This group of animals was formerly regarded as constituting along with the Polyzoa and the Brachio-poda 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.
HISTORY.
More than two thousand years ago Aristotle gave a short account of a Simple Ascidian under the name of Teihywm. He described the appearance and some of the more important points in the anatomy of the animal. From that time onwards to little more than a century ago, although various forms of Ascidians had been briefly described by writers on marine zoology, comparatively little advance was made upon the knowledge of Aristotle. Schlosser and Ellis, in a paper containing a description of Botryllus, published in the Philosophical Transactions of the Eoyal Society for 1756, first brought the Compound Ascidians into notice; but it was not until the commence-ment of the 19th century, as a result of the careful anatomical investigations of Cuvier (/) upon the Simple Ascidians and of Savigny (2) upon the Compound, that the close relationship between these two groups of the Tunicata was conclusively demonstrated. Up to 1816, the date of publication of Savigny's great work (2), the few Compound Ascidians then known had been generally regarded as Alcyonaria or as Sponges; and, although many new Simple Ascidians had been described by 0. F. Muller (4) and others, their internal structure had not been investigated. Lamarck (3) in 1816, chiefly as the result of the anatomical discoveries of Savigny and Cuvier, instituted the class Tunicata, which he placed between the Radiata 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 Peron in 1804, and Salpa, described by Forskal in 1775.
Chamisso in 1820 made the important discovery that Salpa in its life-history passes through the series of changes which were afterwards more fully described by 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. Milne-Edwards (5) also founded the group of "Social" Ascidians, now known as the Clavelinidm, and gave a classification of the Compound Ascidians which was universally accepted for many years. From the year 1826 onwards a number of new and remarkable forms were discovered, as, for instance, some of the Bolteninx (Macleay), Chelyosoma (Broderip and Sowerby, and afterwards Eschricht), Oikopleura (Mertens), Perophora (Lister), Pelonaia (Forbesand Goodsir), Chondro-stachys and Diplosoma (Denis Macdonald), Diazona (Forbes and Goodsir), and Rhodosoma (Ehrenberg, and afterwards Lacaze-Duthiers).
In 1845 Carl Schmidt (<5) 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 Kolliker (7) confirmed the discovery and made some additional observations upon this substance and upon the structure of the test in general. Huxley (8), in an im- Huxley, portant series of papers published in the Transactions of the Boyal 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 Gegenbaur, Vogt, H. Muller, Krohn, and Leuckart. The most important epoch in the history of the Tunicata is the date of the publication of Kowalevsky's celebrated memoir upon the development of Kowa-a Simple Ascidian (9). The tailed larva had been previously levsky. discovered and investigated by several naturalists—notably Tailed H. Milne-Edwards (3), J. P. van Beneden (10), and Krohnlarva' (11); 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 Reiation-1866 to demonstrate the striking similarity in structure ship to and in development between the larval Ascidian and the Vertf " vertebrate embryo. He showed that the relations between ra et" the nervous system, the notochord, and the alimentary canal are much 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), Amphioxus (Cephalochorda), and the Vertebrata. Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by Kupffer (12), Giard Kupffer, (13 and 13), and others. Important observations upon Giard, &c. the process of gemmation and the formation of colonies in (jemma-various forms of Compound Ascidians have been made by tion. Krohn, Metschnikoff, Kowalevsky, Ganin, Giard, Delia Valle, and others, and have gradually led to the establish-ment of the general principle, that all the more important layers of the bud are derived more or less directly from the corresponding regions in the body of the parent.
In 1872 Fol (14) added largely to the knowledge of theFol, &c. Appendiculariidse, and Giard (13) to that of the Compound Ascidians. The latter author described a number of new forms and remodelled the classification of the group. The most important additions which have been made to the Compound Ascidians since Giard's work have been those described by Von Drasche (16) from the Adriatic and those discovered by the " Challenger" expedition (17). The structure and the systematic arrangement of the Simple Ascidians have been mainly discussed of recent years by Alder and Hancock (18), Heller {19), Lacaze-Duthiers (20), Traustedt (21), and Herdman (17, 22). In 1874 UssofF (-?j) investigated the minute structure of the nervous gu). system and of the underlying gland, which was first dis- neural covered by Hancock, and showed that the gland has a S,ali(1
and dorsal
duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or "olfactory") tibercle. tubercle. In an important paper published in 1880 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. He suggests
have of late years been the subject of several very import-Thali- ant memoirs. The researches of Todaro, Brooks (23), acea. Salensky (26), and others have elucidated the embryology, the gemmation, and the life-history of the Salpidx; and Grobben, Barrois (27), and more especially Uljanin (28) have elaborately worked out the structure and the details of the complicated life-history of the Doliolidx. Finally, in an important work published in 1886 on the morpho-Vau logy of the Tunicata, E. van Beneden and Julin (30) have, Beneden mainly as the result of a close comparison of the embryo-Julin *°£>y °^ Ascidians with that of Ampliioxus and other Chordata, added considerably to our knowledge of the position and affinities of the Tunicata, and of the exact relations of their organs to the corresponding parts of the body in the Vertebrata.
ANATOMY.
eharac ters.
