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This group of marine animals was formerly regarded as constituting, along with the Polyzoa and the Brachiopoda, the invertebrate class Molluscoidea. It is now known to be a degenerate branch of the Chordata, and to be more nearly related to the Vertebrata than to any group of the Invertebrata. The Tunicata are found in all seas, from the littoral zone down to abyssal depths. They occur either fixed or free, solitary, aggregated or in colonies. The fixed forms are the " simple " and " compound " Ascidians. The colonies are produced by budding and the members are conveniently known as Ascidiozooids. Some Tunicata undergo alternation of generations, and most of them show a retrograde metamorphosis in their life-history.
History 1 More than two thousand years ago Aristotle gave a short account of a simple Ascidian under the name of Tethyum. Schlosser and Ellis, in a paper on Botryllus, published in the Philosophical Transactions of the Royal Society for 1756, first brought the compound Ascidians into notice; but it was not until the commencement of the 19th century, as a result of the careful anatomical investigations of G. Cuvier (I) upon the simple Ascidians and of J. C. Savigny (2) upon the compound, that the close relationship between these two 1 Only the more important works can be mentioned here. For a more detailed account of the history of the group and a full bibliography see (17) and (35) in the list of works at the end of this article.
groups of the Tunicata was conclusively demonstrated. Lamarck (3) in 1816 instituted the class Tunicata, which he placed between the Radiara and the Vermes in his system of classification. The Tunicata included at that time, besides the simple and the compound Ascidians, the pelagic forms Pyrosoma, which had been first made known by F. Peron in 1804, and Salpa, described by P. Forskal in 1775. A. v. Chamisso, in 1819, made the important discovery that Salpa in its life-history passes through the series of changes which were afterwards more fully described b y J. J. S. Steenstrup in 1842 as " alternation of generations "; and a few years later Kuhl and Van Hasselt's investigations upon the same animal resulted in the discovery of the alternation in the directions in which the wave of contraction passes along the heart and in which the blood circulates through the body. It has since been found that this observation holds good for all groups of the Tunicata. In 1826 H. MilneEdwards and Audouin made a series of observations on living compound Ascidians, and amongst other discoveries they found the free-swimming tailed larva, and traced its development into the young Ascidian.
In 1845 Carl Schmidt (6) first announced the presence in the test of some Ascidians of " tunicine," a substance very similar to cellulose, and in the following year Lowig and A. v. Kolliker (7) confirmed the discovery and made some additional observations upon this substance and upon the structure of the test in general. T. H. Huxley (8), in an important series of papers published in the Transactions of the Royal and Linnean Societies of London from 1851 onwards, discussed the structure, embryology and affinities of the pelagic Tunicates Pyrosoma, Salpa, Doliolum and Appendicularia. These important forms were also investigated about the same time by C. Gegenbaur, C. Vogt, H. Muller, A. Krohn and F. S. Leuckart. The most important epoch in the history of the Tunicata is the date of the publication of A. Kowalevsky's celebrated memoir upon the development of a simple Ascidian (9). The tailed larva had been previously investigated; but its minute structure had not been sufficiently examined, and the meaning of what was known of it had not ben understood. It was' reserved for Kowalevsky in 1866 to demonstrate the striking similarity in structure and in development between the larval Ascidian and the vertebrate embryo. He showed that the relations between the nervous system, the notochord and the alimentary canal are the same in the two forms, and have been brought about by a very similar course of embryonic development. This discovery clearly indicated that the Tunicata are closely allied to Amphioxus and the Vertebrata, and that the tailed larva represents the primitive or ancestral form from which the adult Ascidian has been evolved by degeneration, and this led naturally to the view usually accepted at the present day, that the group is a degenerate side-branch from the lower end of the phylum Chordata, which includes the Tunicata (Urochorda), Balanoglossus, &c. (Hemichorda), Amphioxus (Cephalochorda) and the Vertebrata. Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by C. v. Kupffer (12), A. Giard (13 and 15), and others.
In 1872 H. Fol (14) added largely to the knowledge of the Appendiculariidae, and Giard (15) to that of the compound Ascidians. The most important additions which have been made to the latter since have been those described by Von Drasche (16) from the Adriatic and those discovered by the " Challenger " and other expeditions (17). The structure and the systematic arrangement of the simple Ascidians have been mainly discussed of recent years by J. Alder and A. Hancock (18), C. Heller (19), H. de Lacaze-Duthiers (20), M. Traustedt (21), L. Roule, R. Hartmeyer, C. P. Sluiter, W. Michaelsen and W. A. Herdman (17, 22). In 1874 Ussoff (23) investigated the minute structure of the nervous system and of the underlying gland (first discovered by Hancock), and showed that the duct communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or " olfactory ") tubercle. In 1880 C. Julin (24) drew attention to the similarity in structure and relations between this gland and the hypophysis cerebri of the vertebrate brain, and insisted upon their homology. M. M. Metcalf has since added to our knowledge of these structures. The Thaliacea have of late years been the subject of several very important memoirs. The researches of F. Todaro, W. K. Brooks (25), W. Salensky (26), O. Seeliger, Korotneff and others have elucidated the embryology, the gemmation and the life-history of the Salpidae; and K. Grobben, Barrois (27), and more especially Uljanin (28), have elaborately worked out the structure and the details of the complicated lifehistory of the Doliolidae. Finally, we owe to the successive memoirs of J. Hjort, O. Seeliger, W. E. Ritter, E. van Beneden, C. Julin, C. P. Sluiter, R. Hartmeyer and others the description of many new forms and much information as to the development and life-history of the group.
