion of the egg and the sperm. The ontogenic developmental process in most animals is direct. But in a few cases the development is indirect, i.e., the development is accompanied by metamorphosis.
The phenomenon of metamorphosis includes a series of changes which a larva (an immature individual of a species differing the adult both in morphology and physiology and having free existence) has to undergo before attaining its adulthood.
Metamorphosis, in most animals, is a progressive process. But there are some examples, where the developmental events are largely retrogressive. Ascidia furnishes one of the best examples of retrogressive metamorphosis in which a highly developed tadpole larva, in course of ontogenic processes, transforms into a sessile and degenerated adult (Fig. 3.1 A).
The retrogressive metamorphosis is a peculiar and uncommon process in the chordates, although such phenomenon is observed in some non- chordate animals. The life cycle is divided into several stages.
These are:
a. Pre-larval stages:
i. The eggs are small and almost yolk- less (Fig. 3.4B).
ii. Each egg is covered by some membranes which help the egg to float in water.
iii. The segmentation is holoblastic and nearly equal at the initiation.
iv. At the eight-cell stage, four cells are larger and four cells become smaller. The larger cells become the prospective ectoderm (epiblast) and smaller cells become the prospective endoderm (hypoblast).
v. The eight-celled embryo undergoes three more sets of cleavages.
vi. This results in the formation of a spherical coeloblastula containing a small cavity, called blastcoel.
vii. Before the completion of seventh cleavage (i.e., between the 64-cell and 128-cell stage) the gastrulation begins by epiboly and invagination.
viii. The formed archenteron which is formed by invagination, obliterates the blastocoel.
ix. The blastopore marks the posterior end of the embryo and closes, and the embryo elongates along the anteroposterior axis.
x. The neural plate sinks inward from the ectoderm and rolls up as a tube, called neural tube. The stage during the neural tube formation is called the neurulation.
xi. Along the mid dorsal line, the archenteron gives rise to notochord.
xii. After about 3 days and with further development, the neurula stage transforms into a free-Swimming tadpole larva because of the superficial resemblance of the larval stage of frogs.
b. Larval stages:
The tadpole larva is very active at the beginning which, in course of metamorphosis, transforms into a sessile adult (Fig. 3.4D-F).
Structure of early tadpole larva:
i. A fully grown tadpole larva is highly motile and does not take food from outside, i.e., it is a non-feeding form.
ii. It has an elongated body.
iii. The body is more or less oval in outline.
iv. It is distinctly divisible into two regions—the head and the tail (Fig. 3.4D).
v. The whole of the body is covered over by the tunic.
vi. The head is elliptical and has three adhesive papillae or chin warts. Of the chin warts, one is mid-dorsally placed and the rest two are located ventrolaterally. The adhesive papillae secrete adhesives that are used for settlement at the onset of metamorphosis.
vii. The tail is laterally compressed and pointed terminally. It is provided with a caudal fin. The dorsal and ventral fins are continuous along the tail and are marked with 36 uni-nucleated, striae. The striae are regarded to be precursors of the fin-rays in fishes.
viii. The central nervous system is situated dorsal to the notochord. It enlarges anteriorly into a sensory vesicle which opens into the pharynx near the mouth by neuropore.
ix. A single median eye containing the retina, pigmented layer, cornea and lens, is present in the inner wall of the hollow sensory vesicle.
x. An otocyst (statocyst), the organ of balance, is also situated in its ventral side and the rear wall usually bears an ocellus.
xi. The slender nerve cord is not much differentiated but is hollow throughout. The nerve cord is composed of a single layer of ependymal cells.
xii. The notochord is restricted only in its tail region. It extends anteriorly up to the pharyngeal region and is en-sheathed by gelatinous materials.
xiii. Segmental muscle bands are present in the tail region, which are arranged on both the sides of the nerve cord.
xiv. The mouth is present and the alimentary canal is rudimentary.
xv. The pharynx is sac-like and is well- developed.
xvi. It bears a fully developed endostyle and two pairs of gill-slits.
xvii. Non-functional heart with epicardia lies beneath the endostyle.
xviii. The paired atrial sacs are present. Just after hatching the tadpole larva becomes positively phototactic and negatively geotactic.
Cloney (1982) classified the ascidian tadpole structures into 3 categories and the features are represented in Table 14.
c. Settlement and metamorphosis:
The free swimming tadpole larva, after a short period of free existence, becomes sluggish. It soon fixes itself to sea weeds or stones by adhesive papillae and immediately falls a victim of fast degeneration. At this stage the larva becomes negatively phototactic and positively geotactic. Some authors believe that habitat selection is a main cause for the metamorphosis of ascidian tadpole larva.
Habitat selection:
Berrill (1955) has emphasised the importance of habitat selection in the life of the ascidians. It is interesting to note that many species cannot survive at all if the larvae settle at any substratum other than hard and rocky places. Moreover, just after settlement on the mud and detritus may suffocate these creatures.
In an experiment Goodbody (1961) took two populations A and B in the month of June and maintained these two populations at the depth of four feet. On the other hand, populations D and F were taken in the early part of August and were maintained at the depth of seven feet. The number of animals that could survive was much lesser in the populations D and F after a period of about one month and a half.
This difference in survival probably resulted from competition with populations of cirripeds, serpulids and other colonies of ascidians at the same habitat. Again, populations C and E were taken in August and were reared at the depth of four feet.
After a few days of the experiment, the number of survivors drastically reduced due to profuse algal growth in September. Again Millar (1974) assumes that fixation is not always essential for metamorphosis.