at
the partile
FIG. 1
1 Some writers use a different nomenclature of regions ; see (/7).
Ascidia As a type of the Tunicata, Ascidia mentula, one of the mentula. iarger species of the Simple Ascidians, may be taken. This species is found in most of the European seas, generally in External shallow water on a muddy bottom. It has an irregularly ovate form, and is of a dull grey colour. It is attached to some foreign object by one end (fig. 1). The opposite end of the body is usually nar-row, and it has a terminal opening surrounded by eight rounded lobes. This is the mouth or branchial aperture, and it always indicates the anterior end1 of the animal. About half-wray back from the anterior end, and on a rounded projection, is the atrial or cloacal aperture— an opening surrounded by six lobes—which is always placed upon the dorsal re-gion. When the Ascidian is living and undisturbed, water is being constantly drawn in through the branchial aperture and passed out through atrial. If coloured tides be placed in wTater near the apertures, they are seen to be sucked into the body through the branchial aperture, and after a short time some of them are ejected with consider-able force through the atrial aperture. The current of water passing in is for re-spiratory 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 test. The outer grey part of the body, which is attached at or near its posterior end and penetrated by the two aper-tures, is the " test." This is a firm gelatinous cuticular secretion from the outer surface of the ectoderm, which is a layer of flat cells lining its inner surface. Although at first produced as a cuticle, the test soon becomes organized by the migration into it of cells derived from the ectoderm (see fig. 2). These test cells may remain as rounded or fusiform or stellate cells imbedded in the gelatinous matrix, to which they are constantly adding by secretions on their
.EM
surfaces; or they may develop vacuoles in their proto-plasm, which become larger and fuse to form a huge ovate clear cavity (a "bladder cell "), sur-rounded by a delicate film of protoplasm and having the nucleus still visible at one point; or they may form pigment granules in the pro-toplasm ; 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. Calcareous spicules are found chiefly in the Didemnidx, amongst Compound Ascidians; but pigmented cells may occur in the test of almost all groups of Tunicata. The matrix in which these structures are imbedded is usually clear and apparently homogeneous; but in some cases it becomes finely fibrillated, especially in the family Cynthiidx. 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 num-ber of branches, which extend in all directions and finally 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 ec-toderm cells lined by con-nective tissue (fig. 3, B), and is divided into two tubes by a septum of con-nective tissue. The septum FlQ> does not extend into the terminal bulb, and conse-quently the two tubes com-municate at their ends (fig. 3, A)
The vessels are formed by an outgrowth of a blood sinus (derived originally from the blastoccele of the embryo) from the body wall (mantle) into the test, the wall of the sinus being formed by con-nective tissue and pushing out a covering of ectoderm in front of it (fig. 2, *'). 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 Mantle, of a layer (the somatic layer of mesoderm) of connective tissue, inclosing 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 atrial siphons (fig. 4). In the walls of these siphons the muscle fibres form powerful circular bands, the sphincter muscles. Throughout the rest of the mantle the bands of muscle fibres form a rude irregular network. 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 con-nective tissue of the mantle is chiefly a clear gelatinous
matrix, containing cells of various shapes; it is frequently pigmented and is penetrated by numerous lacunae, in which the blood flows. In-side the mantle, in all parts of the body, except along the ven-tral 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 origin-ally from the ecto-derm and directly continuous with that a layer through the vd-atrial aperture (fig. '" 5); consequently the mantle is covered both externally and inter-nally by ectodermal cells.
Bran- The branchial aper-
chial sac ture (mouth) leads in-
neigh- ^° e branchial si-
bouring phon (buccal cavity FM
ergans. or stomodaeum), and this opens into the anterior end of a very large cavity (the bran-chial sac) which ex-tends nearly to the posterior end of the body (see figs. 4 and 5). This branchial sac is an enlarged and modified pharynx, and is therefore properly a part of the ali-mentary canal. The oesophagus opens from it far back on the dorsal edge (see below, p. 612). The wall of the branchial sac is pierced by a large number of ver-tical slits,—the stigmata, —placed in numerous trans-verse rows. These slits place the branchial sac in communication with the
peribranchial or atrial cavity, which lies outside it (fig. 5, B). Between the stigmata the wall of the branchial sac is traversed by blood-vessels, which are arranged in three regular series (fig. 6),—(1) the transverse vessels, which run horizontally round the wall 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 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 bran-chial sac into the peribranchial ca-vity, and so cause the currents that flow in through the branchial aperture and out through the atrial.
Along its vent-ral edge the wall of the branchial sac is continu-
ous externally with the mantle (fig. 5, B), while internally it is thickened to form two parallel longitudinal folds bounding a groove, the " endostyle," hypobranchial groove, EIKIO-or ventral furrow (figs. 4, 5, end). The endoderm cells style, which line the endostyle are greatly enlarged at the bottom and on parts of the sides of the furrow so as to form projecting pads, which bear very long cilia. It is generally supposed that this organ is a gland for the pro-duction 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; but it has recently been pointed out that there are comparatively few gland cells in the epithelium of the endostyle, and that it is more prob-able that this furrow is merely a ciliated path along which the mucous secretion (produced possibly by the subneural gland) is conveyed posteriorly along the ventral edge of the branchial sac. At its anterior end the edges of the Peri-endostyle become continuous with the right and left halves pharyn-of the posterior of two circular ciliated ridges,—the peri-pharyngeal bands,—which run parallel to one another round the front of the branchial sac. The dorsal ends of the posterior peripharyngeal band bend posteriorly (en- Dorsal closing the epibranchial groove), and then join to form lamina, 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. 4, 5, dl). 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 the oesophagus. In many Ascidians this organ, Dorsal instead of being a continuous membranous fold as in A. languets, mentula, is represented by a series of elongated triangular .processes—the dorsal languets,—one attached iii 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 pos-terior edge of the sphincter muscle,—which bears a circle of long delicate processes, the tentacles (figs. 4, 7, 8, tn). Tcn-These project inwards at right angles so as to form a net-tacles-work across the entrance to the branchial sac. Each tentacle consists of connective tissue covered with epithe-
Hum (endoderm), and contains two or more cavities which are continuous.with blood sinuses in the mantle. In the Subneur- dorsal median line near the anterior end of the body, and al gland, imbedded 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 sug-gested that the function of this organ may possibly be renal.1 The sub-neural gland, which was first noticed by Hancock, communicates anteriorly, as ITssofT (2j) 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 openings, so as to form a complicated projection, called the dorsal tubercle, situated in the dorsal part of the prebranchial zone (fig. 7). The dorsal tubercle in A. mentula is somewhat horse-shoe-shaped (fig. 8) ; it varies in form in most Ascidians according to the genus and species, and in some cases in the individual also. Possibly, besides being the FIG opening of the duct from the sub-neural gland, it may be a sense-organ for testing the quality of the water entering the branchial sac.
Nervous system.
The single elongated ganglion in the median dorsal line of the mantle between the branchial and atrial si-phons is the only nerve-centre in A. mentula and most other Tunicata. It is the degenerate remains of the anterior
1 See also Herdman, Nature, vol. xxviii. p. 284.
at..
which run through the mantle to the neighbourhood of the Sense apertures, where they divide and subdivide. The onlyorSans-sense-organs are the pigment spots between the branchial and atrial lobes, the tentacles at the base of the branchial siphon, and possibly the dorsal tubercle and the languets or dorsal lamina. These are all in a lowly developed con-dition. The larval Ascidians on the other hand have well-developed intra-cerebral optic and auditory sense-organs; and in some of the pelagic Tunicata otocysts and pigment spots are found in connexion with the ganglion.