The new forms described from Puget Sound and Alaska have drawn renewed attention to the similarity of the fauna in that region of the North Pacific and the fauna of north-west Europe. There is probably a common circumpolar Tunicate fauna which sends extensions downwards in both Atlantic and Pacific. As the result of the careful quantitative work of the German Plankton expedition, A. Borgert thinks that the temperature of the water has more to do with both the horizontal and the vertical distribution of pelagic Tunicata in the sea than any other factor. It is probable that the occasional phenomenal swarms of Doliolum which have been met with in summer in the North Atlantic are a result of the curious life-history which, in favourable circumstances, allows a small number of budding forms to produce from the numerous minute buds an enormous number of the next generation. The great increase in the number of species known from nearly all seas during the last twelve or fifteen years of the 19th century enables us now to form a truer estimate of the geographical distribution of the group than was possible when the " Challenger " collections were described, and shows that the Tunicata at least give no support to the " bi-polar theory " of the distribution of animals.
Anatomy As a type of the Tunicata, Ascidia mentula, one of the larger species of the simple Ascidians, may be taken. This species is found in most of the br Characters. European seas, in shal ¦ low water. It has an irregularly ovate form, of a dull grey colour, and is attached to some foreign object by one end (fig. 1). The opposite end of the body has a terminal opening surrounded by eight rounded lobes. This is the mouth or branchial aperture, and it indicates the anterior end of the animal. About half-way back from the anterior end is the atrial or cloacal aperture, surrounded by six lobes and placed upon the dorsal edge. When the Ascidian is living and undisturbed, water is being constantly drawn in through the branchial aperture and passed out through the atrial. If coloured particles be placed in the water near the apertures, they are seen to be sucked into the body through the branchial aperture, and after a short time some of I them are ejected with considerable force through the atrial aperture. The current of water passing in is for respiratory purposes, and it also conveys food into the animal. The atrial current is mainly the water which has been used in respiration, but it also contains all excretions from the body, and at times the ova and spermatozoa or the embryos.
The outer grey part of the body, which is attached at or near its posterior end and penetrated by the two apertures, is the " test." This is a firm gelatinous cuticular secretion upon the outer surface of the ectoderm, which is a layer of flat cells. Although at first produced as a cuticle, the test soon becomes organized by the migration into it of cells derived from the mesoderm. A. Kowalevsky has shown that cells of the mesenchyme of the larva make their way through t in e e `-"m (From Herdman, " Challenger "Report.) FIG. 2. - Diagrammatic section of part of Mantle and Test of an Ascidian, showing the formation of a vessel and the structure of the test.
m, Mantle. blc, Bladder cell. mc, Mantle cells.
e, Ectoderm. s, s', Blood sinus in mantle y, Septum of ves tc, Test cell. being drawn out sel.
tm, Matrix. into test.
the ectoderm to the exterior during the metamorphosis, and become the first cells of the young test. Some of the cells in the adult test may, however, be ectodermal in origin (see fig. 2). These test cells may remain as rounded or fusiform or stellate cells embedded in the gelatinous matrix, to which they are constantly adding by secretions on their surfaces; or they may develop vacuoles which become larger and fuse so that each cell has an ovate clear cavity (a bladder cell), surrounded by a delicate film of protoplasm with the nucleus still visible at one point; or they may form pigment granules in the protoplasm; or, lastly, they may deposit carbonate of lime, so that one or several of them together produce a calcareous spicule in the test. Only the unmodified test cells and the bladder cells are found in Ascidia mentula (fig. 3).
(From The Cambridge Natural History, vol. vii., " Fishss, &c." By permission of Macmillan & Co., Ltd.) FIG. 3. - Section through the surface layer of Test of Ascidia mentula (X 50).
bl, Bladder cells; tc, test cell; tk, terminal knobs of vessels; v, vessels of test.
Calcareous spicules are found chiefly in the Didemnidae amongst compound Ascidians; but pigmented cells may occur in the test of almost all groups of Tunicata. The matrix in which these structures are embedded is usually clear and apparently homogeneous; but in some cases it becomes finely fibrillated, especially in the family Cynthiidae. It is this matrix which contains tunicine. At one point on the left side near the posterior end a tube enters the test, and then splits up into a number of branches, which extend in all directions and finally terminate in rounded enlargements or bulbs, situated chiefly in the outer layer k of the test. These tubes are known as the " vessels " of the test, and they contain blood. Each vessel is bounded by a layer of ectoderm cells lined by connective tissue (fig. 4, B), and is divided into two tubes by a septum of connective tissue. The septum does not extend into the terminal bulb, and consequently the two tubes communicate at their ends (fig. FIG. 4.
4, A). The vessels are formed by an outgrowth of a blood sinus (derived originally from the blastocoele of the embryo) from the body wall (mantle) into the test, the wall of the sinus being formed by connective tissue and pushing out a covering of ectoderm in front of it (fig. 2, s'). The test is turned inwards at the branchial and atrial apertures to line two funnel-like tubes - the branchial siphon leading to the branchial sac, and the atrial siphon leading to the atrial or peribranchial cavity.
