Herdamania: Structure, Locomotion and Systematic Position

In this article we will discuss about Herdamania:- 1. Habit and Habitat of Herdamania 2. External Structures of Herdamania 3. Body Wall 4. Coelom and Atrium 5. Locomotion 6. Digestive and Respiratory Systems 7. Circulatory System 8. Excretory System 9. Nervous System 10. Reproductive System 11. Affinities and Systematic Position.

Contents:

1. Habit and Habitat of Herdamania
2. External Structures of Herdamania
3. Body Wall of Herdamania
4. Coelom and Atrium in Herdamania
5. Locomotion in Herdamania
6. Digestive and Respiratory Systems of Herdamania
7. Circulatory System of Herdamania
8. Excretory System of Herdamania
9. Nervous System of Herdamania
10. Reproductive System of Herdamania
11. Affinities and Systematic Position of Herdamania

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1. Habit and Habitat of Herdamania:

Herdmania is a solitary Aascidian. It is exclusively marine. The genus is recorded to go to the depth of 9 m – 21.6 m of the sea. The adult is a sessile form which remains attached to the substratum by its base or foot. It is a microphagous animal which feeds on micro­scopic animals and plants.

The tunic of Herdmania provides shelter for many organisms. A very common occur­rence is the growth of a green alga on the tunic which sometimes hides the whole animal. The other organisms inhabiting the tunic are the hydroids, anemones, minutes lamellibranches, gastropods and many other animals.

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2. External Structures of Herdamania:

Herdmania (Fig. 3.5A) has an oblong bag­like body. The species inhabiting the sandy bed is provided with a narrow foot, but when the animals are attached to rocks or any molluscan shell, the ‘foot’ is found to be lacking. The attachment is done by a flat base. The animals with the ‘foot’ have their bodies divided into two parts—the body proper and the ‘foot’.

The size and shape of the animals vary greatly. The average size of the adult is recorded to be 9.5 cm in length. The foot, when present, measures about 3-4 cm in length. The free end of the body bears the oral and atrial funnels contain­ing the oral aperture and atriopore, respective­ly. Both these funnels are produced into four distinct lobes (Fig. 3.5A).

The oral funnel is smaller than the atrial funnel. The whole of the body is enclosed by the test or tunic. The tunic is soft and leathery. The test is transparent in young stage which becomes opaque in the adult. In the young stage, a network of blood capillaries is clearly visible through the trans­parent test.

One of the important characteristic features of Herdmania is the presence of red­dish patches on the body, produced by the ampullae at the terminal ends of the blood ves­sels in the test. The test acts as the accessory respiratory structure in Herdmania.

The mouth is situated at the base of oral funnel and the atriopore is contained in atrial funnel. The mouth indicates the anterior end of the body and its opposite end is the poste­rior. The dorsal side is marked by the atrial funnel, the opposite end is designated as the ventral side of the body. The bases of both the oral and atrial funnels are provided with a tentacular ring.

The tentacles that surround the mouth are called the oral or branchial tentacles and that surround the atriopore are called the atrial tentacles. The oral tentacles are comparatively longer than the small and vestigial atrial tentacles.

The oral tentacles form a sort of sieving apparatus which pre­vents the entry of larger particles into the pharyngeal cavity. The internal cavities of both the funnels are lined by a thin layer of test which is folded.

On these folds, red pig­mented patches are present which are sepa­rated by white streaks. There are eight red patches arranged alternately with eight white streaks. Both the patches and streaks extend from the tip of the funnels down to the basal end. In living condition, a strong water cur­rent is found entering the oral funnel and going out through the atrial funnel.

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3. Body Wall of Herdamania:

The test is the protective covering of the body. It is 4-8 mm thick and is composed of tunicin. In the ground substance of the test many structural elements are embedded. These are the meshwork of blood vessels, spicules, interlacing fibres and a few cell-types (Fig. 3.5C). Several cell-types are encountered in Herdmania (Fig. 3.5D).

These are:

i. Amoeboid cells:

These cells are abun­dant in the test.

ii. Eosinophilous cells:

These are spherical cells, and the homogeneous cytoplasm contains fine granules. The granules take bright red stain with eosin.