The mouth and the pharynx (branchial sac) have already Aliment-been described. The remainder of the alimentary canal aiT is a bent tube which in A. mentula and most other Ascid-cana1, ians lies imbedded 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. 4, cea). 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 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 com-pleting a second curve—the rectal loop—:open anteriorly (see fig. 4). The wall of the intestine is thickened inter-nally, 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 Excre-loop, and may extend over the adjacent walls of the in-tory testine, is a renal organ without a duct. Each vesicle is or&alls-the modified remains of a part of the primitive ccelom or body-cavity, and is formed of cells which eliminate nitro-genous 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 Aseidian are very numerous. The nitrogenous waste products are thus de-posited 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 a renal organ.
The heart is an elongated fusiform tube placed on the Blood-ventral and posterior edge of the stomach, in a sjiace (the vascular pericardium) which is part of the original coelom or body- ^dtem cavity, the rest of which exists merely in the form of lacunae cceiom, and of the cavities of the reproductive organs and renal vesicles in the adult Aseidian. The wall of the heart is 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. 9), 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 cardio-visceral vessel, goes to the test, runs along the
7.—Diagrammatic section through anterior dor-sal part of A. mentula, showing the relations of the nerve ganglion, sub-neural gland, &c. Letter-ing as for fig. 4 ; n, nerve ; n', myelon ; pp, peripha-ryngeal band ; sgl, sub-neural gland ; sgd, its duct ; tf, test liningbranch-ial siphon. (Original.)
FIG. 8.—Dorsal tubercle and neighbouring organs of A. mentula. Lettering as before; egr,
epibranchial groove ; z, prebranchial zone. (Original.)
part of the cerebro-spinal nervous system of the tailed larval Aseidian (see below, p. 614). The posterior or spinal part has entirely disappeared in most Tunicata. It persists, however, in the Appendiculariidee, and traces of it are found in some Ascidians (e.g., Clavelina; see Julin). The ganglion gives off distributory nerves at both ends,
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 lacunas finally communicate with
a third great sinus, the viscero-branchial vessel, which runs
forward along the dorsal edge of the branchial sac exter-
nally 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
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
blastoccele of the embryo, and not, as has been usually
Course of supposed, from the coelom (30). When the heart contracts
cireula- ventro-dorsally, the course of the 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 ;a
branchial 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 ccelom, and the spermatozoa are formed from the cells lining the wall. In some Ascidians repro-ductive 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 AND LIFE HISTORY.
In most Ascidians the eggs are fertilized in the peribranchial Embryo cavity, and undergo most of their development before leaving the logy, parent; in some cases, however, the eggs are laid, and fertilization takes place in the surrounding water. The segmentation is com-plete and regular (fig. 10, A) and results in the formation of a spherical blastula, which then undergoes invagination (fig. 10, B). The embryo elongates, and the blastopore or invagination opening comes to be placed on the dorsal edge near the posterior end (fig. 10, C). The hypoblast cells lining the archenteron are columnar in form, while the epiblast cells are more cubical (fig. 10, 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 canal by its side walls growing up, arching over, and coalescing in the median dorsal
Footnotes
According to E. van Beneden and Julia's recent investigations (jo) only the outer wall of the atrium is lined with epiblast, the inner wall being derived from the hypoblast of the primitive branchial sac.
See also Herdman, Nature, vol. xxviii. p. 284.
For structure of other forms, see p. 614 sq. below.
For reproduction by gemmation, see under "Classification," p. 614 sq. below.
en-' the It is pro-from dorsal
òr
the test ters heart, then pelled the
the FIG. 9.
fore
visceral vessels; vb, viscero-branchial or dorsal vessel; vt, vessels to test. (Original.)
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. 4). It contains a cavity which, along with the cavities of the testis, is derived from a part of the original ccelom, 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 com-posed 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 peri-
end of 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 viscero-branchial vessel in an impure condition, and is distributed to the branchial vessels to be purified again. When the heart on the other hand contracts dorso-ventrally, this course of 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 of intermediate colours. Precisely similarly pigmented cells are found throughout the connective tissue of the mantle and other parts of the body.
line (fig. 10, D). This union of the lamina} 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. 10, D), while the anterior end of that cavity remains 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. 10,
FIG. 10.—Stages in the embryology of a Simple Ascidian (after Kowalevsky). 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 gastruia stage. C. Stage after gas-trula, showing commencement of notochord. D. Later stage, showing forma-tion 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 papillas of larva ; at, epiblastic (atrial) involution ; au, auditory organ of larva ; ar, archenteron ; be, blastoccele ; bp, blastopore ; c%, noto-chord ; ep, epiblast ; liy, hypoblast ; nc, neural canal ; nec, neurenteric canal ; oc, ocular organ of larva ; g, gelatinous investment of embryo ; m, muscle cells of tail ; mes, mesenteron ; me, mesoderm cells ; nv, cerebral vesicle at anterior end of neural canal.
D, nee). 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. 10, C, cli) have become separated off, and then arranged to form an elongated band, two cells wide, underlying the posterior half of the neural canal (fig. 10, 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 meta-meric segmentation. The cavities (reproductive and renal vesicles) which are formed later in the mesoblast represent the ccelom. Consequently the body-cavity of the Tunicata is a modified form of enteroecele. 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. 10, 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. 10, 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 ceils formed bv proliferation from the epi-blast.
Larval The embryo is hatched about two or three days after fertilization, stage. in the form of a tadpole-like larva, wdiich swims actively through the sea by vibrating its long tail. The anterior end of the body is provided with three adhering papillae (fig. 10, 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. 10, 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. 10, F, au). The posterior part of the cerebral vesicle thickens to form a solid ganglionic mass traversed by a narrow central canal. 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 nei've 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 mesenteron, wdiich 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 thjToid _ body of Vertébrala. A curved outgrowth from the posterior end of the mesenteron forms the alimentary canal (oesophagus, stomach, and intestine), wdiich at first ends blindly. An anus is formed later by the intestine opening into the left of two lateral epiblastic involutions (the atria), wdiich 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 wdth 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 Vertébrala. Metamor- After a short free-swimming existence the fully developed tailed phosis larva fixes itself by its anterior adhering papilla? to some foreign to adult object, and then undergoes a remarkable series of retrogressive form. changes, wdiich convert it into the adult Ascidian. The tail atro-phies, until nothing is left but some fatty cells in the posterior part of the trunk. The adhering papilla? 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 re-duced to the single small ganglion, placed on the dorsal edge of the pharynx, and a slight nerve cord running for some distance pos-teriorly (Van Beneden and Julin). Slight 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 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 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 (31). The important conclusion drawn from all this is that the Tunicata are the degenerate descendants of a group of the primitive Chordata (see below p. 618).