The body wall, inside the test and the ectoderm, is formed of a layer (the somatic layer of mesoderm) of connective tissue, enclosing muscle fibres, blood sinuses, and nerves. This layer (the mantle has very much the shape of the test outside it, but at the two apertures it is drawn out to form the branchial and n atrial siphons (fig. 5). In the walls of these siphons the muscle fibres form powerful circular bands, the Body Wall sphincter muscles. Throughout the rest of the mantle and Body the bands of muscle fibres form a rude irregular net- Cavities. work. They are numerous on the right side of the body, and almost totally absent on the left. The muscles are all formed of very long fusiform non-striped fibres. The connective tissue of the mantle is chiefly a clear gelatinous matrix, containing cells of various shapes; it is frequently pigmented, giving brilliant red or yellow colours to the body, and is penetrated by numerous lacunae, in which the blood flows. Inside the mantle, in all parts of the body, except along the ventral edge, there is a cavity - the atrial or peribranchial 'cavity - which opens to the exterior by the atrial aperture. This cavity is lined by a layer of cells derived originally from the ectoderm' ' According to E. van Beneden and Tulin (30) oily the outer wall of the atrium is lined with epiblast, the inner wall being derived from the hypoblast of the primitive branchial sac.
t,c 61e FIG. I. - Ascidia mentula, from the right side.
and directly continuous with that layer through the atrial aperture (fig. 6); consequently the mantle is covered both dz externally and internally by to ectodermal cells.
There is no true body cavity or coelom in the mesoderm; and yet the Tunicata are Coelomata in their structure and affinities, although it is very doubtful -br s whether the enterocoele which has been described in M s the development is really P br found. In any case the - coelom if formed is after wards suppressed, and in - the adult is only represented by the pericardium and its derivatives and the small cavities of the renal and reproductive organs.
The branchial aperture (mouth) leads into the bran chial siphon (buccal cavity or Sac Neighbour- stomoda eum), ing organs. and this opens into the anterior end of a very large cavity (the branchial sac) which extends nearly to the posterior end of the body (see figs. 5 and 6). This branchial sac is an enlarged and modified pharynx, and is therefore properly a part of the alimentary canal. The oesophagus opens from it far back on the dorsal edge (see below). The wall of the branchial sac is pierced by a large number of vertical slits - the stigmata - placed in numerous transverse rows (secondary or subdivided gill-slits). These slits place the branchial sac in communication with the peribranchial or atrial cavity, which lies outside it (fig. 6). Between the stigmata the wall of the branchial sac is traversed by blood-vessels, which are arranged in three regular series (fig. 7) - (I) the transverse vessels, which run horizontally round the wall i. ov.
(From The Cambridge Natural History, vol. vii., "Fishes, &c." By permission of Macmillan & Co., Ltd.) FIG. 6. - Semi-diagrammatic transverse sectionofA scidia,passingthrou the atrial aperture, seen from anterior surface, left side uppermost mb, Muscle-bundles.
ov, Ovary.
pbr, Peribranchial cavity. r, Rectum.
ren, Renal vesicles.
sg, Stigmata.
sph, Atrial sphincter.
t, Test.
tr, Transverse vessel.
ty, Typhlosole.
vbls, Ventral blood-sinus.
and open at their dorsal and ventral ends into large longitudinal vessels, the dorsal and ventral sinuses; (2) the fine longitudinal vessels, which run vertically between adjacent transverse vessels and open into them, and which bound the stigmata; and (3) the internal longitudinal bars, which run vertically in Trough (From Herdman, "Challenger " Report.) FIG. 7. - A, Part of branchial sac of Ascidia from inside. B, Transverse section of same.
tr, Transverse vessel. lv, Fine longitudinal vessels.
cd, Connecting duct. Papillae.
P, ' p, p hm, Horizontal membrane. sg, Stigmata. il, Internal longitudinal bar.
(A and B are drawn to different scales.) a plane internal to that of the transverse and fine longitudinal vessels. These bars communicate with the transverse vessels by short side branches where they cross, and at these points are prolonged into the lumen of the sac in the form of hollow papillae. The edges of the stigmata are richly set with cilia, which drive the water from the branchial sac into the peribranchial cavity, and so cause the currents that flow in through the branchial aperture and out through the atrial.
Along its ventral edge the wall of the branchial sac is continuous externally with the mantle (fig. 6), while internally it is thickened to form two parallel longitudinal folds bounding a groove, the endostyle or ventral furrow (figs. 5, 6, 8, end.) corresponding to the hypopharyngeal groove of Amphioxus and the median part of the thyroid gland of Vertebrata. The endoderm cells which line the endostyle are greatly enlarged at the bottom, where they bear very long cilia, and on parts of the sides of the furrow so as to form projecting glandular pads (fig. 8, gl.). It is generally, sup posed that this organ is a gland for the production of the mucous secretion which is spread round the edges of the branchial sac and catches the food particles in the passing current of water. It has, however, been pointed out that there are comparatively few gland cells in the epithelium of the endo style, and that it is vv possible that this fur-;FIG. 8. - Transverse section of the endorow is' merely a ciliated style of an Ascidian. path along which the mucous secretion (produced in part by the subneural gland) is conveyed posteriorly along the ventral edge of the branchial sac. There are sensory bipolar cells in the lateral walls of the endostyle. At its anterior end the edges of the endostyle become continuous with the right and left halves of the posterior of two circular geai Bands. ciliated ridges - the peripharyngeal bands - which run parallel to one another round the front of the branchial sac. The dorsal ends of the posterior peripharyngeal band bend posteriorly (enclosing the epibranchial groove), and then join to form the anterior end of a fold which runs along the dorsal edge of the branchial sac as far as the oesophageal aperture. This fold is the dorsal lamina (figs. 5, 6, di). 1v gh a nd _,t FIG. 5. - Diagrammatic dissection of A. mentula to show the anatomy.
