144 THE VOYAGE OF H.M.S. CHALLENGER. 



on the main stem of tlie Ascidias Sim^^lices near to the point M. in the tabk", and 

 which consisted of one or two hermaphrodite polycarjjs attached to the mantle on each 

 .side of the body. In some of the species of Polycarpa the polycarps, originally 

 hermaphrodite, have become unisexual, and the ovaria and spermaria, as they may 

 then Ije called, have assumed different characters. 



The interesting little group of species composing the family Polystyelidse, which have 

 been variously regarded as Simple Ascidians belonging to the Cynthiidae, and as Social 

 Ascidians allied to Clavelina, have really, I believe, been derived from the Styelinas, l>ut 

 .should now be regarded as Compound Ascidians, .since they seem to reproduce by 

 gemmation so as to form colonies in which the Ascidiozooids are embedded in a common 

 test. INIost of the Polystyelidae have their branchial sacs folded longitudinally, and they 

 .■dl possess polycarps on the mantle, which are in some cases unisexual, consequently 

 there can be little doubt that they are derived from the immediate ancestors of the 

 genus Polycarpa, after the separation oi Styela (see table, p. 150). 



The new genus Chorizocormus,^ obtained during the Challenger Expedition at Eoyal 

 Sound, Kerguelen Island, shows in a most instructive way how the transition from the 

 ancestral Polycarpa to the colonial Polystyelidse was effected. The colony of Chorizo- 

 cormus- consists of a number of small rounded and irregularly-shaped masses joined 

 liy a creeping and branching stolon. In some cases each mass contains only one 

 Ascidiozooid, and then it bears the closest po.ssible resemblance to a Polycarpa, and 

 is, of course, not embedded in any colonial test ; but in other cases a number of 

 Ascidiozooids are placed together in each mass, and they are then completely buried in 

 the test, so as to form a true colony ; and all intermediate forms between these two 

 extreme conditions are also found. 



Chorizocormus, then, is j^rolialily the nearest form known to the ancestral Poly- 

 styelidae (see table, p. 150) ; while the species of Thylaciiua, PolystycJa, and Synstycla 

 form a series of gradations towards the complete colonies with a massive test found 

 in the genus Gooclsiria (see table). This genus," which is found in the Straits of 

 Magellan growing to a size of ujjwards of two feet in length, shows various intermediate 

 conditions between distinct branchial folds like those of the Styelinse, and a smooth 

 branchial sac with no folds, like that found in the Botryllidge. 



In the genera Thylacium and Pohjstyela the Ascidiozooids project above the 

 general surface of the colony, consequently these forms bear much the same 

 relation to the other Polystyelidse that the genus Diazona does to the typical 

 Distomidse. Synstyela. and Gooclsiria, the two most highly evolved forms of the 

 famil}-, have diverged in opposite directions (see table, p. 150). In Synstyela the 

 colony has become thin and incrusting, while in Gooclsiria the test is greatly enlarged, 



^ See this Report, Part II. p. 3-t.5. - See this Report, Part II., pi. xlvi. fig:. 1. 



^ Cunningham, Trans. Linn. Soc. Lond., vol. xxvii. p. 46.'), 1S71 ; and this Report, Part II. p. 327. 



