REPORT ON THE TUNICATA. 283 



allied to Coretta, while Abyssascidia is a link between Corynascidia and Ascidia. Like 



some of the other deep- water forms, — c.//., Hypobythius, Funynlns, and some of the species 

 of Cideolus, — Corynascidia has the body pedunculated, and the test is soft and fragile, 

 both in that genus and in Abyssascidia. The alimentary canal, also, in all these abyssal 

 forms is small relatively to the size of the branchial sac and of the body generally, while 

 the mantle and branchial sac are always delicate. 



The stomach and intestine vary considerably throughout the Ascidiidse in theil 

 relation to the branchial sac, but it is possible to trace the passage of one form into the 

 other. The simplest and central arrangement seems to be that which prevails in the 

 genus Ciona, where the oesophagus continues the antero-postcrior line of the branchial 

 sac, and thus throws the stomach and the first part of the intestine behind the branchial 

 sac. In Ascidia and in Paclujchlama a change has been effected, probably by the 

 branchial sac having extended down on the right side of the stomach and intestine, 

 resulting in the arrangement shown in figure 22, page 202. 



In Corella the relation is very different, and the conditions of affairs found in 

 Corynascidia and Abyssascidia are intermediate between that seen in Corella and the 

 primitive arrangement in Ciona. In Corynascidia the stomach and intestine, which 

 reach as far back as the posterior end of the branchial sac, but do not extend beyond it, 

 are situated along the dorsal edge of the sac, and, if anything, slightly on the right side 

 (PL XXV. fig. 2). This condition might be brought about in a Ciona, by making 

 the posterior end of the branchial sac extend down upon the ventral side of the stomach, 

 instead of upon the right side as in Ascidia. Then in Abyssascidia, where the stomach 

 and intestine are at the posterior end of the right side of the sac (PL XXVII. fig. 3), 

 the process commenced in Corynascidia has advanced still further, and the branchial sac 

 has extended posteriorly upon the left side, in place of the ventral edge. Finally, in 

 Corella (PL XXVI. fig. 3) we find the same relation as in Abyssascidia, but here the 

 stomach and intestine- are still more completely upon the right side of the sac, so as to 

 form a perfect contrast to the arrangement in Ascidia; and in this respect Corella 

 japonica seems more advanced than Corella pa. rail el o<j ran una, where the disposition of 

 the intestine is more like that seen in Abyssascidia. 



It is more natural, I think, to attribute these changes to a posterior prolongation of 

 the branchial sac, the stomach and intestine remaining comparatively passive, than to 

 suppose that the arrangement found in Ciona has become modified into those in Ascidia 

 and Corella, by the stomach and intestine having moved up in the one case upon the 

 left side of the branchial sac, and in the other upon the right, while in Corynascidia 

 they have occupied the dorsal edge. 



Ciona shows also the simplest arrangement in the structure of the branchial sac. In 

 most of the species of Ascidia, the complication described under the name of longitudinal 

 plication has been produced, while in Corella and some other forms the stigmata have 



(ZOOL. CHALL. EXP. — PART XVII. — 1682.) K 3( 