It probably serves to direct the stream of food particles entangled in a string of mucus from the anterior part of the dorsal lamina to Dorsal the oesophagus. In many Ascidians this organ, instead Do rsal . of being a continuous membranous fold as in A. mentula, is represented by a series of elongated triangular processes - the dorsal languets - one attached in the dorsal median line opposite to each transverse vessel of the bra nchial sac. The anterior peripharyngeal band is a complete circular ridge, having no connexion with either the endostyle or the dorsal lamina. In front of it lies the prebranchial zone, which separates the branchial sac behind from the branchial siphon in front. The prebranchial zone is bounded anteriorly by a muscular band - the posterior edge of the sphincter muscle - which bears a circle of long delicate processes, the tentacles (figs. 5, 9, to, tn). These project Tentacles. inwards at right angles so as to form a network across the entrance to the branchial sac. Each tentacle consists of connective tissue covered with epithelium (endoderm), and contains two or more cavities which are continuous with blood sinuses in Subneural the mantle. In the Gland. dorsal median line near the anterior end of the body, and embedded in the mantle on the ventral surface of the nerve ganglion, there lies a small glandular mass - the subneural gland - which, as Julin has shown (24), there is reason to regard as the homologue of the hypophysis cerebri of the vertebrate brain. Julin and E. van Beneden have suggested that the function of this organ may possibly be renal. The subneural gland, which was first noticed by Hancock, communicates anteriorly, as Ussoff (23) pointed out, by means of a narrow duct with the front of the branchial sac (pharynx). The opening of the duct is enlarged to form a funnelshaped cavity, which may be folded upon itself, convoluted, or even broken up into a number of smaller Dorsal openings, so as to form a complicated projec- Tubercle. tion, called the dorsal tubercle, situated in the dorsal part of the prebranchial zone. (fig. 9). The dorsal tubercle in A mentula is somewhat horseshoe-shaped (fig. 10); it varies in form in most Ascidians according to the genus and species, and in some cases in the individual also. The function of the neural gland must still be regarded as doubtful. The secretion is formed by the degeneration and disintegration of cells proliferated from the walls of the duct or its branches, and no concretions are found. The ciliated funnel of the dorsal tubercle is a sense-organ, innervated by a large nerve from the ganglion; it may be a sense-organ for testing the quality of the water entering the branchial sac.
The single elongated ganglion in the median dorsal line of the mantle between the branchial and atrial siphons is the only nerve- Nervous centre in A. mentula and most other Tunicata. It is the degenerate remains of the anterior part of the cerebro- System. spinal nervous system of the tailed larval Ascidian (see below). The posterior or spinal part has entirely disappeared in most Tunicata. It persists, however, in the Appendiculariidae and traces of it are found in some Ascidians (e.g. Clavelina). The ganglion gives off distributory nerves at both ends, which run through the mantle to the neighbourhood of the apertures, where Sense- they divide and subdivide. The only sense-organs are Organs. the pigment spots between the branchial and atrial lobes, the tentacles at the base of the branchial siphon, the dorsal tubercle, and possibly the languets or dorsal lamina. These are all in a lowly developed condition. Nerve-endings have also been found in the endostyle, the peripharyngeal bands and other parts of the wall of the pharynx. The larval Ascidians, on the other hand, have welldeveloped intracerebral optic and otic sense-organs; and in some of the pelagic Tunicata otocysts and pigment spots or eyes are found in connexion with the ganglion. Atrial tentacles (which may also be sensory) have now been found in a number of the gregarious Cynthiidae and Polystyelidae.
The mouth and the pharynx (branchial sac) have already been described. The remainder of the alimentary canal Alimentary is a bent tube which in A. mentula and most other Canal. Ascidians lies embedded in the mantle on the left side of the body, and projects into the peribranchial cavity. The oesophagus leaves the branchial sac in the dorsal middle line near the posterior end of the dorsal lamina (see fig. 5, œa). It is a short curved tube which leads ventrally to the large fusiform thick-walled stomach. The intestine emerges from the ventral end of the stomach, and soon turns anteriorly, then dorsally, and then FIG. io. - Dorsal Tubercle and neighbouring organs of A. mentula. Lettering as before.
egr, Epibranchial groove; z, prebranchial zone.
posteriorly so as to form a curve - the intestinal loop - open posteriorly. The intestine now curves anteriorly again, and from this point runs nearly straight forward as the rectum, thus completing a second curve - the rectal loop - open anteriorly (see fig. 5). The wall of the intestine is thickened internally to form the typhlosole, a pad which runs along its entire length. The anus opens into the dorsal part of the peribranchial cavity near to the atrial aperture. The walls of the stomach are glandular; and a system of delicate tubules with dilated ends, which ramifies over the outer wall of the intestine and communicates with the cavity of the stomach by means of a duct, is probably a digestive gland.
A mass of large clear vesicles which occupies the rectal loop, and may extend over the adjacent walls of the intestine, is a renal organ without a duct. Each vesicle is the modified remains Excretory of a part of the primitive coelom or body cavity, and is formed of cells which eliminate nitrogenous waste Organs. matters from the blood circulating in the neighbouring blood lacunae and deposit them in the cavity of the vesicle, where they form a concentrically laminated concretion of a yellowish or brown colour. These concretions contain uric acid, and in a large Ascidian are very numerous. The nitrogenous waste products are thus deposited and stored up in the renal vesicles in place of being excreted from the body. In other Ascidians the renal organ may differ from the above in its position and structure; but in no case has it an excretory duct, unless the subneural gland is to be regarded as an additional renal organ.
