Generally speaking, in proportion as these marginal tubes advance into the common tegumentary tissue around them, they divide into several branches, and the extremity of each of them becomes inflated and claviform; soon there appears, towards the summit of each terminal swelling, a small granular mass wherein the forms of an Aseidian gradually develope themselves, and which in time becomes a new animal, resembling those already existing in the common mass, of which it is destined itself to become a new inhabitant. Ultimately the communication between the parent and the young individual becomes obliterated; but still the newly-formed animals, thus derived from the same parent, remain for some time united by their pedicle; and, apparently, to this circumstance their mode of arranging themselves in groups is due.

(1276). The ovary is a whitish glandular mass, imbedded with the liver among the folds of the intestine; its position in fig. 243 is indicated by the letter m; and at o, fig. 244, it is seen separated from the surrounding structures. The oviduct, which is occasionally very tortuous, accompanies the rectum, and terminates near the anal aperture (fig. 243, m, fig. 244,p); so that the ova ultimately escape through the common excretory orifice.

(1277). Since the publication of the former editions of this work, important additions have been made to our knowledge relative to the generative system of the class under consideration, and a distinct male apparatus, the existence of which was formerly denied, has been satisfactorily described by many skilful observers. The arrangement of these organs, as they exist in Cynthia ampulla, dissected by Van Beneden, is shown in the accompanying figure (fig. 246, a, b.) In this species the sexual parts appear at first sight to form but a single organ, imbedded in a fold of the alimentary canal (a, b); but by the assistance of a microscope, this is readily seen to consist of two portions - one male, and the other female. The testicle (c) surrounds the base of the ovary, and is distinguishable by its milky-white colour; its substance is entirely made up of innumerable short convoluted caeca, visible to the naked eye, and resembling the seminiferous tubes of many of the higher animals. Three or four glandular prolongations (f) arise from the surface of this organ, which are hollow internally, and contain a milky fluid which is poured into the cloaca, and which the microscope reveals to be almost entirely composed of spermatozoa with disciform heads and filamentary tails. The ovary is of a dark colour, and is imbedded, as it were, in the testes; its oviduct (e) opens into the cloaca by the side of the anus.

(1278). In Ascidia grossularia, the eggs, as seen through the walls of the ovary, are of a fine red colour, and are contained in separate sacs; so that the ovary when distended resembles a bunch of grapes. By the side of the ovary is another series of sacculi (fig. 246, b, b), the contents of which abound with spermatozoa, intimating their identity with the male apparatus above described.

Generative system of Cynthia ampulla.

Fig. 246. Generative system of Cynthia ampulla.

(1279). Deprived as these animals are of any of the higher organs of sense, and almost cut off from all relation with the external world, we can look for no very great development of the nervous centres. There is one ganglion, however, lodged in the substance of the mantle, distinctly recognizable, situate in the space between the branchial and excretory openings, from which large nerves are given off; but of other details connected with the nervous system of the Tunicata little has been made out.

(1280). In the seas of tropical latitudes, many forms of Tunicated Mollusca are met with abundantly which, although allied to Ascidians in the main points of their economy, present certain peculiarities of structure that require brief notice in this place. These, grouped by authors under the general name of Salpae, are many of them so transparent that their presence in a quantity of sea-water is not easily detected; and their viscera, if coloured, are readily distinguishable through their translucent integument, which in texture seems to be intermediate between cartilage and jelly. The body is oblong, and open at both extremities, the posterior opening being very wide, and furnished with a crescentic valve, so disposed that water is freely drawn into the interior through this aperture, but cannot again be expelled by the same channel; so that, being forced by the contractions of the body in powerful gushes from the opposite end, it not only supplies the material for respiration, but impels the delicate animal through the water in a backward direction.

The branchial chamber of Ascidia is consequently in this case represented by a wide membranous canal, which traverses the body from end to end; but, instead of the network of vessels lining the respiratory sac of Ascidians, a singular kind of branchial organ is placed within it. This consists of a long vascular riband attached by both its extremities to the walls of the canal, through which the water rushes; and of course, being freely exposed to the influence of the surrounding medium, the blood contained in this curious branchial apparatus is perpetually renovated, and afterwards distributed, by a heart resembling that met with in the genus last described, to all parts of the body.

(1281). The viscera, which occupy comparatively a very small space, are lodged in a distinct compartment between the membranous respiratory channel and the external gelatinous investment, or soft shell, as we might properly term it. The mouth is a simple aperture, situated near the upper extremity of the branchial organ; and probably, as in Ascidia, ciliary currents rushing over the respiratory surface bring into it a sufficient supply of nutritive molecules. The stomach is capacious, and covered with parallel rows of large white filaments, that seemingly represent the liver; and , the alimentary canal, which is perfectly simple, runs to the posterior extremity of the animal, and terminates there by a wide opening*. Two oblong bodies, each consisting of a granular substance, are seen upon the ventral surface of the body, lodged between the external and internal membranes; these, no doubt, are the ovaria, and form a reproductive system as devoid of complication as that of the sessile Ascidians.