The eosinophilous cells are of two varieties:

(a) larger variety with a big vesicular nucleus and

(b) smaller type with an eccentric nucleus.

iii. Granular cells:

These cells are oblong in shape and the nucleus is large. These cell-types are surrounded by nerve fibres and are regarded as receptor cells.

iv. Spherical vacuolated cells:

These cells contain numerous small vacuoles, and the nucleus is not visible.

v. Nerve cells:

These are small cells with conspicuous nucleus and 2 to 6 dendrites. Besides, there are interlocking fibres run­ning in the test. These fibres resemble the mus­cle fibres of the mantle. The test is traversed by blood vessels with their terminal ends dilated to form the terminal knobs or vascular ampul­lae.

These ampullae impart red-coloured patches on the outer side of the body. Another peculiar structure present in the test is the spicules.

The spicules are of two varieties:

(a) Microscleres and

(b) Megascleres.

The microscleres are minute bodies, each having a spherical head and a long body. The body is beset with 5 to 25 equidistant rings of small spines. But the head is smooth in most cases. The average size is about 50 micra.

The mega­scleres are larger than the microscleres and are grouped under two categories—depending on the shape. The first category of spicules has spindle-shaped bodies, so these are called spindle-shaped spicules and the second cate­gory of spicules are called pipette-shaped spicules.

The spindle-shaped spicules, like microscleres, are provided with 20-60 equally spaced rings of spines. The size is variable, but the average size is 1.5 mm. These spicules usu­ally occur in bundles. The pipette-shaped spicules, as the name suggests, have a large round median swelling. These spicules are also provided with numerous rings of spines. The length of the spicules extends up to 3.5 mm.

Beneath the test lies the mantle. The man­tle is the true cellular wall which surrounds the body. It is not uniformly developed at all regions of the body. The mantle is highly developed in the anterodorsal side of the body.

The mantle is composed of:

(a) An outer epidermis,

(b) A median layer of muscles and fibres, and

(c) An inner ectodermal layer lining the atrium.

The epidermis is formed of hexagonal cells. The median layer is com­posed of muscles and connective tissue ele­ments.

There are two sets of muscles—longitu­dinal and annular. The longitudinal sets of muscles are more numerous than the annular sets. The muscles are non-striated. Each mus­cle is made up of many non-striated muscle fibres which are en-sheathed by a common connective tissue sheath.

Each muscle fibre is a flat ribbon-like cell with a conspicuous nucleus. The connective-tissue cells present in the middle layer are mostly the amoeboid and vacuolated cells. The ectodermal layer lining the atrium is composed of flat cells.

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4. Coelom and Atrium in Herdamania:

Due to overdevelopment of the atrium or peribranchial cavity, the true coelom in Herdmania is absent except in certain doubtful deri­vatives, like the pericardial cavity, gonad, etc. Like that of Ascidia, the space between the pharynx and the mantle is occupied by the atrium.

The atrium is divided into right and left halves due to the attachment of the pharynx with the mantle on the ventral and dorsal sides. However, both the halves are continu­ous dorsally and open to the exterior through the atriopore.

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5. Locomotion in Herdamania:

Herdmania, in adult stage, is sessile. The visible movement is observed during the con­traction of the body for the squirting out of water through the oral and atrial funnels. This movement is caused by three sets of special muscles.

These are divided into the:

(a) Oral muscle group,

(b) Atrial muscle group and

(c) Trio-oral muscle group.

Both the oral and atrial muscle groups surrounding the funnels are formed of annular muscles and the longitu­dinal muscles. The annular muscles are com­posed of strong circular muscles and sphincter muscles (oral and atrial sphincters).

The longi­tudinal muscles are more extensively devel­oped than the annular muscles. The atrio-oral muscle group is composed of one muscle band running between the oral and atrial funnels and two pairs of muscles situated one pair on each side of the neural gland.

These muscles extend longitudinally from one funnel to the other. The contraction of the body is caused by the longi­tudinal muscles while that of the funnels is effected by the annular muscles.

