54 8 



TH. MORTENSEN : DEVELOPMENT OF 



It is interesting to note that it has quite typically pelagic larva of the 

 common Clypeastroid shape. As it was only during the last days 

 of my stay in Misaki that I got the ripe material of this species, I 

 naturally have succeeded only in getting the first stages of tho larva. 



I should take the occasion here to mention that, while in general I 

 think that deep-sea Echinoderms have no pelagic larva; but undergo 

 more or less direct development, there exist — perhaps only among 

 the archibcnthal forms, and not among those from very deep waters — 

 some that have pelagic larva:. Thus, I venture to maintain from 

 the small size of the eggs, that a small species of Orechinus (?) 

 which I have got from the Sagami Sea, will, like the Laganum men- 

 tioned above, prove to have pelagic larva. The same I should think 

 will hold good also for Suicida pacifica, Hemipedina a.o. Unfortunately 

 I could not get sufficient material of these forms for studying their 

 development. 



Lagaimm dccagonale proved to be especially interesting as re- 

 gards its development. The eggs are comparatively large, measuring 

 nearly 0,5 mm. in diameter, and are yolky, in accordance with which fact 

 the development is shortened, the whole metamorphosis being accom- 

 plished in the course of only 3-4 days. The larva are pelagic, but very 

 much reduced in organization. Generally they have only two processes, 

 the postoral ones; more rarely a pair of postero-do sal processes are 

 found. For the rest they are very variable as to their shape. The 

 two postoral processes may have coalesced, so that they appear like one 

 single process, and instead of the two postero-dorsal processes there 

 may be only one. In fact, we may find the larva having one, two, 

 three or four processes, or even none at all ; but the metamorphosis 

 goes on just the same, whatever may be the number of processes 

 developed. (Fig. 2). There is no vibratile chord, but a general ciliation 

 of the whole body. The larval intestine is rudimentary ; the mouth 

 opening is very small and there appears to be no anal opening. 

 Evidently the larva does not take any food, but subsists on the yolk 



