572 r. IKED A : 



c'Mvity is not yet extonded dorsally as far as in iiii;. 58. Tluis, in 

 fig. 63 e it is represented Ijy a mesoldastie eell-niass (.s.r.r.) ^Yhicll 

 is placed just ninler the seeond tentacle («!."), and encloses no 

 lumen in itself as yet. It is evident therefore that the adnlt 

 collar cavity extends itself during devdopnient from the venti'al 

 towards the dorsal side of the body, as is also the case witli the 

 tentacles. 



As the larva readies the end of the swimming period, the 

 adult collar cavity in the adult tentacles hccomes wider and wider, 

 nearly filliug up the interior of the latter, while the larval ten- 

 tacular cavity is henceforth gradually reduced to a narrow space 

 appressed to the upper roof of the tentacle. Fig. 58 d represents 

 a median sagittal section of a larval tentacle of a larva of type D. 

 The portion belonging to the adult tentacle (.'?.^.) is characterised 

 hy a very thick ectol^lastic layer forming the ventral wall of that 

 tentacle. The adult cavity appears as a remarkably wide space 

 {».c.c^ beginning at the somatic insertion of the postoral septum 

 (wes.) and ending at the tip of the adult tentacle (-s.^.) The nar- 

 row cellular band ii^.c-c) I'esting upon the adult cavity {s.c.c.) 

 corresponds to the larval collar cavity of the tentacle. The 

 lai'val cavity is clearly seen in cross sections of the tentacle ; in 

 fig. 58 e it is visible as a small space [p.c.c.) iuclosed by the adult 

 cavity {s.c.c.) except at the median dorsal point. Tracing the 

 cavity {p.c.c.) in the above figure to the base of the tentacle, we 

 see that it communicates by a tiny opening with the larval 

 collar cavity. That portion of the tentacle, which is thrown off 

 during the metamorphosis (see fig. 58 d) is distinctly difterent from 

 the persistent portion (the adult tentacle) in that the former has 

 no trace of the adult tentacular cavity {s.c.c). I have ascertained 

 after repeated examinations, that the retrogressing larval tentacular 