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6. Digestive and Respiratory Systems of Herdamania:

The digestive tract starts from the oral funnel. It has a tubular cavity containing the mouth at the basal end (Fig. 3.6). The base of the funnel is provided with a circlet of tenta­cles. The tentacles are extensively branched delicate structures and form a sort of strainer to prevent the entry of larger particles into the pharyngeal cavity.

The tentacles are richly supplied with nerves, the presence of which led many workers to suggest that these are also testing organs. The tentacles are broadly divided into four varieties depending on size.

Although the number, size and shape of the tentacles show individual variation, the typi­cal tentacular formula stands as: 8 large (5 mm in length), 8 median (2.5 mm long), 16 small (1.5 mm in length) and 32 minute (0.5 mm long). Each tentacle bears numerous paired lateral branches, called tentaculets which also carry small secondary branches. The secondary branches bear tertiary bran­ches.

The mouth leads into a spacious sac-like pharynx or branchial sac which occupies the major part of the body. As stated earlier, the pharynx is enclosed by the atrium except the ventral and dorsal sides. The wall of the pha­rynx is perforated by numerous stigmata. The roof of the pharyngeal cavity is marked by the presence of a thin fold called dorsal lamina which is situated along the mid-dorsal line.

The free margin of the dorsal lamina produces 20-30 tentacle-like outgrowths, called languets. The dorsal lamina is lined outside by the non-ciliated cells while that of the languets are ciliated. The connective tissue core of both these structures contains blood sinuses. The endostyle runs as a shallow longitudinal groove running along the mid-ventral line of the pharynx.

It resembles closely that of Ascidia in structure, function and origin. At the anterior end of the pharyngeal cavity there are two pairs of ciliated bands (peripharyngeal bands) which are spaced by a peripharyngeal groove. The anterior pair of peripharyngeal bands unite to form a complete ring, while the posterior pair are interrupted to become con­tinuous with the dorsal lamina and with the folded margins of the endostyle.

Both the peripharyngeal bands and groove are lined by tall ciliated cells. The inner wall of the pha­rynx becomes folded longitudinally to increase the surface area. The inner cavity of the pharynx is lined by non-ciliated cells while the cells in the region of the stigmata are tall and ciliated (Fig. 3.7).

The pharynx (Fig. 3.7) is supplied by two sets of blood vessels. The inner side of the pharynx is supplied by internal longitudinal vessels and the outer side by the external transverse vessels. These two types of vessels are so arranged that they cross one another at right-angles.

As a consequence, many square areas are produced which are bounded by transverse and longitudinal vessels. Such areas are often designated as the stigmatic areas. About 5-6 stigmata are present in each stig­matic area. Many small transverse blood vessels traverse the stigmatic areas.

The pharynx leads into a very short curved tube called oesophagus. The opening of the oesophagus is guarded by two semicircular folds. These folds are produced by the modi­fication of the posterior most part of the pha­rynx.

The stomach is not so much dilated as seen in Ascidia. The stomach leads into the intestine which forms a single loop. The distal limb of the loop is called rectum which opens through the anus into the atrial funnel (Fig. 3.7).

Digestive glands:

A large digestive gland with two unequal lobes (liver) (Fig. 3.6) is present in Herdmania. This gland is closely associated with the stom­ach. The liver is composed of an accumulation of a large number of caeca embedded in a ground substance.

This is made up of connec­tive tissues and blood sinuses. The products of the liver are poured into the stomach by eleven independent ducts. Each duct is formed by the union of many ductules coming from the caeca.

The other digestive gland is the pyloric gland. It is an extensively branched structure and composed of numerous tubules. This gland remains in close contact with the stom­ach and intestine. The tubules unite to form a number of ducts which, ultimately open into the middle part of the intestine by a single opening.

The exact function of the pyloric gland is not properly known. It acts as a pancreas and also as an excretory organ.

The mechanism of food collection, diges­tion and respiration is similar to those of Ascidia. Only striking difference in the diges­tive system is the presence of a well-developed liver in Herdmania which is lacking in Ascidia. The digestive enzymes secreted by the distal parts of the liver-tubules are poured into the stomach.

The digestive enzymes are the amy­lase, protease, lipase, invertase, lactase and maltase. The liver stores carbohydrates in the form of starch. The exact relationship of the liver of Herdmania with that of vertebrates has not been experimentally established.

