49 



Fig. 13. Egg in the 

 4-cell stage of Str. 

 franciscanus; show- 

 ing double mem- 

 brane. 



adjoining sketch Fig. 13 (which was not drawn with the 

 camera). I may state that I found the same pecuUar 

 membrane-formation in Str. drobachiensis (by a fertil- 

 ization made at the same time, which, however, did 

 not result in any rearing of the larvse, the breeding 

 season of that species being then already past, only 

 few eggs remaining) ; 1 regret not having noticed whether 

 the same phenomenon occurs in Str. pulcherrimus. 



The gastrula stage was reached after about 24 hours, 

 and in the course of the next day the embryos had 

 assumed the shape of young Plutei. The postero-dorsal arms began to 

 appear at the age of 8 days. Beyond this stage 1 did not succeed in 

 rearing the larvse, the culture dying off when the larvse were 12 days 

 old. A new fertilization was tried in the later part of June, but then 



the breeding season was past and only few 

 eggs could be found, which did not develop 

 normally. However, I can give some informa- 

 tion also of the fully formed larva of Str. 

 franciscanus, having found a specimen in the 

 plankton. As no other species of Echinoids 

 occur there, besides Dendraster excentricus — 

 the larva of which 1 have also reared, and 

 which has no resemblance to the present larva 

 — and a Spatangoid, Brisaster latifrons (A. 

 Ag.) — which was not ripe and which has, 

 moreover, probably not pelagic larva — there 

 cannot be any doubt that this larva really 

 belongs to Str. franciscanus. 



The larva of S. franciscanus in its first stage 

 differs from that of drobachiensis and pulcher- 

 rimus in the posterior end of its body being 

 distinctly less elongated and more rounded 

 (PI. IX, Figs. 1 — 3). The arms are not so broad 

 as in the said species. As in these species the 

 body skeleton does not form a basket struc- 

 ture. The body rod is clubshaped, rather 

 strongly thorny at the thickened end, which 

 curves somewhat inwards; it is distinctly 

 shorter than in the two other species. Further- 

 more this larva differs from the two other 

 larvse known of this genus in having a well 



7 



Fig. 14. Skeleton of larva of 



Strongylocentr. franciscanus, 1st 



stage. Side view. '"%. 



Letters as in fig. 5. 