The heart is an elongated fusiform tube placed on the ventral and posterior edge of the stomach, in a space (the pericardium) which is part of the original coelom or body cavity, the Blood rest of which exists merely in the form of lacunae and ascalar of the cavities of the reproductive organs and renal Sy st em and vesicles in the adult Ascidian. The wall of the heart is Coelom. formed of a layer of epithelio-muscul a r cells, the inner ends of which are cross-striated; and waves of contraction pass along it from end to end, first for a certain number of beats in one direction and then in the other, so as to reverse the course of circulation periodically. At each end the heart is continued into a vessel (see fig. I I), which is merely a large sinus or lacuna lined with a delicate endothelial layer. The sinus leaving the ventral end of the heart is called the branchio-cardiac vessel,' and the heart itself is merely the differentiated posterior part of this sinus and is therefore a ventral vessel. The branchio-cardiac vessel, after giving off a branch which, along with a corresponding branch from the cardiovisceral vessel, goes to the test, runs along the ventral edge of the branchial sac externally to the endostyle, and communicates laterally with the ventral ends of all the transverse vessels of the branchial sac. The sinus leaving the dorsal end of the heart is called the cardio-visceral vessel, and this, after giving off to the test the branch above mentioned, breaks up into a number of sinuses, which ramify over the alimentary canal and the other viscera. These visceral lacunae finally communicate with a third great sinus, the ' On account of the periodic reversal of the circulation none of the vessels can be called arteries or veins.
FIG. 9. - Diagrammatic section through anterior dorsal part of A. mentula, showing the relations of the nerve ganglion, subneural gland, &c.
Nerve.
n', Myelon.
pp, Peripharyngeal band. sgl, Subneural gland.
sgd, Its duct.
1, Test lining branchial siphon.
viscero-branchial vessel, which runs forward along the dorsal edge of the branchial sac externally to the dorsal lamina and joins the dorsal ends of all the transverse vessels of the branchial sac. Besides these three chief systems, there are numerous lacunae in all parts d .?.
br.
ant.
vv ventral FIG. I I. - Diagram of the Blood Circulation in an Ascidian. The test is solid black.
at, Atrial aperture. da, Dorsal aorta.
br, Branchial aperture. ht, Heart.
b y , Branchio-visceral vessel. vv, Ventral or branchio-cardiac cv, Cardio-visceral system. vessel.
of the body, by means of which anastomoses are established between the different currents of blood. All these blood spaces and lacunae are to be regarded as derived from the blastocoele of the embryo, and not, as has been usually supposed, from the coelom (30). When of the heart contracts ventro-dorsally the course of the Course, i crculation is as follows: the blood which is flowing Circulation through the vessels of the branchial sac is collected in an oxygenated condition in the branchio-cardiac vessel, and, after receiving a stream of blood from the test, enters the heart (ht). It is then propelled from the dorsal end of the heart into the cardio-visceral vessels, and so reaches the test and digestive and other organs; then, after circulating in the visceral lacunae, it passes into the branchio-visceral vessel in an impure condition, and is distributed to the branchial vessels (fig. II, da) to be purified again. When the heart on the other hand contracts dorso-ventrally, this course of the circulation is reversed. As the test receives a branch from each end of the heart, it follows that it has afferent and efferent vessels whichever way the blood is flowing. In some Ascidians the vessels in the test become very numerous and their end branches terminate in swollen bulbs close under the outer surface of the test. In this way an accessory respiratory organ is probably formed in the superficial layer of the test. The blood corpuscles are chiefly colourless and amoeboid; but in most if not all Ascidians there are also some pigmented corpuscles in the blood. These are generally of an orange or reddish brown tint, but may be opaque white, dark indigo-blue, or even of other colours. Precisely similarly pigmented cells are found throughout the connective tissue of the mantle and other parts of the body.
A. mentula is hermaphrodite, and the reproductive organs lie, with the alimentary canal, on the left side of the body. The ovary is a ramified gland which occupies the greater part of the intestinal loop (see fig. 5). It contains a cavity which, along with the cavities of the testis, is derived from a part of the original coelom, and the ova are formed from its walls and fall when mature into the cavity. The oviduct is continuous with the cavity of the ovary and leads forwards alongside the rectum, finally opening near the anus into the peribranchial cavity. The testis is composed of a great number of delicate branched tubules, which ramify over the ovary and the adjacent parts of the intestinal wall. Those tubules terminate in ovate swellings. Near the commencement of the rectum the larger tubules unite to form the vas deferens, a tube of considerable size, which runs forwards alongside the rectum, and, like the oviduct, terminates by opening into the peribranchial cavity close to the anus. The lumen of the tubules of the testis, like the cavity of the ovary, is a part of the original coelom, and the spermatozoa are formed from the cells lining the wall. In some Ascidians reproductive organs are present on both sides of the body, and in others (Polycarpa) there are many complete sets of both male and female systems, attached to the inner surface of the mantle on both sides of the body and projecting into the peribranchial cavity.' Embryology 2 And Life-History We owe to W. E. Castle (1896) the most complete account which has yet been given of the early stages of development in an Ascidian. His careful study of the cell lineage in Ciona has made it clear that some of the conflicting statements of his predecessors arose from incorrect orientation of the embryos. One of the most important of his conclusions is that the mesoderm of Ascidians, and probably that of the archaic Vertebrates, is derived from both primary layers, ectoderm and endoderm. Further, he finds that Ciona produces both ova and spermatozoa at the same time, but selffertilization very rarely occurs. The eggs are laid just before dawn, ' For structure of other forms, see below.