The respiratory system in Herdmania is more efficient than that of Ascidia. The pharynx is a highly vascular structure, and the inter-stigmatic and intra-stigmatic blood vessels help in the exchange of respiratory gases.

The presence of internal foldings increases the surface area of the pharynx. Besides the pharynx, the test also acts as an accessory res­piratory organ. It is richly supplied with blood vessels and vascular ampullae.

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7. Circulatory System of Herdamania:

The circulatory system is highly deve­loped in Herdmania. Due to extensive vascularization of the test, the circulatory system is modified. The blood contains a few colourless amoeboid and plenty of pigmented corpus­cles. Six types of corpuscles are claimed to be present in Herdmania. The pigment present in the corpuscles is either orange or yellowish-green or yellow or brown. Red colour is absent.

The heart is a cylindrical tube-like struc­ture enclosed by tubular pericardium. The heart is attached to one side of the pericar­dium by a thin connective tissue bridge. It is situated ventral to the right gonad as an obliquely placed tube at about the middle of the endostyle.

The heart is a highly contractile tube though the pericardial wall is non- contractile. The wall of the heart is composed of a thin outer striated muscle layer which encloses an epithelial layer. The heart con­tracts and dilates with the activity of the heart muscles. The wave of contractions of the heart is alternately reversed as seen in other asci­dians.

The larger blood vessels have definite wall while the small ones lack definite wall and these are mere sinuses (haemocoel). Origina­ting from the heart, a larger vessel extends ante­riorly as well as posteriorly below the whole length of the endostyle.

This is the strongest vessel in Herdmania and is called ventral or sub-endostyle vessel. The ventral vessel, throughout its course, gives paired transverse vessels to the pharynx and a stout ventral test vessel to the test. The latter originates just from the point of origin of the ventral vessel from the- heart.

Another large blood vessel located dorsal to the dorsal lamina is the dorsal vessel which receives transverse vessels from the pharynx along its course and anteriorly sends branches to the neural gland. Originating from the dorsal vessels there is a small branchio-visceral vessel.

This vessel bifurcates into right and left vessels on the respective sides of the pharynx. The left one is the ventrointestinal vessel supplying blood to the oesophagus, stomach and left lobe of liver while the right oesophageal vessel supplies the right lobe of liver and oesophagus.

From the dorsal side of the heart a cardio-visceral vessel emerges out. This vessel imme­diately divides into two branches. One of the branches is the hepatic vessel ramifying within the liver and the other vessel immediately bifurcates to form a slender test vessel and a stout oesophageal vessel.

The cardio-visceral vessel, after sending these branches, gives off one dorsal, a median and a ventral vessel. The dorsal branch ends in a ring-like sub-tentacular vessel. It encircles the base of the atrial funnel and sends branches to the wall of the funnel. The median branch ends in the left gonad. The ventral branch divides into dorsointestinal vessel and a sub-intestinal test vessel.

The pharynx and the test are supplied by many blood vessels from different sources. The anterior part of the pharynx is provided with peripharyngeal vessel which originates from the ventral vessel. The internal longitudinal blood vessels of the pharynx originate from the peripharyngeal vessel.

Thus the circulatory system in Herdmania is very well-developed and the course of cir­culation, like other Ascidians, is periodically changed by reversion of peristalsis. This phe­nomenon of reversal of peristalsis is not observed in any other animal except in certain larval insects.

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8. Excretory System of Herdamania:

The neural gland in Herdmania is regar­ded as the excretory organ. It is an oval- shaped gland situated dorsal to the nerve gan­glion. But in Ascidia its position is ventral. This gland is composed of branched tubules and many desquamated cells containing dark gran­ules (probably the excretory products) are found in the cavity of the gland.

The actual homology of the gland is much disputed. Julin (1893) and Metcalf (1901) regarded this gland to be homologous with the hypophysis of the vertebrates. Georges (1971) assumes that the gland influences the process of reproduction and Godeaux (1964) claims the phagocytotic activity of the gland. But Das (1936) strongly inclines to think that the neural gland, at least in Herdmania, is an excretory organ.

