The terms 'mantle' and 'branchial sac,' often used in speaking of the body walls and pharynx respectively, are better discarded, as it is a well-established fact that there is ho relationship between the Urochorda and Mollusca.
The muscle fibres are unstriped and have the form of fusiform or filiform fibres. There is a well-developed sphincter both at the oral and atrial apertures, and the test at these spots is generally lobed - eight lobes round the oral, six round the atrial, aperture of the Ascidiidae. The part of the test between the thickened lobes is very thin and bendable. The mode in which the muscle fibres are disposed in the body walls is characteristic in the sub-groups of Urochorda. The connective tissue corpuscles of the body walls are often pigmented.
The tentacles spring from a ridge which coincides with the line in the oral aperture where the test ends. They are simple and filiform. Posteriorly to the circle of tentacles is a plane area - the praebranchial zone - limited behind by two ciliated ridges bounding a ciliated groove. The ridges are the two peripharyngeal bands. The posterior is continuous ventrally with the ridges bordering the endostyle, and the groove of the latter is continuous with the peripharyngeal groove. The posterior is also continuous dorsally with the dorsal lamina. The anterior band forms a complete circle.
The cavity of the pharynx is lined with endoderm. Its outer or atrial surface is covered by the invaginated ectoderm. The stigmata lying between the transverse vessels and the fine longitudinal vessels are usually straight, but sometimes curved. Under some conditions it appears that the direction of the ciliary current may be reversed, and water be expelled at the oral instead of the atrial aperture. The system of internal longitudinal vessels is very well developed in Ascidiidae, and it is characteristic of the family to have a prominent papilla developed from the free surface of these vessels at spots opposite to the short vessels which connect them to the transverse system.
The endostyle is a groove, at the bottom of which are cells bearing extremely long cilia. The sides of the groove are ridged, and the ridges are caused by the large size of certain cells which appear to secrete mucus. The mucus collects into small lumps or balls which are carried forwards to the peripharyngeal groove. They traverse this groove and are then conveyed by the ciliated dorsal lamina backwards to the stomachal opening. The dorsal lamina has anteriorly a median epipharyngeal groove, the extent of which varies in different Ascidians. The free edge of the lamina is curved, usually to the right. It ends posteriorly in a low ridge continuous with the endostyle.
The ganglion is single, generally swollen at either end where it gives off an anterior and posterior set of nerves. The central part is fibrous, and the ganglion cells are placed peripherally. The ganglion lies in the body walls, and always between the oral and the atrial apertures, on the opposite side of the body to the endostyle. As to organs of special sense, the tentacles must be regarded as tactile; and in many Ascidians there are orange-coloured visual (?) spots between the lobes of the oral aperture. The sub-neural gland consists of caecal ramified tubes. They open into a duct which lies between the gland and the ganglion. This duct runs forwards and opens into a ciliated depression of the prae-branchial zone. The margins of the depression are prominent, and the aperture has usually a crescentic shape, the concavity of the crescent being turned forwards. The margins may be much modified in shape. They constitute what is often spoken of as the ciliated sac or dorsal tubercle. The gland has been homologised by Julin with the pituitary body of the brain in higher Vertebrata, and it has been suggested that it has a renal function.
In some cases it has secondary ducts opening into the atrial cavity.
The digestive portion of the alimentary canal is disposed variously in different groups of Urochorda. The liver (?) may be represented by glandular tissue coating the stomach, and sometimes attaining considerable size, or by a system of clear tubes ramifying over the stomach and part of the intestine and opening into the pyloric portion of the former. A renal organ is probably represented by clear walled vesicles containing concretions in which uric acid has been found, and situated round the intestine and in the body walls. These vesicles have no ducts.
The heart is more or less fusiform, and is inclosed in a delicate pericardium. It lies in Ascidia along the posterior ventral edge of the stomach on the left side. It gives off (1) a ventral vessel which sends a branch through the mantle to the test and then runs forwards beneath the endostyle, and is connected to the transverse system of vessels in the walls of the pharynx; (2) a dorsal vessel from its opposite end which sends a branch through the mantle to the test parallel to the one above mentioned. These two branches divide in the test, end in ampullae, and intercommunicate. The remaining branches of the dorsal vessel go to the body walls; and to the stomach intestines and reproductive organs from which the blood is collected into a large vessel running along the dorsal edge of the pharynx and connected to its transverse vessels. There are branches also (=connectives of Hancock) from the pharynx to the body walls, thence to the test and back again. When the heart pulsates so as to drive the blood from the ventral to the dorsal surface, it draws arterialised blood from the pharynx and venous only from the test; when in the opposite direction the blood it draws has previously passed through the viscera, body walls, and test.
