IIKPUKT ON Till: TUXK'ATA. 55 



and the ducts lead up alongside the rectum just as in Ascidia. The ovary shows a mass 

 of ova of different sizes, while the testis has its ducta much branched dichotomously and 

 ending in elongated vesicles united in pairs. 



In the Ascidiae Composite the position of the genital organs varies, as Mdne- 

 Edwards showed, according to the position of the heart. In Botryllus they are placed 

 alongside the branchial sac. In Didemnvm and allied forms tiny are, as in Clavelina, 

 in the abdomen on the side of the intestine ; and finally, in Amaroucium and a number 

 of other genera, the genital glands, along with the heart, constitute the long post-abdomen 

 which projects behind the loop of the intestine. 



In Pyrosoma each ascidiozooid of the colony has a rosette-shaped testis and a female 

 gland or " ovisac," containing a single ovum, and communicating with the atrial cavity 

 by a short oviduct. In Doliolum there is a long tubular testis placed ventrally and 

 opening into the atrial cavity. The ovary is small and lies at the posterior end of the 

 testis. In Salpa only the forms united in chains develop genital organs — the solitary 

 forms reproducing by gemmation. As in Pyrosoma a single ovum is formed in an 

 ovisac united to the atrial cavity by a short oviduct. The testis is a large branched 

 organ forming part of the visceral nucleus. Brooks considers that the Salpce united in 

 chains are the males produced by gemmation from the solitary form, which is the true 

 female, and which has deposited an ovum in the body of each male, where it matures, 

 becomes impregnated, and develops into a female solitary Salpa. 



In the Appendiculariidse the genital glands lie in the posterior part of the body, behind 

 the intestine, and have no efferent ducts. The testis alone is generally present in this 

 position in the adult. The ovary, which was discovered by Fol, developes later than the 

 testis. 



As this section is intended to be merely anatomical, and as the Challenger collection 

 has thrown no direct light upon the embryology of the group, it is considered unnecessary 

 to attempt here a brief account of the process of development in an Ascidian, the more so, 

 as such an excellent epitome of all the best work upon this subject has been published 

 recently in Balfour's "Comparative Embryology," vol. ii. pp. 8 to 19. 



Gemination. 



Reproduction by means of more or less complicated processes of gemmation is preva- 

 lent in all groups of the Tuuicata, with the exception of the Appendiculariidse and the 

 Ascidife Simplices. Among the latter it is found, however, in the family Clavelinidse, 

 where it results in the formation of small societies (PI. XXXVI. fig. 2), the members 

 of which may either remain connected by a common vascular system, or more rarely {e.g., 

 sometimes in Clavelina), may lose this connection and become independent. 