2 For reproduction by gemmation see under " Classification " below.
and the larva is hatched during the following night. The test cells adhering to the young homogeneous test have, it is now well known, no connexion with the cells found later in the adult test. The larvae are free-swimming for from one to several days. They avoid the light. The spermatozoon enters at the ventral hemisphere, and that point determines the median plane and the posterior end of the embryo. The ventral is the animal pole. The cleavage is from the beginning bilateral. The first cleavage plane is vertical, and separates the right and left halves of the embryo. The four smaller dorsal cells with yolk give rise to the endodermal hemisphere; the four larger, more protoplasmic, cells form the ventral ectodermal hemisphere. The cells of the latter hemisphere divide more rapidly, and form the future aboral surface. When the dorsal hemisphere has twenty-two cells the ventral has fifty-four. The gastrulation is a combination of epiboly and invagination. The ventral ectoderm grows over, so as to envelop the dorsal hemisphere, while the latter sinks down and becomes saucer-shaped. In the centre of the dorsal surface ten cells form the future endoderm. Round these comes a ring of cells, the chordamesenchyme ring, from which the notochord and mesenchyme arise. Outside this ring is a row of cells, the neuro-muscular ring. The more anterior of these cells form the medullary plate, the more posterior the longitudinal musculature of the larva. The remainder of the cells (in the 112-cell stage) form ectoderm. By growth at the anterior end the blastopore gets pushed posteriorly, and the anterior chorda cells are covered up, and come to lie in the dorsal wall of the archenteron, sixteen cells in two rows, one over the other. The blastopore closes in the posterior part of the dorsal surface. In front of it is the medullary plate, with a continuation backwards at the sides of the blastopore. This region forms the trunk of the larva, the part posterior to it being drawn out to form the tail. The chorda cells pass back into the tail, while the mesenchyme cells shift forwards into the trunk. The muscle cells, derived from the neuro-muscular ring, lie behind the blastopore, and form the muscles of the tail. The closure of the medullary canal takes place from the blastopore forwards, and then the nerve cord is grown over by ectoderm. After closure of the blastopore the mesenchyme cells lie as lateral masses in the trunk; later they become the blood corpuscles and the mantle cells, &c.
Castle also discusses some important theoretical questions. He points out that, in Ciona at least, the chorda-mesenchyme ring takes part along with endoderm in the primary invagination, and so belongs to the primary endoderm; while the rest of the mesoderm, the muscle cells of the neuro muscular ring, are carried in by a secondary invagination, and belong to the outer layer of the young gastrula, or primary ectoderm. He considers that the. chorda must be regarded as a mesodermal organ. He agrees with former observers in seeing no trace of enterocoele formation, and he doubts whether any Chordata are Enterocoela. He does not believe in distinguishing those Metazoa with a mesoderm from those with a " mesenchyme." He considers that embryology gives no support to the Annelid hypothesis as to the origin of Chordates.
A long-continued discussion as to the origin, nature and fate of certain cells, the " testa-zellen," which make their appearance between the young embryo and its follicle (fig. 12), has ended in (After Pizon.) FIG. 12. - Portion of Mature Ovum of Ascidian, showing F, follicle, and f, r, " test-cells." practical agreement that these small cells are derived from the follicle-cells, and have nothing to do with the test. In Salpa, however, certain follicle-cells enter the embryo, and perform important functions in guiding the development for a time.
In most Ascidians the eggs are fertilized in the peribranchial cavity, and undergo most of their development before leaving the parent; in some cases, however, the eggs are laid, and Embryology. fertilization takes place in the surrounding water. The segmentation is complete and regular (fig. 13, A) and results in the formation of a spherical blastula, which then undergoes invagination (fig. 13, B). The embryo elongates, and the blastopore or invagination opening comes to be placed on the dorsal edge near the posterior end (fig. 13, C). The hypoblast cells lining the archenteron are columnar in form, while the epiblast cells are more cubical (fig. 13, B, C, D). The dorsal surface of the embryo now becomes flattened and then depressed to form a longitudinal groove, extending forwards from the blastopore to near the front of the body. This " medullary groove " now becomes converted into a closed dorsal nt. by. ductive Organs. canal by its side walls growing up, arching over, and coalescing in the median dorsal line (fig. 13, D). This union of the laminae dorsales to form the neural canal commences at the posterior end behind the blastopore and gradually extends forwards. Consequently the blastopore comes to open into the posterior end of the neural canal (fig. 13, D), while the anterior end of that cavity remains (After Kowalevsky.) FIG. 13. - Stages in the Embryology of a Simple Ascidian.
A to F, Longitudinal vertical sections of embryos, all placed with the dorsal surface uppermost and the anterior end at the right.
A, Early blastula stage, during segmentation.
B, Early gastrula stage.
C, Stage after gastrula, showing commencement of notochord.
D, Later stage, showing formation of notochord and of neural canal.
E, Embryo showing body and tail and completely formed neural canal.
F, Larva just hatched; end of tail cut off.
G, Transverse section of tail of larva.
adp, Adhering papillae of larva. nec, Neurenteric canal.
at, Epiblastic (atrial) involution. oc, Ocular organ of larva.
au, Auditory organ of larva. g, Gelatinous investment of ar, Archenteron. embryo.
bc, Blastocoele. in, Muscle cells of tail.
bp, Blastopore. mes, Mesenteron.
ch, Notochord. mc, Mesoderm cells.
ep, Epiblast. nv, Cerebral vesicle at anterior hy, Hypoblast. end of neural canal. nc, Neural canal.