Retrogressive changes:
i. Just after fixation the length of the tail becomes greatly diminished.
ii. The nerve cord becomes restricted to the trunk region and is ultimately reduced to a solid nerve ganglion.
iii. The notochord becomes coiled, dis-organised, and finally disappears.
iv. The trunk becomes broadened.
v. The numbers of the striae are diminished and become restricted to certain regions. The muscle bands also become degenerated. Gradually the tail is further shortened without having any striae.
vi. The mouth is shifted to 90° from the point of attachment (Fig. 3.4F).
vii. Shifting of the mouth is caused by the rapid growth of the region between the adhesive papillae and mouth, and also due to inhibition of growth of the original dorsal side.
viii. As development goes on, the tail becomes still more shortened and is partially withdrawn into the test. The rotation of the mouth is clearly marked.
Progressive changes:
During metamorphosis, from the tadpole larva to the adult, all the changes are not necessarily degenerative but some structures, which are intimately associated with their survival, become more elaborated and specialised.
The progressive changes are:
1. The branchial chamber becomes enlarged and the number of stigmata are enhanced.
2. The post-pharyngeal portion of the gut gets regionated into different parts.
3. The atrium becomes more extensive.
4. The development of velum is observed.
5. The gonads and gonoducts appear from the mesoderm.
The foregoing description shows a peculiar scheme of retrogression of a complex and well-organised larval form to a simpler degenerated adult. The drastic changeover is really a strange phenomenon because no external agency is recorded to act upon it.
In the whole of animal kingdom, a parasitic crustacean— Sacculina—can be brought to the same status as far as the retrogression is concerned, if the external agency of parasitism is assumed to be absent from its life.
The essence of retrogressive metamorphosis in Ascidia is (i) the differential growth and disappearance of histologically differentiated larval tissues and (ii) the formation of the adult structures from the residual larval tissues.
In ascidian metamorphosis, two sets of changes occur. First, the disappearance of notochord, dorsal nerve cord and tail, and secondly, the elaboration of some adult structures likes the formation of stigmata, specialisation of pharynx, overdevelopment of atrium, etc.
Experimental analysis for the metamorphosis of ascidian tadpole:
Some physical and chemical factors are concerned with the metamorphosis of ascidian tadpole. Herdman regards phagocytosis as the primary cause of larval tissue destruction while Berrill (1929) advocates that the relative tissue starvation is responsible for larval tissue destruction. Grave (1935) believes that a metabolic product of swimming activity is essential for metamorphosis.
Grave (1935) concludes that normal metamorphosis of tadpole is conditioned by two factors:
(i) Ageing of the larva after the liberation, and
(ii) Swimming activity.
Glaser and Angslow (1949) attempted to assign the controlling role of copper during metamorphosis, released in the tadpole tissues. Some other factors like metabolic ions, iodine, low concentrations of vital dyes, some amino acids, thyroxine extracts, exposure to hypotonic sea water, dimethyl sulpoxide, acetylcholine and favourable lightening condition may be recorded to accelerate the metamorphosis.
Molecular analysis of Ascidian metamorphosis:
Scientists have succeeded to isolate some genes which are related to ascidian metamorphosis. Five novel genes — manx, lynx, cymric, p58 and bobcat have isolated from the ascidian species. Lynx and Cymric are maternal and experiments are underway to see their roles in the development.
Manx, p58 and bobcat are expressed both maternally and zygotically in the tailed species, and antisense experiments suggest a role in the specifying the body plan during development. Bobcat is also seen to be expressed in the neural tube of chordate embryos.
In addition, 132 different protein coding sequences have been isolated, of which 65 of these transcripts show significant matches to Gen Bank proteins. Some of these genes have putative functions relevant to metamorphic events, related to the differentiation of muscles, blood cells, heart tissue and adult nervous systems.
One set of genes that are activated at metamorphosis are the innate immunity genes. These genes are related to the expression of immune system which are critical for remodeling the body plan during metamorphosis. In addition, this immune system may be necessary for phagocytosis and restructuring of larval tissues.
12. Zoological Importance of Tadpole Larva in Ascidia:
The presence of tadpole larva in the life- history of Ascidia is very significant. From the taxonomical standpoint, the tadpole larva helps us to include the Ascidians under the Phylum Chordata, otherwise their inclusion under the phylum would have been questionable.
The existence of tadpole in their life- cycle appears to be either a case of recapitulation of the past racial history or it must be the birthright of a chordate. However, the tadpole larva can be regarded as a relic of the ancestral free-swimming chordate.
Two contradictory views exist on this particular issue. The first view holds that the existence of tadpole larva is an interpolation in the life-history of Ascidia which—by the suppression of metamorphosis and further evolution— might have given origin to the ancestral chordate. The other view holds that the existence of tadpole larva is not a case of interpolation.
It has evolved within the group to meet certain Ascidian needs. The tadpole larva holds the key position, from which the vertebrates have emerged out in space and time. However, in recent days, the tadpole larvae are not regarded as the ancestors of vertebrates.
13. Classification of Ascidia:
Historical resume:
Aristotle (384-322 B.C.) described the first simple Ascidian, Tethyum. Schlosser and Ellis (1756) discovered the compound ascidians. Since then many ascidians have been added to our knowledge up to the early part of the nineteenth century. But the systematic status of the group remains a disputed issue. Lamarck (1816) gave the name of the group— Tunicata and placed the ascidians between the Radiata and Vermes.
Cuvier placed them within Mollusca, but Lamarck isolated the group from the molluscs. Milne Edwards instituted a new class—Molluscoidea—to include the Brachiopoda, Polyzoa and Tunicata.
All the disputes regarding the systematic position of the group were put to an end when the remarkable memoir entitled, ‘Development of a Simple Ascidian’ was published by Kowalvesky in the year 1886. Since then the inclusion of the group under the Phylum Chordata is established with a degree of certainty.