1210 



TUNICATA. 



surface of this organ : they are hollow in the 

 centre, and emit a milky fluid, which is shed 



Fig. 779. 



A, generative organs of Cynthia ampulla, situated 

 in the fold of the intestine. {After Van Beneden.) 



a, stomach and its aperture (destitute of osso- 

 phagus) ; b, rectum and anus ; c, ovary, with its 

 outlet into the cloaca; d, testicle, enveloping the 

 edges of the ovary. 



B, magnified portion of the reproductive organs of 



Ascidla grossularia. {After Van Beneden. ) 



a, testicle ; b, ovary and ova. 



into the cloaca, and contains, or rather is 

 almost composed of, spermatozoa, with dis- 

 ciform heads and filamentous tails. The ovary 

 is blackish, and is situated in the midst of the 

 testicle. Its situation corresponds to that of the 

 ovary of the Limaces among the Gasteropods, 

 which is surrounded by the liver. It is easily 

 distinguished from the testicle by its colour 

 and by the appearance of its contained eggs. 

 The oviduct opens into the cloaca by the side 

 of the anus. In Ascidla grossularia the eggs, 

 seen through the walls of the ovary, are of a 

 fine red colour, and are contained in separate 

 sacs, the ovary appearing like a bunch of grapes 

 (fig. 779. B). Van Beneden has distinctly 

 seen in all of these ova the two germinal vesi- 

 cles. The vitellus is at first white, but during 

 development it becomes of a deep red. By the 

 side of the ovary is another series of sacs 

 without ova, and some free cells containing a 

 great number of other more minute cells 

 moving about in their interior, and which 

 when shed swarm about like spermatozoa. 

 These appear to constitute the male organ, 

 and its disposition accords with that of the 

 Amaroucium argus, and with that of the 

 Bryozoa (Van Beneden). 



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 Beneclen'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 AscidiadtE 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 Acephala, is of 

 the organic or unstriped type. 



In the AscidiadcB, 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 (jig* 780.). 



Fig. 780. 



Muscular sac or mantle of Boltenia reniformis. 

 (Original.) 



In Ascidiee 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 some 