open to the exterior. In this way the archenteron communicates indirectly with the exterior. The short canal leading from the neural canal to the archenteron is known as the neurenteric canal (fig. 13, D, nec). Previous to this stage some of the hypoblast cells at the front edge of the blastopore and forming part of the dorsal wall of the archenteron (fig. 13, C, ch) have become separated off, and then arranged to form an elongated band, two cells wide, underlying the posterior half of the neural canal (fig. 13, D, E, ch). This is the origin of the notochord. Outgrowths from the sides of the archenteron give rise to laterally placed masses of cells, which are the origin of the mesoblast. These masses show no trace of metameric segmentation. The cavities (reproductive and renal vesicles) which are formed later in the mesoblast represent the coelom. Consequently the body cavity of the Tunicata is a modified form of enterocoele. The anterior part of the embryo, in front of the notochord, now becomes enlarged to form the trunk, while the posterior part elongates to form the tail (fig. 13, E). In the trunk the anterior part of the archenteron dilates to form the mesenteron, the greater part of which becomes the branchial sac; at the same time the anterior part of the neural canal enlarges to form the cerebral vesicle, and the opening to the exterior at the front end of the canal now closes. In the tail part of the embryo the neural canal remains as a narrow tube, while the remains of the wall of the archenteron - the dorsal part of which becomes the notochord - are converted into lateral muscle bands (fig. 13, G) and a ventral cord of cells, which eventually breaks up to form blood corpuscles. As the tail grows longer, it becomes bent round the trunk of the embryo inside the egg-membrane. About this period the epiblast cells begin to form the test as a cuticular deposit upon their outer surface. The test is at first devoid of cells and forms a delicate gelatinous investment, but it shortly afterwards becomes cellular by the migration into it of test cells formed by proliferation from the epiblast.' The embryo is hatched about two or three days after fertilization, in the form of a tadpole-like larva, which swims actively through the sea by vibrating its long tail. The anterior end of the body is provided with three adhering papillae (fig. 13, Larval F, adp.) in the form of epiblastic thickenings. In the free-swimming tailed larva the nervous system, formed from the walls of the neural canal, becomes considerably differentiated. The anterior part of the cerebral vesicle remains thin-walled (fig. 13, F), and two unpaired sense-organs develop from its wall and project into the cavity. These are a dorsally and posteriorly placed optic organ, provided with retina, pigment layer, lens and cornea, and a ventrally placed auditory organ, consisting of a large spherical partially pigmented otolith, attached by delicate hair-like processes to the summit of a hollow crista acoustica (fig. 13, F, au). The posterior part of the cerebral vesicle thickens to form a solid ganglionic mass traversed by a narrow central canal: this becomes the ganglion of the adult Ascidian. The wall of the neural canal behind the cerebral vesicle becomes differentiated into an anterior thicker region, placed in the posterior part of the trunk and having a superficial layer of nerve fibres, and a posterior narrower part which traverses the tail, lying on the dorsal surface of the notochord, and gives off several pairs of nerves to the muscles of the tail. Just in front of the anterior end of the nervous system a dorsal involution of the epiblast breaks through into the upturned anterior end of the mesenteron and thus forms the mouth opening. Along the ventral edge of the mesen-' teron, which becomes the branchial sac, the endostyle is formed as a narrow groove with thickened side walls. It probably corresponds to the median portion of the thyroid body of Vertebrata. A curved outgrowth from the posterior end of the mesenteron forms the alimentary canal (oesophagus, stomach and intestine), which at first ends blindly. An anus is formed later by the intestine opening into the left of two lateral epiblastic involutions (the atria), which rapidly become larger and fuse dorsally to form the peribranchial cavity. Outgrowths from the wall of the branchial sac meet these epiblastic involutions and fuse with them to give rise to the first formed pair of stigmata, which thus come to open into the peribranchial cavity; and these alone correspond to the gill clefts of Amphioxus and the Vertebrata.
FIG. 14. - Sketches of Ascidian Larvae. A, Ascidia; 5, Styela; M, Anurella; C, Compound Ascidian.
Fig. 14 shows a few characteristic forms of Ascidian " tadpoles," or free-swimming larvae. A and S are typical simple Ascidians; M is the aberrant tailless form found in some Molgulidae; and C is the larva of a typical compound Ascidian.
After a short free-swimming existence the fully developed tailed larva fixes itself by its anterior adhering papillae to some foreign object, and then undergoes a remarkable series of retro- gressive changes, which convert it into the adult 'h' to Ascidian. The tail atrophies, until nothing is left but dult some fatty cells in the posterior part of the trunk. The adhering papillae disappear and are replaced functionally by a growth of the test over neighbouring objects. The nervous system with its sense organs atrophies until it is reduced to the single small ganglion, placed on the dorsal edge of the pharynx, and a slight nerve cord running for some distance posteriorly (van Beneden and J ulin). Changes in the shape of the body and a further growth and differentiation of the branchial sac, peribranchial cavity and other organs now produce gradually the structure found in the adult Ascidian.
The most important points in connexion with this process of development and metamorphosis are the following: (1) In the 1 Some of the first test cells are also probably derived from the epithelium of the egg follicle.
Ascidian embryo all the more important organs (e.g. notochord, neural canal, archenteron) are formed in essentially the same manner as they are in Amphioxus and other Chordata. (2) The free-swimming tailed larva possesses the essential characters of the (From The Cambridge Natural History, vol. vii., " Fishes, &c." By permission of Macmillan & Co., Ltd.) FIG. 15. - Metamorphosis of an Ascidian (modified from Kowalevsky and others).
A, Free-swimming tailed larva. B, The metamorphosis - larva attached. C, Tail and nervous system of larva degenerating. D, Further degeneration and metamorphosis of larva into E, the young fixed Ascidian.
at, Atrial invagination. m, Mouth.
ch, Notochord. mes, Mesenteron.
hy, Hypoblast cells. nc, Neural canal. i, Intestine.
nv, Neural vesicle with sense-organs.
Chordata, inasmuch as it has a longitudinal skeletal axis (the notochord) separating a dorsally placed nervous system (the neural canal) from a ventral alimentary canal (the archenteron); and therefore during this period of its life-history the animal belongs to the Chordata. (3) The Chordate larva is more highly organized than the adult Ascidian, and therefore the changes by which the latter is produced from the former may be regarded as a process of degeneration (30. The important conclusion drawn from all this is that the Tunicata are the degenerate descendants of a group of primitive Chordata (see below).