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9. Nervous System of Herdamania:

The nervous system, in adults, is represen­ted by an elongated solid nerve ganglion, called brain or cerebral ganglion. It is situated ventral to the neural gland. From the anterior end of the ganglion, three nerves emerge to innervate the oral funnel. The nerve ganglion gives off poste­riorly two nerves to the atrial funnel.

Sense Organs of Herdamania:

Definite organs of special sense are absent, but several structures function as the sensory receptors.

a. Receptor cells – Many receptor cells (receive contact stimuli) are present throu­ghout the test except at the vascular areas.

b. The epithelial cells covering the vascular ampullae of the test are tactile receptors.

c. The pigmented spots or ocelli located in the margins of the oral and atrial funnels are the photoreceptors.

d. The oral tentacles are able to test water that is drawn in through the oral funnel.

e. The dorsal tubercle is regarded to be either an olfactory or gustatory organ. The dorsal tubercle is composed of two conical eleva­tions. Each conical elevation is produced by a conical lobe which becomes spirally coiled. The dorsal tubercle is situated near the junction of the peripharyngeal bands with the dorsal lamina.

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10. Reproductive System of Herdamania:

The development of Herdmonia pallida has been worked out by Sebastian (1953). Herdmania is hermaphrodite. The herma­phroditic gonads consist of two large lobulated bodies. The right gonad is situated on the right side and just dorsal to the heart while the left gonad is lodged within the loop of the intes­tine. Each gonad is formed of 10-25 lobes.

These lobes are arranged in two rows, one on either side of a central axis (see Fig. 3.5D). Two gonoducts run within the central axis. Each gonadal lobe consists of two distinct zones which differ in cellular structure and coloura­tion.

The inner zone (ovarian region) contains different stages of egg formation while the peripheral zone (testicular region) contains spermatogonia, spermatocytes and spermato­zoa.

Each gonad is thus an ovotestis. Each ovarian region of each gonadal lobe is connected with oviduct through a narrow short tubule, called ovarian ductule. The oviduct opens to the atrial cavity very near to the anus. Similarly, the male gonoduct or vas deferens receives a sperm-ductule from each testicular region of the gonadal lobe.

The vas deferens opens near to the oviductal aperture. The number of ovarian ductules and sperm- ductules usually corresponds to the number of gonadal lobes. But the sperm-ductules from two adjacent gonadal lobes may sometimes join prior to opening into the vas deferens.

A mature spermatozoon is a minute struc­ture of about 4 micra in length. Each has a head capped by an acrosome, a neck and an extremely elongated straight tail. A mature egg measures 0.30 mm in diameter. Like that of Ascidia, the egg is enveloped by membranes which help in floatation.

The cytoplasm of the ovum contains a dense mass of yolk material. The nucleus with an excentric nucleolus is situ­ated on one side of the cell-body. This is lined by vitelline membrane which constitutes the wall of the ovum. There are two more layers — the inner chorion and outer chorion. Many vacuolated outer follicular cells are attached to the outer chorion.

The space between the outer chorion and inner chorion is filled with interchorional fluid. The space between the inner chorion and the vitelline membrane is filled up with perivitelline fluid. There are many inner follicle cells floating in the perivitelline fluid and the rest adhere to the vitelline membrane.

Although Herdmania is a hermaphroditic animal, self-fertilization is rare, because the ovarian regions mature prior to the testicular regions (protogynous condition). When the gametes become mature, they are expelled out to the sea water. Fertilization is thus external. The developmental sequences and metamor­phosis are exactly similar to that of Ascidia, already described.

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11. Affinities and Systematic Position of Herdamania:

The systematic status of urochordates has long been an unsettled issue. But the discovery of tadpole larva in the life history has removed all difficulties extant on the field. Prior to its dis­covery, claims have been put forward to indi­cate several animal groups as their relatives.

Non-chordate affinities:

Entire food shifting and aerating complex in ascidians have close functional parallel with that of sponges and oysters.

Other non-­chordate features of ascidians are:

(a) The presence of larval eyes and otocyst,

(b) The habit of budding chain of new zooids,

(c) The colonial adaptation of the simple and compo­site fixed ascidians and

(d) The presence of typhlosole in the intestine. But none of these above non-chordate features appears to be convincing to establish the relationship.

