THIS group of animals was formerly regarded as constituting along with the Polyzoa and the Brachiopoda the invertebrate class Molluscoidea. It is now known to be a degenerate branch of the Chordata, and to be more nearly related to the Vertebrata than to any group of the Invertebrata History. ...
More than two thousand years ago Aristotle gave a short account of a Simple Ascidian under the name of Tethyum. 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 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 Cuvier (I ) 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 O. F. Müller (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 Péron 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 Clavelinidae, 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 Bolteninae (Macleay), Chelyosoma (Broderip and Sowerby, and afterwards Eschricht), Oikopleura (Mertens), Perophora (Lister), Pelonaia (Forbes and Goodsir), Chondro-stachys and Diplosoma (Denis Macdonald), Diazona (Forbes and Goodsir), and Rhodosoma (Ehrenberg, and afterwards Lacaze-Duthiers).
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 Löwig and Kölliker (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 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 Gegenbaur, Vogt, H. Müller, 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 a Simple Ascidian (p). The tailed larva had been previously discovered and investigated by several naturalists—notably H. Milne-Edwards (5 ), J. P. van Beneden (zo), and Krohn (II ); 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 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 (13 and 15 ), and others. Important observations upon the process of gemmation and the formation of colonies in various forms of Compound Ascidians have been made by Krohn, Metschnikoff, Kowalevsky, Ganin, Giard, Della Valle, and others, and have gradually led to the establishment 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 the Appendiculariidae, and Giard (15) 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 (zp), Lacaze-Duthiers (20 ), Traustedt (21 ), and Herdman (17 , 22). In 1874 Ussoff (23 ) investigated the minute structure of the nervous system and of the underlying gland, which was first discovered by Hancock, and showed that the gland has a duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or “olfactory”) 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 that they perform a renal function. The Thaliacea have of late years been the subject of several very important memoirs. The researches of Todaro, Brooks (25 ), Salensky (26 ), and others have elucidated the embryology, the gemmation, and the life-history of the Salpidae; 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 Doliolidae. Finally, in an important work published in 1886 on the morphology of the Tunicata, E. van Beneden and Julin (30 ) have, mainly as the result of a close comparison of the embryology of Ascidians with that of Amphioxus 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.
As a type of the Tunicata, Ascidia mentula, one of the larger species of the Simple Ascidians, may be taken. This species is found in most of the European seas, generally in 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 narrow, and it has a terminal opening surrounded by eight rounded lobes. This is the mouth or branchial aperture, and it always indicates the anterior end of the animal. About half-way 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 region. 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 them are ejected with considerable force through the atrial aperture. The current of water passing in is for respiratory purposes, and it also conveys food into the animal. The atrial current is mainly the water which has been used in respiration, but it also contains all excretions from the body, and at times the ova and spermatozoa or the embryos.
The outer grey part of the body, which is attached at or near its posterior end and penetrated by the two apertures, is the “test.” This is a firm gelatinous cuticular secretion 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 surfaces; or they may develop vacuoles in their protoplasm, which become larger and fuse to form a huge ovate clear cavity (a “bladder cell ”), surrounded by a delicate film of protoplasm and having 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. 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 imbedded is usually clear and apparently homogeneous; but in some cases it becomes finely fibrillated, especially in the family Cynthiidae. It is this matrix which contains tunicine. At one point on the left side near the posterior end a tube enters the test, and then splits up into a number of branches, which extend in all directions and finally terminate in rounded enlargements or bulbs, situated chiefly in the outer layer 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. 3, 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 com municate at their ends (fig. 3, A). The vessels are formed by an outgrowth of a blood sinus (derived originally from the blastocoele of the embryo) from the body wall (mantle) into the test, the wall of the sinus being formed by connective tissue and pushing out a covering of ectoderm in front of it (fig. 2, s '). The test is turned inwards at the branchial and atrial apertures to line two funnel-like tubes, —the branchial siphon leading to the branchial sac and the atrial siphon leading to the atrial or peribranchial cavity.
The body wall, inside the test and the ectoderm, is formed of a layer (the somatic layer of mesoderm) of connective tissue, 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 connective 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. 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 and directly continuous with that layer through the atrial aperture (fig. 5); consequently the mantle is covered both externally and internally by ectodermal cells.
