ON THE DEVELOPMENT OF SOME BEITISH ECHINODERMS. 17 



Krohn gives it (op. cit., p. 209). It may be mentioned that from the 

 appearance of the first processes, the posterior and postoral, it ceases 

 rotating round its longitudinal axis. The first appearance of the postero- 

 lateral processes occurred on the 7th July, viz. in the two weeks old 

 larva ; the antero-dorsal processes are the latest to appear — when the 

 metamorphosis is beginning, in ca. three weeks old larvae. 



All processes, except the preoral, have a conspicuous red point ; other- 

 wise there are scattered red pigment cells on the body and processes, more 

 numerous on the body and the posterior process, and at the frontal 

 €dge. 



VI.— Holothuria nigra. 



The development and the larval form of this species being hitherto 

 quite unknown, I was very anxious to try fertilizing and rearing it. Ripe 

 specimens were found together with such as were very far from having 

 ripe sexual products. The difficulty was with the eggs ; I never found a 

 specimen with the eggs quite ready for fertilization. The nucleus was 

 nearly always large, and while in the other Echinoderms used by me for 

 fertilization the eggs were found to ripen after having been some hours 

 in sea- water (the nucleus disappearing), this was not the case in 

 Holothuria nigra ; even after the eggs had been twenty-four hours or 

 more in the sea-water only in quite a small percentage of them had the 

 nucleus disappeared. The eggs are a beautiful red, with a radiating 

 striated membrane, as is known in other Holothurians. 



On the 24th June I tried fertilization, having found a female in which 

 more eggs than usual had no visible nucleus. (Eipe males were easy to 

 get.) The fertilization was, contrary to my expectations, successful; on 

 the next day I found that ca. 200 eggs were apparently in cleavage — 

 but the cleavage looked so irregular that I was nearly certain the develop- 

 ment was not going on normally. However, these eggs were isolated, 

 and on the 27th I found that they had developed into swimming gas- 

 trulae. On the 1st July the vibratile chord had begun to differentiate, 

 and on the 4:th July they had developed into a typical Auricularia, with 

 a star-shaped spicule at the posterior end. Beyond this stage the develop- 

 ment had not proceeded when I left Plymouth, and as I had only very 

 few larvae left, I could not get material of it preserved later on. It 

 appears from this that there will be no difficulty in rearing the larvae of 

 this species, contrary to Selenka's experiences with Holothuria tuhulosa 

 <" Zur Entwicklung der Holothurien. \H. tuhulosa und Cucumaria 



XEW SERIES, — VOL. X. NO. 1. NOVEMBEK, 1913. B 



