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TUNICATA

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Originally appearing in Volume V27, Page 383 of the 1911 Encyclopedia Britannica.
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TUNICATA. This group of marine animals was formerly regarded as constituting, along with the PoIyzoa 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 genera- tions, and most of them show a retrograde metamorphosis in their life-history. HISTORY 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 ' 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 Sal pa, 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 by 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. Milne-Edwards 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. KSlliker (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 been 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 I88o 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 life-history 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 iauna 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 External found in most of the 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. I). 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 "t 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, FIG. I.—Ascidia mentula, from `Which is attached at or near its the right side. The Test, posterior end andpenetrated by the two apes at, Atrial aperture; br, bran- tures, is the " test." This is a chial aperture; t, test. 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 c e`., 8 Pt t"in t e • e,• (From Herdman," Challenger "Report.) 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). 7i. t.k. (From The Cambridge Natural History, vol. vii., "Fishes, &c." By permission of Macmillan & Co., Ltd.) bi, 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 fally terminate in rounded enlargements or bulbs, situated chiefly in the outer layer 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 A, A vessel from the test. an outgrowth of a blood sinus B, Diagrammatic transverse sec-(derived originally from the bias- ttomof a vessel. tocoele of the embryo) from the ec, Ectoderm. body wall (mantle) into the test, c t, Connective tissue. the wall of the sinus being formed s,s',The two tubes. by connective tissue and pushing y septum. out a covering; of ectoderm in t k ,Terminal bulb. 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, enclosin 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 Mantle, atrial siphons (fig. 5). In the walls of these siphons L3 an 0' the muscle fibres form powerful circular bands, the and ' Wal sphincter muscles. Throughout the rest of the mantle Cavitie . s the bands of muscle fibres form a rude irregular net- 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 Julin (30) only the outer wall of the atrium is lined with epiblast, the inner wall being derived from the hypoblast of the primitive branchial sac. br dt ,ln Branchial sac. Dorsal lamina. Dorsal tubercle. Endostyle. Heart. Intestine. Mantle. Nerve ganglion. Oesophagus. Oesophageal aperture. Ovary. Peribranchial cavity. Rectum. Stomach. Test. Tentacles. Vas deferens. Subneural gland. of a, brs, dl, dt, end, h, m, ng, a', tea, ov, pbr, st, ln, vd, ngl, at I: At. Dorsal. At, Atrial aperture. all, Atrial lobe. Brs, Branchial sac. cl, Cloaca. con, Connective. dbls, Dorsal blood-sinus. dl, Dorsal lamina. end, Endostyle. gd, Genital ducts. n', Intestine. 1v, Interstigmatic vessel. m, Mantle. ov, pbr, r, ren, sg, sph, t, tr, ty, vbls, TUNICATA 381 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 trans-verse vessels and open into them, and which bound the stigmata; and (3) the internal longitudinal bars, which run vertically in (From Herdman,"Challenger" Report.) tr, Transverse vessel. lv, Fine longitudinal vessels. cd, Connecting duct. p,p', Papillae. hm, Horizontal membrane. sg, Stigmata. 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 Endosiye, 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 pass- ing current of water. v `! It has, however, been pointed out that there • are comparatively few ' ` • • • - gland cells in the epi- thelium ,Ra'a~. y m +a~„ sc c _ ~1l• .a,: ~~ b of the endo- — ". style, and that it is `'" possible that this fur- Fio. 8.—Transverse section of the endorow is merely a ciliated style of an Ascidian. path along which the br., Branchial sac; end., lips of endomucous secretion (pro- style; gl., glandular tracts; m.b., muscle duced in part by the bands; pbr., peribranchial cavity; sg., subneural gland) is stigma; v.v, ventral vessel. 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 Peripharynright and left halves of the posterior of two circular gerlBands. 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 Dorsal form the anterior end of a fold which runs along the Lamica dorsal edge of the branchial sac as far as the oesophageal aperture. This fold is the dorsal lamina (figs. 5, 6, dl). with that layer through the atrial consequently the mantle is covered both externally and internally by ectodermal cells. There is no true body cavity or coelom in the mesoderm ; and yet the Tunicata are Coelomata in their structure and affinities, al-though it is very doubtful whether the enterocoele which has been described in the development is really 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 re-productive organs. The branchial aperture (mouth) leads into the bran-Branchial chial siphon Sacand (buccal cavity or 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 r, ov. c be. (From The Cambridge Natural History, vol. vii., "Fishes, &c." By permission of Macmillan & Co., Ltd.) Peribranchial cavity. Rectum. Renal vesicles. Stigmata. Atrial sphincter. Test. Transverse vessel. Typhlosole. Ventral blood-sinus. 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 Dorsal . 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 branchial 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 Tentacles. processes, the tentacles (figs. 5, 9, 10, 111). These project inwards at right angles so as to form a network across the entrance to the branchial sac. Each tentacle consists of con- nective tissue covered with epithelium (endoderm), and contains two or more cavities which are continuous with blood sinuses in the mantle. In the subneural dorsal median line near Gland. 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 funnel-shaped cavity, which may be folded upon itself, convoluted, or even broken up into a number of smaller Dorsal openings, so as to form orercle, a complicated projec- 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. Io); it varies in form in most Asci- dians 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 disintegra- tion of cells proliferated from the branchial 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- centre in A. mentula and most other Tunicata. It is the Nervorts degenerate remains of the anterior part of, the cerebro- systea• 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 well-developed 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 A/laentary described. The remainder of the alimentary ' canal 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 peribraachial cavity. Theoesophagus leaves the branchial sac in the dorsal middle line near the posterior end of the dorsal lamina (see fig. 5, tea). 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 art 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 Bzcretory of a part of the primitive coelom or body cavity, and is Organs. formed of cells which eliminate nitrogenous waste 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 rest of which exists merely in the form of lacunae andB lab of the cavities, of the reproductive organs and renal System and vesicles in the adult Ascidian. The wall of the heart iseoetom~ formed of a layer of epithelio-muscular 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 1), 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 there-fore 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. n, Nerve. n', Myelon. pp, Peripharyngeal band. sgl, Subneural gland. sgd, Its duct. t, Test lining 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 dorsal Vv. ventral at, Atrial aperture. da, Dorsal aorta. br, Branchial aperture. ht, Heart. bv, 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 Course of the heart contracts ventro-dorsally the course of the circulation. circulation is as follows: the blood which is flowing 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 Repro- is a ramified gland which occupies the greater part of Repro- the intestinal loop (see fig. 5). It contains a cavity Organ . 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.'
End of Article: TUNICATA
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