Classification And Characters Of Groups Order I. - Larvacea Free-swimming pelagic forms provided with a large locomotory appendage (the tail), in which there is a skeletal axis (the urochord). A relatively large test (the " house ") is formed with great rapidity as a secretion from the ectoderm; it is of Larvacea. merely a temporary structure, which is cast off and replaced by another. The branchial sac is simply an enlarged pharynx with two ventral cili ated openings (stigmata) leadin to the exterior. There is no separate peribranchial cavity. The nervous system consists of a large dorsally placed ganglion and a long nerve cord, which stretches backwards over the alimentary canal to reach the tail, along which it runs on the left side of the urochord. The anus opens ventrally on the surface of the body in front of the stigmata. No reproduction by gemmation or metamorphosis is known in the life-history.
This is one of the most interesting groups (fig. 16) of the Tunicata, as it shows more cornpletely than any of the rest the char acters of the original ancestral forms. It has undergone little or no degeneration, and consequently corresponds more nearly to the tailedlarval condition than to the adult forms of the other groups. The order includes a single family, the Appendiculariidae, all the members of which are minute and freeswimming. They occur on the surface of the sea in most parts of the world. They possess the power to form with great rapidity an enormously large investing gelatinous layer (fig. I I), which corresponds to the test of other groups. This was first described by von Mertens and by him named " Haus." It is only loosely attached to the body and is frequently thrown off soon after its formation and again reformed. H. Lohmann has made a careful study of the mode of formation of this " house " from certain large ectoderm cells, the " oikoplasts," and he considers that it probably fulfils the following functions: Its complicated apparatus of passages with partial septa form a finely perforated network, through which a relatively large volume of water is strained so as to entrap microscopic food particles; it helps in locomotion by its hydrostatic effect, and it is also a protection to the animal, which may escape from enemies by throwing off the house, which is many times its own size. The tail in the Appendiculariidae is attached to the ventral surface of the body (fig. 18), and usually points more or less anteriorly. The supposed traces of vertebration in the muscle bands and the nerve cord are probably artifacts, and do not indicate true metameric segmentation. Near the base of the tail there is a distinct elongated ganglion (fig. 18, ng'). The anterior (cerebral) ganglion has connected with it an otocyst, a pigment spot, and a tubular process opening into the branchial sac and representing the dorsal tubercle and associated parts of an ordinary Ascidian. The branchial aperture or mouth leads into the branchial sac or pharynx. There are no tentacles. The endostyle is short. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly. The wall of the branchial sac has only two ciliated apertures (fig. 19). They are homologous with the primary stigmata of the typical Ascidians and the gill clefts of vertebrates. They are placed o 4 0 ?0.?0 ?? ?? ....« ' ' .4.1.4 ? ?. ..r ' (From The Cambridge Natural History, vol. vii., " Fishes," &c. By permission of Macmillan & Co., Ltd.) FIG. 16. - Sketch of the chief kinds of Tunicata found in the sea. XXVII. 13 (After Fol.) FIG. 17. - Oikopleura cophocerca in " House," seen from right side, magnified six times. The arrows indicate the course of the water.
x, Lateral reticulated parts of " House." far back on middle line, open on the These tubes the ventral surface, one on each side of the and lead into short funnel-shaped tubes which surface of the body behind the anus (fig. 18, at). correspond to the right and left atrial involutions ov, Ovary.
pp, Peripharyngeal band.
ng, Cerebral ganglion.
ng', Caudal ganglion.
ng", Enlargement of nerve cord in tail.
so, Sense-organ (tactile) on lower lip.
sg, Ciliated aperture in pharynx.
st, Stomach.
tes, Testis.
u, Urochord.
u', Its cut end.
which, in an ordinary Ascidian, fuse to form the peribranchial cavity. The heart, according to Lankester, is formed of two cells, which are placed at the opposite ends and connected by delicate contractile protoplasmic fibrils. The large ovary and testis are placed at the posterior end of the body. The remainder of the structural details can be made out from figs. 18 and 19.
FIG. 19. - Transverse Section of Oikopleura; anterior part of body and tail.
At, Atrial passage.
b.s, Blood sinus.
br.s, Branchial sac (pharynx). ec, Ectoderm.
en, Endoderm.
The family Appendiculariidae comprises amongst others the following genera: Oikopleura (Mertens), and Appendicularia (Cham.), in both of which the body is short and compact and the tail relatively long, while the endostyle is straight; Megalocercus (Chun) containing M. abyssorum, a huge deep-sea form from the Mediterranean (30 mm. long); Fritillaria (Quoy and Gaimard), in which the body is long and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, and an ectodermal hood is formed over the front of the body; and Kowalevskia (Fol), a remarkable form described by Fol (14), in which the heart and endostyle are said to be absent, while the branchial sac is provided with four rows of ciliated tooth-like processes.
Order Ii. - THALIAcEA Free-swimming pelagic forms which may be either simple or compound, and the adult of which is never provided with a tail or a notochord. The test is permanent and may be either a li a cea. well developed or very slight. The musculature of the mantle is in the form of more or less complete circular bands, by the contraction of which locomotion is effected. The branchial sac has either two large or many small apertures, leading to a single peribranchial cavity, into which the anus opens. Blastogenesis. takes place from a ventral endostylar stolon. Alternation of generations occurs in the life-history, and may be complicated by polymorphism. The Thaliacea comprises two groups Cyclomyaria and Hemimyaria.
or 