BY TH. MOItTENSKN. 205 



far as I have been able to ascertain on the quite intransparent 

 living embryo, there is no trace of a larval skeleton. 



'I'here is a general ciliation of the whole body of the embryo, 

 but no special ciliated bands. A ring of red pigment is formed 

 round the mouth, and gradually more or less of a reddish pigment 

 develops all over the body. Some specimens, however, have 

 scarcely any pigment at all; such probably give rise to the very 

 light-coloured specimens, which are often found among the other- 

 wise generally dark-coloured specimens of this species. 



The young sea-urchin develops on one side of the embryo, just 

 above the mouth, and here the primary tube-feet are soon seen 

 to protrude. The aboral part of the body, which serves as a 

 reservoir of nourishment for the embryo, gradually shortens, and 

 ultimately becomes completely overgrown by the young urchin, 

 and enclosed within its body. The first spines to appear are of 

 the trifid, embryonal type; but soon the spines of the usual, 

 pointed shape, make their appearance. 



When the first rudiments of the sea-urchin begin to appear, 

 the embryo generally sinks to the bottom, remaining there in 

 the usual position, mouth downwards; many specimens, however, 

 remain swimming until the metamorphosis is nearly completed; 

 especially a number of specimens remained at the surface, at the 

 contact-line between the water and the glass. The whole meta- 

 morphosis is completed in the course of 4-5 days. 



The details of the formation of the enteroccel and all the 

 internal transfoi-mation-processes, upon the whole, can only be 

 studied by means of sections. That must be postponed till 

 after my return to Copenhagen. Likewise, the development of 

 the skeleton of the young urchin will have to be studied carefully 

 later on, the conditions, under which I had to carry out my 

 experiments, being far too unfavourable for such minute and 

 partly very difficult research. 



This is the most reduced development hitherto known in any 

 Echinoid with free-swimming embryos. The development of 

 Laganum decagonale^ described in my paper "On the develop 

 ment of some Japanese Echinoderms" (Annot. Zool. Japonenses, 

 Vol. viii., 1914), is not nearly so reduced, the typical larval shape 



