27* 



THE VOYAGE OF H.M.S. CHALLENGER. 



in the former. I incline to this view since it permits us to place in close relationship all 

 the pedunculated forms, whether possessing an ordinary or a modified branchial sac, and 

 does not necessitate the independent origin, in two distinct groups, of two sets of very 

 different characters, namely, compound tentacles and a peduncle, but merely of one — the 

 modification of the branchial sac. And this modification has probably taken place after 

 the separation of the Bolteninae from the Cynthinse (both these groups have arisen from 

 the StyelinaB previously), because we do not find any traces of the modified branchial sac 

 among the Cynthinse. The following scheme shows what I imagine to have been the 

 sequence of these stages in the evolution of the Cynthiidaj : — 



Branchial sac not folded. 

 Tentacles simple. 

 No peduncle. 



B. Branchial sac folded. 



I 

 (Cynthiid^e). 



Ascidia. 

 Ciona, $c. 



(Styelin^e). 



Styela. 

 Polyearpa. 



C. Tentacles compound. 



(Cynthinj;). 



E. Branchial sac 

 modified. 



I 



Bathyoncus. 



D. Peduncle. 



I 

 (Boltexix^e). 



Cynthia. 

 Microcosmus. 



E'. Branchial sac 

 modified. 



I 

 Culeolus. 



Fungulus. 



Boltenia. 



The whole family seems to have sprung originally from a simple sessile form (marked 

 A. in the scheme), with the branchial sac not folded, and unbranched tentacles, of which the 

 present Ascidia, or more probably Ciona, is the comparatively little modified descendant. 

 From such a form the ancestral branch of the Cynthiidas would be first distinguished by a 

 longitudinal folding of the branchial sac, a condition which we find common to the three 

 sub-families. This hypothetical form (B. in the scheme) had still simple tentacles, and is 

 represented at the present day by the Styelinse. From this bne the Cynthinse and 

 Boltenime were later distinguished by the tentacles having become branched in their 

 common ancestor (C. in the scheme), and we find the comparatively slightly modified 

 descendants of this form in the Cynthia and Microcosmus of the present day. The next 

 stage in differentiation was the formation of a peduncle in the ancestral form of the Bol- 

 tenina3 (D. in the scheme), represented now by Boltenia. Hence the Bolteninae have been 