CLASSIFICATION AND CHARACTERS OF GROUPS. Order I.—LARVACEA. Free-sw-imming pelagic forms provided wdth a large loeomotory Char-appendage (the tail), in wdiich there is a skeletal axis (the urochord). acters of A relatively large test (the
the "Haus") is formed with Larvacea-great rapidity as a secretion from the ectoderm ; it is merely a temporary structure, which is cast off and replaced by another. The branchial sac is simply an enlarged pharnyx with two ventral ciliated openings (stigmata) leading to the exterior. There is no se-parate 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 wdiich it runs on the left side of the urochord. The anus opens ventrally on the surface of „ the body in front of the stig-mata. No reproduction by gemmation or metamorphosis is known in the life-history.
This is one of the most in-
teresting groups of the Tuni-
cata, as it shows more com-
pletely than any of the rest
the characters of the original
ancestral forms. It has un-
y dergone little or no degen-
/ eration, and consequently o
corresponds more nearly to the tailed-larval condition than to the adult forms of the other groups. The order . Oikoplmra eophoeerca in " Haus " includes a single family, the (after Fol), seen from right side, magnified APPENDICULARIID*, all the six times. The arrows indicate the course members of wdiich are minute " Haus™*6''! *'Meral reticulated parts of and free - swimming. They
occur on the surface of the sea in most parts of the world. They possess the power to form Struc-with great rapidity an enormously large investing gelatinous layer ture of (fig. 11), wdiich corresponds to the test of other groups. This was Appen-
first described by Von llertens and by him named "Haus." It is only loosely attached to the body and is frequently thrown off soon after its formation. The tail in the Appendiculariidaz is at-tached to the ventral surface of the body (fig. 12), and usually
FIG. 12.—Semi-diagrammatic view of Appendwularia from the right, a, anus ; at, one of the atrial apertures; app, tail; or, branchial aperture; brs, branchial
, sac; at, dorsal tubercle; end, endostyle ; h, heart; i, intestine; m, muscle band of tail; n, nerve cord in body; n', nerve cord in the tail; ce, oesophagus ; ot, otocyst; 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; Us, testis ; u, urochord; u', its cut end. (Original.)
Thali-acea.
Characters of Cyclo-myaria.
Struc-ture of Doliolum.
points more or less anteriorly. It shows distinct traces of meta-meric segmentation, having its muscle bands broken up into myotomes, while the nerve cord presents a series of enlargements from which distributary nerves are given off (fig. 12, ng"). Near the base of the tail there is a distinct elongated ganglion (fig. 12, 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 i only two ciliated apertures. They are homologous with the primary stigmata of the typical Ascidians and the gill clefts of Vertebrates. They are placed far back on the -ven-tral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (fig. 12, at). These tubes correspond to the right and left atrial involutions 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 fig. 12.
The family Appendiculariida} comprises the genera,—OikopUura (Mertens), and Appendicularia (Cham.), in both which the body is short and compact and the tail relatively long, while the endostyle is straight; Fritillaria (Q. and G.), 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, endostyle, and intestine 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 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 wdiich 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. Alternation of generations occurs in the life-history, and may be complicated by polymorphism. The Thaliacea comprises two groups, Gydomyaria and Hemimyaria.
Sub-order 1.—Cyclomyaria.
Free-swimming pelagic forms which exhibit alternation of generations in their life-history but never form permanent colonies. The body is cask-shaped, with the branchial and atrial apertures at the opposite ends. The test is more or less well developed. The mantle has its musculature in the form of circular bands surrounding the body. The branchial sac is fairly large, occupying the anterior half or more of the body. Stigmata are usually present in its posterior part only. The peribranchial cavity is mainly posterior to the branchial sac. The alimentary canal is placed ventrally close to the posterior end of the branchial sac. Hermaphrodite reproductive organs are placed ventrally near the intestine.
This group forms one family, the DOLIOLIDJE, including two genera, Doliolum (Quoy and Gaimard) and AncMnia (C. Vogt).
Doliolum, of which several species are known from various seas, has a cask-shaped body, usually from 1 to 2 cm. in length. The terminal branchial and atrial apertures (fig. 13) are lobed, and the lobes are provided with sense organs. The test is very slightly developed and contains no cells. The mantle has eight or nine circular muscle bands surrounding the body. The most anterior and posterior of these form the branchial and atrial sphincters. The wide branchial and atrial apertures lead into large branchial and peribranchial cavities, separated by the posterior wall of the branchial sac, which is pierced by stigmata ; consequently there is a free passage for the water through the body along its long axis, and the animal swims by contracting its ring-like muscle-bands, so as to force out the contained water posteriorly. Stigmata may also be found on the lateral walls of the branchial sac, and in that case there are corresponding anteriorly directed diverticula of the peribranchial cavity. There is a distinct endo-style on the ventral edge of the branchial sac and a peripharyngeal band surrounding its anterior end, but there is no representative of the dorsal lamina on its dorsal edge. The oesophagus commences rather on the ventral edge of the posterior end of the branchial sac, and runs backwards to open into the stomach, which is followed by a curved intestine opening into the peribranchial cavity. The alimentary canal as a whole is to the right of the middle line. The hermaphrodite reproductive organs are to the left of the middle line alongside the alimentary canal. They open tween the posterior end of the endostyle and the oesophageal aperture. The nerve ganglion lies about the middle of the dorsal edge of the body, and gives off many nerves. Under it is placed the subneural gland, the duct of which runs forward and opens into the anterior end of the branchial sac by a simple aperture, surrounded by the spirally twisted dorsal end of the peripharyngeal band (fig. 13, dt).
