330 Transactions. — Zoology. 



origin of the group, its few calcareous spicules and simple re- 

 productive organs being evidence of its generalised character. 

 From it on the one hand have arisen the species forming the 

 genus Didcmnum, in which the number of spermatic vesicles 

 is reduced to one, around which the vas deferens coils spirally, 

 and having often very numerous spicules in the test. On the 

 other hand, the genus Polysyncraton has sprung from a Di- 

 2)losoma having several spermatic vesicles and an uncoiled vas 

 deferens which has become spirally twisted. 



Or, perhaps better still, we may assume a common ancestor 

 for the three genera, in which no spicules were present, and 

 the reproductive organs were numerous, and arranged as in the 

 Distomidae. Gradually the power of forming calcareous bodies 

 in the test was acquired as in Cystodytes among the Disto- 

 midffi, and the reproductive organs became variously special- 

 ised as we find them to-day. 



Lastly, I would briefly enumerate a few points on which I 

 think my researches shed new light. 



The alimentary tract among the Leptoclinids is here more 

 fully dealt with than by Herdman, and its variations are of 

 undoubted specific import. Further, in L. densum a peculiar 

 arrangement of the branchial pore is noted. The siphon is 

 clearly six-lobed, but the pore itself three-lobed (cf. my figures 

 and references with those of Herdman). A distinction must 

 therefore be drawn between a plain branchial siphon {i.e., one 

 whose rim is plain) and a plain branchial pore. I have endea- 

 voured in my descriptions to make this distinction, which is 

 one of some value. 



The question of the protogyny or protandry of the Ascidiae 

 Compositas is an iiiteresting one, about which opinions differ. 

 Herdman contends for protogyny — the young zooids pro- 

 ducing ova, and the older spermatozoa. But is this necessary 

 to secure cross-fertilisation ? The same result would evidently 

 be attained if, each season, ova were produced first, and then 

 spermatozoa when the ovaries were exhausted. In the ab- 

 sence of much definite information I can only speak with 

 diffidence upon this point, but I certainly saw no ova in any 

 young zooids, though sperm-bodies were frequently well 

 developed, and in many cases ova were found among the 

 viscera along with fully- developed spermatic vesicles. 



The ova probably are fertilised either in the peribranchial 

 cavity of the parent or in the cloacal canals. No oviducts 

 were seen in any zooid, so that they must escape directly into 

 the mantle-cavity of the thorax from the mantle-cavity of the 

 abdomen, unless, indeed, they become fertilised in the abdo- 

 men, and pass out below through the mantle : in the Di- 

 demnids examined this seems possible, as no ova are found in 



