THE STAEFISH SOLASTEE ENDECA. 63 



or partly behind the stomach. Meantime we must suppose that in the development 

 of the yolky Solaster egg the larval alimentary canal has been reduced to the stomach, 

 and even the development of this part has been greatly post-dated, vphile at the same 

 time a sliding-back of the walls of the cavity (or cavities) giving rise to the hypogastric 

 ccelom has taken place. The peculiar foldings of the whole blastula-wall, including that 

 part which becomes invaginated in gastrulation, may provide opportunity during their 

 formation and disappearance for a considerable amount of rearrangement of the hypo- 

 blastic tissue ; but, so far, I have not been able in the ontogeny of Solaster to trace 

 adequately the steps by which the backward migration of the ccelom in question may 

 be supposed to have taken place. Probably a hint remains in the deficiency dorsally of 

 the thickened zone A B, referred to on p. 37, while, later, the oblique direction of the 

 dorsal part of the enteric slit, the greater size of the right enteric fold, and the pouch- 

 ing to the right of the enteron wall are indications that the backward migration has 

 been concerned — chiefly, at any rate — with a structure belonging to the left side. But 

 if this view is accepted, one could not refuse to admit as possible the further inference 

 that some degree of backward migration had taken place ventrally also, and that it 

 probably affected a right-sided structure. 



The posterior ccelom of Solaster and Crihrella must correspond with whatever gives 

 rise in other Starfish to the hypogastric ccelom. This, according to the best evidence 

 available (that of Macbride for Asterina, 15), is the left posterior ccelom alone, though, 

 as is well known both in Asterina {10 a) and Asterias pallida, Goto (9, 10) states that 

 a part is contributed from the right side. 



The evidence on this point to be extracted from my account of S. endeca only serves 

 to keep open the question, on which, however, further light may be expected from 

 an exact comparison of the corresponding stages in S. papposa and possibly also from 

 a fresh examination of more abundant S. endeca material prepared with a view to the 

 elucidation of this special point. 



Such other comparative notes as it seemed necessary to include in this paper have 

 been introduced in connection with the structures to which they refer; e.g., hydrocoele 

 (p. 21), nervous system (p. 46), anus (p. 23), external oral periha^mal sinus, etc. (p. 35). 



New points brought out in regard to the adult anatomy of Solaster are discussed 

 in a separate paper (8 a). 



References. 



1. Becker, S. — Die Horblasclien von Leptosynapta bergensis. Biolog. Centralblatt, xxix. p. 413. 



2. Chubb, C. C. — The Growth of the Oocyte in Antedon. Proc. Roy. Soc. Ixxvii. p. 384. 



3. CuENOT, L. — Formation des Organes Genitaux et Depetidances de la Gland Ovoide chez les 



Asterides. Compt. Rend. vol. 104, 1887, p. 88. 



4. . Etudes Morphologiques sur les Echinodermes. Arch, de Biol. vol. xi. 1891, p. 623. 