The ova of the sexual generation produce tailed larvae ; these Develop-develop into forms known as " nurses" (blastozooids), which
into the peribranchial cavity. The ovary is nearly spherical, while the testis is elongated, and may be continued anteriorly for a long distance. The heart is placed in the middle line ventrally, be-
which are ment of asexual, and are characterized by the possession of nine muscle Dolio-bauds, an auditory sac on the left side of the body, a ventrally- lum. placed stolon near the heart, upon which buds are produced, and a dorsal outgrowth near the posterior end of the body. The buds give rise eventually to the sexual generation, which is polymorphous, having three distinct forms, in two of which the reproductive organs remain undeveloped. The buds while still very young migrate from their place of origin on the stolon, divide by fission, and become attached to the dorsal outgrowth of the body of the nurse, where they develop. The three forms produced are as follows. (1) Nutritive forms (trophozooids), wdiich remain permanently attached to the nurse and serve to provide it with food ; they have the body elongated dorso - ventrally, and the musculature is very slightly developed. (2) Foster forms (phorozooids), which, like the preceding, do not become sexually mature, but, unlike them, are set free as cask-shaped bodies with eight muscle bands and a ventral outgrowth, which is formed of the stalk by which the body was formerly united to the nurse. On this outgrowth the (3) forms (gonozooids) which become sexually mature are attached while still young buds, and after the foster forms are set free these reproductive forms gradually attain their complete development, and are eventually set free and lose all trace of their connexion with the foster forms. They resemble the foster forms in having a cask-shaped body with eight muscle bands, but differ in having no outgrowth or process, and in having the reproductive organs fully developed.1
Anchinia, of which only one species is known, A. rubra, from Anchinia.
the Mediterranean, has the sexual forms permanently attached
to portions of the dorsal outgrowth from the body of the unknown
nurse. The body is elongated dorso-ventrally. The test is well
developed and contains branched cells. The musculature is not
so well developed as in Doliolum. There are two circular bands
at the anterior end and two at the posterior, and tw7o on the
middle of the body. The stigmata are confined to the obliquely
placed posterior end of the branchial sac. The alimentary canal
forms a U-shaped curve. The reproductive organs are placed on
the right side of the body. The life-history is still imperfectly
known. As in the case of Doliolum the sexual generation is
polymorphous, and has three forms, two of which remain in a ,
rudimentary condition so far as the reproductive organs are concerned. In AncMnia, however, the three forms do not occur to-gether on one stolon or outgrowth, but are produced successively, the reproductive forms of the sexual generation being independent of the " foster forms " (see Barrois, 27).
Sub-order 2.—Hemimyaria.
- For further details see Uljanin (¿5).
Free-swimming pelagic forms wdiich exhibit alternation of genera. Charac-tions in their life-history and in the sexual condition form colonies, ters of The body is more or less fusiform, with the long axis antero-posterior, Hemi-and the branchial and atrial apertures nearly terminal. The test myaria. is well developed. The musculature of the mantle is in the form of a series of transversely-running bands, which do not form com-plete independent rings as in the Cyclomyaria. The branchial and
peribronchial cavities form a continuous space in the interior of the body, opening externally by the branchial and atrial apertures, and traversed obliquely from the dorsal and anterior end to the ventral and posterior by a long narrow vascular band, which represents the dorsal lamina, the dorsal blood-vessel, and the neighbouring part of the dorsal edge of the branchial sac of an ordinary Aseidian. The alimentary canal is placed ventrally. It may either be stretched out so as to extend for some distance anteriorly, or—as is more usual—be concentrated to form along with the reproductive organs a rounded opaque mass near the posterior end of the body, known as the visceral mass or "nucleus." The embryonic development is direct, no tailed larva being formed.
Fio. 14.—Salpa runcinata-fusiformis. A. Aggre- buds or embryos.
This sub-order contains two very distinct families, the SALC-IM:, which are the typical members, and the OCTACNEMIDJE, including a single very remarkable form (Octacnemus bylhius), which in some respects does not conform with the characters given above. Salpidee. The Salpidm includes the single genus Salpa (Forskal), which, however, may be divided into two well-marked groups of species,—(1) those, such as S. pinnata, in which the alimentary canal is stretched out along the ventral surface of the body, and (2) those, such as S.fusiformis (fig. 14, A), in which the aliment-ary canal forms a compact globular mass, the " nucleus," near the posterior end of the body. About fifteen species altogether are known; they are all pelagic forms and are found in nearly all seas. Each species occurs in two forms—the solitary asex-ual (proles solitaria) and the aggregated sexual (proles gregaria)—which are usually quite unlike end ""IBSSf -3 one another. The soli-tary form (fig. 14, B) gives rise by internal d I gemmation to a complex tubular stolon, which contains processes from all the more important organs of the parent body and which becomes seg-mented into a series of
As
gated form. B. Solitary form. Lettering as the stolon elongates, the
before; 1-9, muscle hands; em embryo/, £«, emblyos ]lear t]le free
gemmiparous stolon; m, mantle vise, visceral , o>. . . ,
mass (nucleus). (Original.) end which have become
advanced in their deve-lopment are set free in groups, which remain attached together by processes of the test, each enclosing a diverticulum from the mantle so as to form "chains" (fig. 15). Each member of the chain is a Salpa of the sexual or aggregated form, and when mature may—either still attached to its neighbours or se-parated from them (fig. 14, A)— produce one or several embryos, which develop into the solitary _ Salpa. Thus the two forms alter-Struc- nate regularly. The more import-ture of ant points in the structure of a Salpa. typical Salpa are shown in fig. 16.
sg d n g
! I ! *
and laterally, but the major-ity do not reach the ventral sur-face. In many cases neigh-bouring bands join in the med-ian dorsal line, F[a lg _ gemi.,iiagrammatic representation of Salpa from (fag. 14). the leftside. Lettering as before; emb, embryo ; m, mantle ; anterior end of I, languet; sgd, duct of subneural gland ; 1-11, muscle the dorsal la- hands of mantle; t, thickening of test over nucleus; dl, gill or branchia. (Original.)