The blood itself is a clear plasma containing rounded nucleated corpuscles, many of which are pigmented, generally yellow, red, or brown, but white and blue are sometimes found.
All Urochorda are hermaphrodite, but in most instances the male and female organs are not ripe at the same time. These organs in Ascidia are racemose glands placed on the left side of the body between the intestine and stomach. The testis is composed of delicate white tubules, ramifying dichotomously, and spread over the ovary, stomach, and intestine. Both oviduct and vas deferens run along the dorsal edge of the intestine and open near the anus.
The Ascidian is an example of an animal which has lost, more or less completely, the structure typical of the phylum, and even, strictly speaking, of the class to which it belongs, at the same time acquiring marked peculiarities of its own. It is an instance of what has been termed by Professor Ray Lankester Degeneration, or 'a gradual change of structure in which the organism becomes adapted to less varied and less complex conditions of life.' Elaboration is the converse: but 'elaboration of some one organ may be a necessary accompaniment of Degeneration in all the others: in fact this is very generally the case.' It is when the total result of both processes combined leaves the organism 'in a lower condition, that is, fitted to less complex action and reaction in regard to its surroundings than was the ancestral form with which we are comparing it (either actually or in imagination), that we speak of that animal as an instance of Degeneration.'
The causes of degenerative evolution are, according to the same authority, four in number: (1) parasitism; (2) fixity or immobility; (3) vegetative nutrition; (4) excessive reduction of size. Instances are, of the first, various Copepoda, e. g. Lernaea; of the second, a barnacle (Lepas); of (1) and (2) combined, the parasitic Cirripedia or Rhizocephala; of the third, the Turbellarian worm Convoluta; and of the fourth male Rotifers, etc.
There can be no reasonable doubt that an Ascidian is one of the Chordata. The structure and development of the larva suffice to establish this point fully. The central nervous system originates as a neural groove the side of which closes over to form a neural tube; there is a cerebral or myelonic eye; the caudal notochord is derived from the archenteron; the pharynx becomes a respiratory organ pierced by slits, with walls richly vascular. These structures reach a certain degree of perfection. But with the fixation of the larva a series of regressive changes sets in. The notochord disappears with the swimming tail. The nervous system is reduced to a fraction of what it was in the larva. The eye is lost. The pharynx however becomes much more complicated and enlarged relatively to the remaining organs. The larval characteristics are however more or less retained and specialised in the free swimming order Larvacea.
Tunicata.Herdman, Challenger Reports, vi. 1882; and Bronn, Klass. und Ordn. des Thierreichs, Malacozoa, iii. i. 1862.
Notes on British Tunicata.Herdman, J. L. S. xv. 1881. Ascidies Simples des cotes de France, de Lacaze Duthiers, A. Z. Expt. iii. 1874; vi. 1877. Heller Untersuchungen, etc., Dk. Wien. Akad. xxxiv. 1875; xxxvii. 1877; and SB. Wien. Akad. lxxvii. Abth i. 1878.
Degeneration.Ray Lankester, Nature Series, 1880.
Various points.R. Hertwig, J. Z. vii. 1872.
Test.O. Hertwig, J. Z. vii. 1872. Semper, Verhandl. Phys. Med. Gesellsch. zu Wurzburg, vii. 1875.
Tunicin = Cellulose. Cf. Watt's Dictionary of Chemistry, v. p. 918, and Suppl. 2, 1875, p. 271.
Nervous System.E. van Beneden et Julin, Archives de Biol. v. 1884. Neural gland. Julin, Archives de Biol. ii. 1881. Herdman, Nature, xxviii. 1883.
Endostyle.Fol, M. J. i. 1876.
Tubular gland opening into stomach. Chandelon, Bull. Acad. Royale Belgique (2) xxxix. 1875. Ulianin, on Doliolum, Fauna and Flora des Golfes von Neapel, x. 1884.