There remains much to be elucidated with regard to the generative functions of the Asci dians. The male and female organs are always associated together, and are apparently each provided with efferent ducts. It appears, however, doubtful whether or not occasion ally the ova and the spermatozoa may not be brought into contact either in the organs them selves or in the ducts, as well as in the cloaca. We may notice that the disposition of the sexual organs in the Bryozoa, to which in so many respects the Tunicata bear reference, appears to be, from Van Beneden's observa tions, as follows. A male and a female organ are separately developed in the peri-intestinal cavity. Each is formed of cells, and in these cells are formed others, which become either vitellus or spermatozoa. When the latter cells are matured, the walls of the ex terior or mother-cells are burst, and their con tents are shed into the fluid filling the cavity around the intestine. It is here the male and female elements come into contact, and the ova are subsequently discharged by an orifice at the side of the anus.
In some compound Ascidians the peduncle like post-abdomen forms a receptacle for the ova ; and so does the pedicle of Clavel lina, as in Cirrhipeds ; but the pedicle of Boltenia and similar forms does not appear to be used for that purpose.
Muscular system. — The Ascidiadm being fixed in their adult state, have no muscles of general locomotion ; but they have numerous muscular bands by which they can effect cer tain movements of contraction and extension. The muscles do not consist, as in Bryozoa, of isolated fibres folding themselves irregularly during contraction ; but each muscle is com posed of many fibres united. They make their appearance in the young individuals contem poraneously with the appearance of the respi ratory sac. The muscular fibre of Tunicata, like that of the whole of the Aceplzala, is of the organic or unstriped type.
In the Ascidiadce, as in other Tunicates, the muscular tissue is chiefly developed in the mantle ; the muscles of the heart and in tesinal canal (if present) have not been observed. The muscles of the mantle are very thin and narrow bands, generally at considerable distances from one another, sur rounding all the body of the animal from the anterior to the posterior extremity, and uniting in part with one another on the median line of the dorsal walls (fig. 780.).
In Ascidim the mantle is more muscular on the anterior and largest part, and more membranous posteriorly. Externally there is a layer of longitudinal muscular bands, and internally of transverse bands. In sonic (A. intestinal's, &c.) the exterior layer is much the thickest, and is composed of about twelve distinct fascicles, the fibres of which are somewhat separated from one another, insensi bly disappearing on the anterior and superior borders. In others (Cynthia microcosmus, &c.),
this external layer is very slight, and formed of distinct fibres, whilst the internal fibres are stronger, transversely oblique, and interlacing. Circular bands of muscles generally surround the tube of each orifice, and are rather stronger towards its base. The fibres forming these collars of the two tubes occasionally in terlace with each other at the interval between the tubes, in a figure-of-eight pattern. Some times these muscular collars or sphincters are very indistinct, and not so apparent as another set of fascicles, which run in a diagonally transverse direction across the sac, and pass up the sides of the tubes, converging at the orifices. The latter bands diverge from the two orifices in two sets, which in part cross each other obliquely, and form an open net work.
In Chelyosonza (fig.777.a,b) each of the six triangular valvules that surround either orifice is furnished with a fan-shaped set of muscular fibres, adhering at one end to the inner sur face of the test, and at the other extremity to a small papillary process on the valvule. Be sides this set of muscles, and within them, is another set which passes laterally from one papilla to another, forming a sphincter, the base of which is hexagonal. There are other strong subcutaneous muscular fibres passing from the edge of the upper part of the tunic to that of the lower, and also from the edge of each of the coriaceous plates forming the upper surface (c, c).
By the action of these muscles, the body or inner sac of the animal is transversely con tracted and somewhat lengthened; the tubular processes also of the mantle are closed, and more or less retracted. When the animal is alarmed, the contraction takes place rapidly, causing the water contained in the respiratory and cloacal cavities to be ejected by one or other of the orifices to the height of even three feet.
There is considerable analogy between the muscular system of the Aseidiw and that of the Bryozoa. In the latter there are retractor muscles of the sheath and the intestinal tube ; and in the interior of the cells there exist also transverse oblique cords, traversing the peri intestinal cavity, and attached to one or the other surface of the skin. These transverse muscles contribute to open the tentacular circlet; and in the Aseidiee, by their contraction, produce the jet that escapes from each of the two apertures.