mma is pro-longed to form a prominent tentacular organ, the languet, pro-
The branchial and atrial apertures are at opposite ends of the body, and each ieads into a large cavity, the branchial and peribranchial Fio sacs, which are in tree communica-tion at the sides of the obliquely-running dorsal lamina or "gill." The test is well developed and adheres closely to the surface of the mantle. The muscle bands of the mantle do not completely encircle the body. They are present dorsally
jecting into the branchial sac. The nerve ganglion, subneural gland, dorsal lamina, peripharyngeal bands, and endostyle are placed in the usual positions. A pigment spot and an otocyst are found in connection with the ganglion. The large spaces at the sides of the dorsal lamina (often called the gill or branchia of Salpa), by means of which the cavity of the branchial sac is placed in free communication with the peribranchial cavity, are to be regarded as gigantic stigmata formed by the suppression of the lateral walls of the branchial sac. Fig. 16 represents an aggre-gated or sexual Salpa which was once a member of a chain, since it shows a testis and a developing embryo. The ova (always few in number, usually only one) appear at a very early period in the developing chain Salpa, while it is still a part of the gemmiparous stolon in the body of the solitary Salpa. This gave rise to the view put forward by Brooks (*$o), that the ovary really belongs to the solitary Salpa, which is therefore a female producing a series of males by asexual gemmation, and depositing in each of these an ovum, wdiich will afterwards, when fertilized, develop in the body of the male into a solitary or female Salpa. This idea would of course entirely destroy the view that Salpa is an example of alterna-tion of generations. The sexual or chain Salpa, although really hermaphrodite, is always protogynous : i.e., the female elements or ova are produced at an earlier period than the male organ or testis. This prevents self-fertilization. The ovum is fertilized by the Develop-spermatozoa of an older Salpa belonging to another chain, and ment of the embryo is far advanced in its development before the testis is Salpa. formed. At an early period in its development a part of the embryo becomes separated off, along with a part of the wall of the cavity in which it lies, to form the " placenta," in which the embryonic and the maternal blood streams circulate in close proximity (or actually coalesce during one period) and so allow of the passage of nutriment to the developing embryo. At a somewdiat later stage a number of cells placed at the posterior end of the body alongside the future nucleus become filled up wdth oil-globules to form a mass of nutrient material—the elasoblast—which is used up later on in the develop-ment. Many suggestions have been made as to the homology of the ekeoblast. The most probable is that it is the disappearing rudiment of the tail found in the larval condition of most Aseidians.
The family Octacncmidaz includes the single remarkable form Octa-Octacnemus bythius, found during the " Challenger " expedition, and enemidee first described by Moseley(2o). It is apparently a deep-sea representative of the pelagic Sal-pidaz, and may pos-sibly be fixed. The body is somewdiat discoid, with its margin prolonged to form eight taper- Fl0' ing processes, on to which the muscle bands of the mantle are con-tinued. The ali-mentary canal forms a compact nucleus (fig. 17); the endostyle is very short; and the dorsal lamina is apparently absent. The re-production and life-history are entirely unknown.
Order III.—ASCIDIACEA.
Fixed or free-swimming Simple or Compound Aseidians which in Ascid-the adult are never provided with a tail and have no trace of a iocea.. notochord. The free-swimming forms are colonies, the Simple Aseidians being always fixed. The test is permanent and well developed ; as a rule it increases with the age of the individual. The branchial sac is large and well developed. Its walls are per-forated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the atrial aperture. Many of the forms reproduce by gemmation, and in most of them the sexually-produced embryo develops into a tailed larva.
The Ascidiacea includes three groups,—the Simple Aseidians, the Compound Aseidians, and the free-swimming colonial Pyrosoma.
Sub-order 1.—Ascidise Simplices.
Fixed Aseidians which are solitary and veiy rarely reproduce by Simple gemmation ; if colonies are formed, the members are not buried in Ascid-a common investing mass, but each has a distinct test of its own. ians. No strict line of demarcation can be drawn between the Simple and the Compound Aseidians, and one of the families of the former group, the Olamliiiidx (the Social Aseidians), forms a transition from the typical Simple forms, which never reproduce by gemmation, to the Compound forms, which always do (see p. 618 below). The Ascidite Simplices may be divided into the following families:—
Family I.—CLAVELINID^E. Simple Aseidians wdiich reproduce by gemmation to form small colonies in wdiich each ascidiozooid has a distinct test, but all are connected by a common blood-system.
15. — Posterior part of solitary form of Salpa demoeratlea-mueronata, showing a chain of embryos nearly ready to be set free, gem, young aggregated Salpse forming the chain ; st, stolon; m, muscle band of the mantle. (Original.)
Buds formed on stolons which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm, and endoderm of the ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. This family contains three genera: Eeteinascidia (Herdman), with internal longitudinal bars in branchial sac ; Clavelina (Savigny), with intestine extending behind branchial sac; and Perophora (Wieginann), with intestine alongside branchial sac.
Family II.—ASCIDIIM:. Solitary fixed Ascidians with gelatinous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed. Branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple. This family is divided into three sections :—
Sub-family 1.—HYPOBYTHIN*. Branchial sac with no internal longitudinal bars. One genus, Hypdbythius (Moseley).
Sub-family 2.—ASOLDIN*. Stigmata straight. Many genera, of which the following are the more important:—Ciona (Fleming), dorsal languets present; Ascidia (Linnajus, —Phallusia, Savigny), dorsal lamina present (see figs. 1 to 10); Ehodosoma (Ehrenberg), anterior part of test modified to form operculum; Aiyssascidia (Herdman), intestine on right side of branchial sac.
Sub-family 3.—COKELLIN.E. Stigmata curved. Three genera :— Corella (Alder and Hancock), test gelatinous, body sessile ; Coryn-ascidia (Herdman), test gelatinous, body* pedunculated; Chelyo-soma (Brod. and Sow.), test modified into horny plates.
Family III.—CYNTHIIDJS. Solitary fixed Ascidians, usually with leathery test; branchial and atrial apertures usually both four-lobed. Branchial sac longitudinally folded ; stigmata straight; tentacles simple or compound. This family is divided into three sections :—
Sub-family 1.—STYEMNJE, not more than four folds on each side of branchial sac ; tentacles simple. The more important genera are —Styela (Macleay), stigmata normal, and Bathyoncus (Herdman), stigmata absent or modified.
Sub-family 2.—CYNTHINJB, more than eight folds in branchial sac; tentacles compound; body sessile. The chief genus is Cynthia (Sa-vigny), with a large number of species.
Sub-family 3. — BOL-TENIM, more than eight folds in branchial sac ; tentacles compound ; body pedunculated (fig.