The branchial aperture (mouth) leads into the branchial siphon (buccal cavity or stomodaeum), 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. 4 and 5). 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, p. 612). The wall of the branchial sac is pierced by a large number of vertical slits, — the stigmata, —placed in numerous transverse rows. These slits place the branchial sac in communication with the
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peribranchial or atrial cavity, which lies outside it (fig. 5, P). 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 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. 5, B ), while internally it is thickened to form two parallel longitudinal folds bounding a groove, the “endostyle,” hypobranchial groove, or ventral furrow (figs. 4, 5, end). The endoderm cells 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 production of the mucous secretion which is spread round the edges of the branchial sac and catches the food particles in the passing current of water; 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 probable 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 endostyle become continuous with the right and left halves of the posterior of two circular ciliated ridges,—the peripharyngeal bands,— which run parallel to one another round the front of the branchial sac. The dorsal ends of the posterior peripharyngeal band bend posteriorly (enclosing the epibranchial groove), and then join to form the anterior end of a fold which runs along the dorsal edge of the branchial sac as far as the oesophageal aperture. This fold is the dorsal lamina (figs. 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, instead 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 processes, the tentacles (figs. 4, 7, 8, tn). These project inwards at right angles so as to form a network across the entrance to the branchial sac. Each tentacle consists of connective tissue covered with epithelium (endoderm), and contains two or more cavities which are continuous with blood sinuses in the mantle. In the dorsal median line near the anterior end of the body, and 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 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 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 opening of the duct from the subneural gland, 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 degenerate remains of the anterior part of the cerebro-spinal nervous system of the tailed larval Ascidian (see below, p. 614). 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 ; see Julin). The ganglion gives off distributory nerves at both ends, which run through the mantle to the neighbourhood of the apertures, where they divide and subdivide. The only 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 condition. 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 been described. The remainder of the alimentary canal is a bent tube which in A. mentula and most other Ascidians 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, oe a). 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 completing a second curve—the rectal loop—open anteriorly (see fig. 4). 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...
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THISTLE. This term, as generally employed, is of vague application, being given to almost any herbaceous plant that is of a spiny character. More strictly, it is ap plied to the species of Carduus. These are Composite herbs with very spiny leaves, and similar bracts surrounding a head of purplish-white, tubular, 5-parted flowers seated on a pitted and hairy receptacle. The anthers have appendages both at the apex and at the base. The style has a ring of hairs at the point of bifurcation of the two stigmata. The fruit is surmounted by a tuft of silky white hairs. The species are numerous, and some are of great beauty, though not unnaturally looked on with disfavour by the farmer. The Cotton Thistle, remarkable for its covering of white down, is Cnopordon Acanthium ; the Blessed Thistle is Carduus benedictus ; the Holy Thistle, the leaves of which are spotted with white, is C. Marianus. The common C. lanceolatus seems to be the most suitable prototype for the Scotch Thistle, though that honour is also conferred on Cnopordon Acanthium, the cotton thistle, a doubtful native, and on other species. The great objection to thistles from an agricultural point of view resides in the freedom with which they produce seed, and in the vigour of their underground growth, which makes their uprooting a matter of difficulty. Partial uprooting may indeed, in the case of the perennial species, increase the mischief, for each fragment left behind may grow into a distinct plant. Annual species might be kept in check were they cut down before the flowers appear, but unless all the cultivators in a particular district co-operate the efforts of individuals are of little avail. The Globe Artichoke and Cardoon are very near allies of the thistles. The Safflower, Carthamus, another thistle, yields ...a serviceable dye; the Burdock, Arctium lappa, has an edible root; and numerous allied species have medicinal properties.
THISTLE, Order of the. See Knighthood, vol. xiv. p. 123.
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THISTLEWOOD CONSPIRACY, or Cato Street Conspiracy, a plot formed in 1820 to murder Lord Castlereagh and other ministers of the British crown, and to seize the Bank and Mansion-House and proclaim a provisional government. Its chief instigator was Arthur Thistle wood, or properly Thistlewaite, born in 1770, the son of a civil engineer in Lincolnshire, who had held a commission in the militia and afterwards in the line in the West Indies. In America and in France he had imbibed revolutionary views, and, having lost his wife’s fortune in speculation and on the turf, had planned the desperate scheme probably for his own benefit as well as the good of the nation. The intention was to murder the ministers in the house of the earl of Harrowby in Mansfield Street on the evening of the 23d February. For this purpose between twenty and thirty men assembled in a stable in Cato Street, Edgeware Road, but while they were arming themselves they were pounced upon by the police, and a large number captured, though the majority, including Thistlewood, escaped. A reward of £1000 having been offered for Thistlewood, he was arrested next day at 10 White Street. After a trial Thistlewood and four others were executed on the 1st May, while five were transported. On being asked on the scaffold if he repented, Thistle wood replied, “No, not at all; I shall soon know the last grand secret.”
See the Trials of Arthur Thistlewood, James Ings, John Thomas Brunt, Richard Tidd, William Davidson, and others at the Session of the Old Bailey 17-28 April 1820, 2 vols., 1820; and the Gentleman’s Magazine for the same year.
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