Chordate affinities:

The inclusion of the ascidians under the Phylum Chordata is beyond any question, because the tadpole larva possesses all the basic characteristics of, the chordates, viz., gill-slits, notochord and dorsal tubular nerve cord. As stated earlier, occurrence of these three charac­teristics in the urochordate is the result of their emergence from a common chordate ancestor.

Relationship with Hemichordata:

The nearest existing relatives of the uro­chordates are probably the hemichordates. This particular contention is based on scienti­fic facts and is supported by having many structural and functional similarities.

The simi­larities are:

(a) The structural plan of pharynx with all its accessories are strikingly similar in both.

(b) Occurrence of restricted notochord, although the claim of notochordal nature of the buccal diverticulum is quite doubtful in hemi­chordates.

(c) The development of the central part of the nervous system is closely similar.

Remarks:

The relationship cannot be fully justified because the inclusion of the hemichordates under the Phylum Chordata itself remains a debatable issue. The aforesaid similarities can be interpreted as the result of remote phylogenetic relation with the ancestral chordate stock.

Relationship with Cephalochordata:

On the basis of adult anatomy of the cephalochordates and the study on the deve­lopment of ascidians, the relationship between urochordates and cephalochordates cannot be denied. The adult forms of both the groups exhibit many structural similarities.

They are:

(a) Similar food concentration mechanism.

(b) Presence of velum and velar tentacles.

(c) Similarly constructed endostyle and its associated parts.

(d) Presence of similar atrial complex.

The ascidian tadpole and the adult cephalochordates also furnish many striking similarities which afford convincing evidence of their evolution from a common chordate ancestry. Early developmental stages are so closely similar that the sequences seem to be almost a duplication. The tadpole larva, before degeneration sets in, is similar to cephalochordate in many respects.

Besides the basic chordate features, both the forms exhibit the following resemblances:

(a) The presence of dorsal and ventral fins,

(b) The presence of endostyle and

(c) Similarities in the atrial com­plex.

Besides these facts, the specializations of the cephalochordates are exactly in the same direction as seen in the case of urochordates. This coincidence indicates that the cephalo­chordates have been departed from the main line of chordate evolution.

But for the un-segmented organisation in urochordates and for the difference in the developmental history, the above relationship becomes difficult to esta­blish. The common characteristics, particularly in the feeding and aerating mechanisms are due to similar habitat.

Other similarities are the result of their remote phylogenetic connection with the stock from which the urochordates and the cephalochordates have descended.

Relationship with Vertebrata:

From the standpoint of evolution, the ascidians are related to the vertebrates. The tadpole larva is compared with the larval fish. The neural gland is considered homologous with the hypophysis of vertebrates. It has experimentally been shown that the mam­malian gonadotropins stimulate the ascidian neural gland.

Various far-reaching hypotheses have been based upon the discovery of the tadpole larva in the developmental history of the ascidians. The most remarkable one is to place them in the direct line of the vertebrate ancestry. But recent workers on this do not support the idea of emergence of the verte­brates directly from the ascidian tadpole. The similarities in structure and physiology are due to common ancestry.

Systematic Position:

As regards the inclusion of ascidians under the phylum Chordata, there is little doubt at present, because the tadpole larva is basically a chordate having notochord, dorsal tubular nerve cord and gill-slits. But the degree of degeneration undergone by this group becomes difficult to interpret. Two views exist on this particular issue.

The urochordates, according to one view, are extremely degener­ated forms and have descended from the ancestors which had all the chordate charac­ters. In the opinion of the others, the ancestors of the urochordates are much lower in the status than any existing chordate, even lower in scale than the cephalochordates and have not yet acquired the distinctive characteristics of the higher forms.

The protourochordates arose from the invertebrate chordates which are not far from the ancestor of the cephalochordates. As regards the interrelationships within the uro­chordates, it can be regarded that the Larvacea are the most primitive forms.

The Thaliaceans have derived from the fixed ascidians, not far from the ancestral compound ascidians which gave origin to the Pyrosomida. The composite ascidians are polyphyletic in origin.