550 



TM. MORTENSEN: DEVELOPMENT OF 



tion of the entoderm takes place not as a real invagination, but 

 more as a strong growth of cells at the oral end, and there is 

 no gastrula mouth. The formation of the cnterocele could not be seen 

 in the living embryos on account of their intransparency ; sections 

 are necessary for studying the whole inner transformation. 



By putting some sand in the dishes, wherein I kept these larvae, 

 and thus giving them a natural substrate, I succeeded in keeping the 

 metamorphosed young sea-urchins alive for some time; when this was 

 not done, they perished very soon, assuming the green colour so 

 characteristic of the dying tissue of Clypeastroids. 



Asteroidea. — I have succeeded in getting the complete develop- 

 ment of As ter ina pectinifera and Astropecten scopar Ut s , and the 

 younger developmental stages of Astropecten polyacanthus and 

 Astcrias calamariaP 



Asterina pectinifera. — A special interest is attached to the 

 development of this species. The only species of Asterina, the 

 development of which was hitherto known, is the common European 

 species, the almost classical Asterma gibbosa, which has large eggs 

 and non-pelagic larvae. Asterina pectinifera has small eggs, which 

 develop into typical pelagic larva;. In the younger stages the larva 

 looks very much like an Astropecten-larva. ; the preorai vibratile 

 chord is well developed also at the apical end — a character that I 

 have thought to be peculiar to those larva; which have no Brachio- 

 laria-stage. I was therefore rather surprised in seeing that this larva has 

 a Brachiolaria-stage. The paired Brachiolarian processes are small, 

 with a few papillae at the end; the median process is of the type hav- 

 ing papillae along the edges. The sucking disk is rather large and 

 distinct. For the rest the larva is small and inconspicuous, without 

 colouration. The postero-lateral processes are small and non-contractile. 



i) I am indebted to Professor S. Goto for the specific names of these 

 Asteroids. 