18, A). The chief genera
are—Boltenia (Savigny), J
branchial aperture four-
lobed, stigmata normal;
and Caleolus
man), branchial aper-
ture with less than four
lobes, stigmata absent or
modified (fig. 18, B).
This last is a deep-sea
genus discovered by the " Challenger" expedition (see 17).
Family IV.—MOLGULIDÍE. Solitary Ascidians, sometimes not fixed ; branchial aperture six-lobed, atrial four-lobed. Test usually incrusted with sand. Branchial sac longitudinally folded; stigmata more or less curved, usually arranged in spirals; tentacles compound. The chief genera are—Molgula (Forbes), with distinct folds in the branchial sac, and Eugyra (Aid. and Hanc), with no distinct folds, but merely broad internal longitudinal bars in the branchial sac. In some of the Molgulidse (genus Anurella, Laeaze-Duthiers, 20) the embryo does not become converted into a tailed larva, the development being direct, without metamorphosis. The embryo when hatched assumes gradually the adult structure, and never shows the features characteristic of larval Ascidians, such as the urochord and the median sense-organs.
Sub-order 2.—Ascidise Compositse. Com- Fixed Ascidians which reproduce Joy gemmation, so as to form pound colonies in which the ascidiozooids are buried in a common invest-Ascid- ing mass and have no separate tests. This is probably a somewhat ians. artificial assemblage formed of two or three groups of Ascidians which produce colonies in which the ascidiozooids are so intimately united that they possess a common test or investing mass. This is the only character wdiich distinguishes them from the Clavelinidse, but the property of reproducing by gemmation separates them from the rest of the Ascidim Simplices. The Ascidise Compositse may be divided into the following families :—
Family I.—DISTOMID.E. Ascidiozooids divided into two regions, thorax and abdomen ; testes numerous ; vas deferens not spirally coiled. The chief genera are—Distoma (Gaertner); Distaplia (Delia Valle); Colella (Herdman), forming a pedunculated colony (see fig.
19, A) in which the ascidiozooids develop incubatory pouches,
connected wdth the peribronchial cavity, in which the embryos
undergo their development (/7) ; and Chondrostachys (Macdonaíd).
Family II.—CCELOCORMID^. Colony not fixed, having a large axial cavity with a terminal aperture. Branchial apertures five-lobed. This includes one species, Ccelocormus huxleyi (Herdman), which is a transition form between the ordinary Compound Ascidians {e.g., Distomidse) and the Ascidiee Salpiformes (Pyrosoma).
Family III.— DIDEMXID.E. Colony usually thin and incrusting Test containing stel-late calcareous spi-cules. Testis single, large ; vas deferens spirally coiled. The chief genera are—Di-demnum (Savigny), in which the colony is thick and fleshy and there are only jr,G. lfl three rows of stig-mata on each side of the branchial sac ; and Lep>toclinum (Milne-Edwards), in wdiich the colony is thin and incrusting (fig. 19, B) and there are four rows of stigmata on each side of the branchial sac.
Family IV. —DIPLOSOMIEVE. Test reduced in amount, rarely con-taining spicules. Vas deferens not spirally coiled. In Diplosoma (Macdonald), the most important genus, the larva is gemmiparous.
Family V..—POLYCLINIIXE. Ascidiozooids divided into three regions,—thorax, abdomen, and post-abdomen. Testes numerous ; vas deferens not spirally coiled. The chief genera are—Pharyngo-dictyon (Herdman), with stigmata absent or modified, containing one species, PA. mirabile (fig. 19, C), the only Compound Ascidian known from a depth of 1000 fathoms ; Polyclinum (Savigny), wdth a smooth-walled stomach ; Aplidium (Savigny), with the stomach wall longitudinally folded ; and Amaroueium (Milne-Edwards), in wdiich the ascidiozooid has a long post-abdomen and a large atrial languet.
Family VI.—BOTRYLLID.&. Ascidiozooids having the intestine and reproductive organs alongside the branchial sac. Dorsal lamina present ; internal longitudinal bars present in branchial sac. The chief genera are—Botrylius (Gaertn. and Pall.), wdth simple stellate systems (fig. 19, D), and Botrylloides (Milne - Edwards), with elongated or ramified systems.
Family VII.—POLYSTYELIM. Ascidiozooids not grouped in systems. Branchial and atrial apertures four-lobed. Branchial sac may be folded ; internal longitudinal bars present. The chief genera are—Thylaeium (Carus), wdth ascidiozooids projecting above general surface of colony; Gooolsiria (Cun-ningham), wdth ascidiozooids completely imbedded in investing mass; and Chorizo-cormus (Herdman), with ascidiozooids united in little groups which are connected by stolons. The last genus contains one species, Oh. reticulatus, a transition form between the other Polystyelidse and the Styelinie amongst Simple Ascidians.
The methods of reproduction by gemma-tion differ in their details in the various groups of Compound Ascidians ; but in all cases the process is essentially a giving off from the parent body of groups of cells re-presenting the ectoderm, the mesoderm, and the endoderm, which develop into the corresponding layers of the bud. The first ascidiozooid of the colony produced by the tailed larva does not form sexual repro-ductive organs, but reproduces by gemma-tion so as to make a colony. Thus there is alternation of generations in the life-history. In the most completely formed colonies (e.g., Botrylius) the ascidiozooids are arranged in groups (systems or cceno-bii), and in each system are placed with their atrial apertures towards one another, and all communicating wdth a common cloaeal cavity which opens to the exterior in the centre of the system (fig. 19 D).
Sub-order 3.—Ascidisa Salpiformes.
Free-swimming pelagic colonies having the form of a hollow cylinder closed at one end. The ascidiozooids forming the colony FlG 20 _pyrosoma are imbedded in the common test in such a natural size. A. Side "view manner that the branchial apertures open of entire colony. B. End on the outer surface and theatrial apertures Spinal.)01*™ extremity-on the«inner surface next to the central ° cavity of the colony. The ascidiozooids are produced by gemmation from a rudimentary larva (the cyathozooid) developed sexually.
-Colonies of Ascidiie Composite (natural size). A. Colella quoyi. B. Lepioclimtm neglectum. C. Pha-ryngodietyon mirabile. D. Botrylius, showing arrangement of ascidiozooids in circular systems each of which has a central common cloaca. (After Herd-man, Challenger Report.)
Struc- This sub-order includes a single family, the PYROSOMIM, con-ture of taining one well-marked genus, Pyrosoma (Peron), with several Pyro- species. They are found swimming near the surface of the sea, soma, chiefly in tropical latitudes, and are brilliantly phosphorescent.
Their dorsal surfaces are turned towards the open end of the colony. The more important points in the structure of the ascidiozooid of are shown
A fully developed Pyrosoma colony may be from an inch or two to upwards of four feet in length. The shape of the colony is seen in fig. 20. It tapers slightly towards the closed end, which is rounded. The opening at the opposite end is reduced in size by the presence of a membranous prolongation of the common test (fig. 20, B). The branchial apertures of the ascidiozooids are placed upon short papilla? projecting from the general surface, and most of the ascidiozooids have long conical processes of the test projecting outwards beyond their branchial apertures (figs. 20, 21, and 22). There is only a single layer of ascidiozooids in the Pyrosoma colony, as all the fully developed ascidiozooids are placed with their antero-posterior axes at right angles to the surface and communicate by their atrial apertures with the central cavity of the colony (fig. 21).
in fig. 22. A circle of tentacles, of which one, placed ventrally (fig. 22, tn), is larger than the rest, is found just inside the branchial aperture. Prom this point a wdde cavity, with a few circularly-placed muscle bands run-ning round its Avails, _ leads back to the large branchial sac, which occupies the greater part of the body. The stigmata are elongated trans-versely and crossed by internal longitu-dinal bars. The dor-sal lamina is repre-sented by a series of eight languets (I). The nerve ganglion (on which is placed a small pigmentedg s sense organ), the sub-neural gland, the dor-sal tubercle, the peri-pharyngeal bands, and the endostyle are placed in the usual positions. On each side of the anterior end of the branchial sac, close to the peri-pharyngeal bands, is a mass of rounded gland cells which are the source of the phosphores-cence. The alimentary canal is placed posteriori}' to the branchial sac, and the anus opens into a large peribronchial (or atrial^eavity, of which only the median posterior part is shown (pbr) in fig. 22. The reproductive organs are developed in a diverticulum of the peri-branchial cavity, and consist of a lobed testis and a single ovum at a time. The development takes place in a part of the peribronchial Develop-cavity (fig. 21, em). The segmentation is meroblastic, and an ment of elongated embryo is formed on the surface of a mass of yolk. The Pyro-embryo, after the formation of an alimentary cavity, a tubular soma. nervous system, and a pair of laterally placed atrial tubes, divides into an anterior and a posterior part. The anterior part then segments into four pieces, wdiich afterwards develop into the first ascidiozooids of the colony, while the posterior part remains in a rudimentary condition, and was called by Huxley the "cyatho-zooid " ; it eventually atrophies. As the four ascidiozooids increase in size, they grow round the cyathozooid and soon encircle it (fig. 21, asc and cy). The cyathozooid absorbs the nourishing yolk upon wdiich it lies, and distributes it to the ascidiozooids by means of a heart and system of vessels which have been meanwhile formed. When the cyathozooid atrophies and is absorbed, its original atrial aperture remains and deepens to become the central cavity of the young colony, which now consists of four ascidiozooids placed in a ring, around where the cyathozooid was, and enveloped in a common test. The colony gradually increases by the formation of buds from these four original ascidiozooids.
PHYLOGENY.
The accompanying diagram shows graphically the probable Phylo-origin and course of evolution of the various groups of Tunicata, geny. and therefore exhibits their relations to one another much more correctly than any system of linear classification can do. The ' ancestral Proto-Tunicata are here regarded as an offshoot from the Proto-Chordata—the common ancestors of the Tunicata (Uro-chorda), Amphioxus (Geplialochorda), and the Verte-brata. The ancestral Tunicata were probably free-swimming forms, not very unlike „ the existing Appendiculariidee, and % are represented in the life-history
of nearly all sections of the Tunicala by the tailed lar-val stage. The Larvacea are the first off-shoot from the ancestral forms which gave rise to
the two lines of descendants, the Proto- Thaliacea and the Proto-Ascidiacea. The Proto- Thaliacea then split into the ancestors of the existing Cyclomyaria and Heminiyaria. The Proto-Ascidiacea gave up their pelagic mode of life and became fixed. This ancestral process is repeated at the present day when the free-swdmming larva of the Simple and Compound Ascidians becomes attached. The Proto-Ascidiacea, after the change, are probably most nearly repre-sented by the existing genus C'lavelina. They have given rise directly or indirectly to the various groups of Simple and Com-pound Ascidians and the Pyrosomidse. These groups form two lines, wdiich appear to have diverged close to the position of the family Clavelinidse. The one line leads to the more typical Compound Ascidians, and includes the Polyclinidse, Distomidse, Didemnidm, Diplosomidse, Cœlocormidm, and finally the Ascidiœ Salpiformes. The second line gave rise to the Simple Ascidians, and to the Botryllidss and Polystyelidse, wdiich are, therefore, not closely allied to the other Compound Ascidians. The later Proto-Ascidiacea were probably colonial forms, and gemmation was re-tained by the Clavelinidse and by the typical Compound Ascidians (Distomidse, &e. ) derived from them. The power of forming colonies by budding wras lost, howrever, by the primitive Simple Ascidians, and must, therefore, have been regained independently by the ancestral forms of the Botryllidse and the Polystyelidse. If this is a correct interpretation of the course of evolution of the Tunicata, we arrive at the following important conclusions. (1) The Tunicata, as a wdiole, form a degenerate branch of the Proto-Ohordata ; 2) the Ascidim Salpiformes (Pyrosoma) are much more closely related to the typical Compound Ascidians than to the other pelagic Tunicata, viz., the Larvacea and the Thaliacea; and (3) the Aseidise Composiim form a polyphyletic group, the sections of which have arisen at several distinct points from the ancestral Simple Ascidians.
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Footnotes
FIG. 22.—Mature ascidiozooid of Pyrosoma, from left side (partly after Keferstein). Lettering as before ; cm, cellular mass, the seat of phosphorescence; cm', posterior cellular mass ; gs, gemmiparous stolon ; mb, muscle band ; ngl, subneural gland ; pig, pigment spot on ganglion ; tp, process of test.
By Dohm and others their point of origin is placed considerably further up on the stem of the Chordata, thus causing the Tunicata to be regarded as very degenerate Vertebrata (see 32).