1 iO WYVILLE THOMSON, ON SYNAPTA INHiBRENS. 



in ttie body cavity. I have said that the growth of these 

 animals was extremely slow. It was not till abont three 

 months after they were first observed that the number of 

 tentacles increased. One could easily imagine a second series 

 of five, raising the number to ten, but I was curious to know 

 how twelve could be produced. The two odd tentacles came 

 first. The shafts of two of the spicules of the calcareous ring, 

 almost at opposite sides of the ring, began to look spongy, 

 and at length divided into two ; and opposite each point where 

 the division occurred, a csecal process, easily recognised by 

 its small size, passed up to the angle between two tentacles ; 

 the sarcode gave Avay and projected before it, and in three 

 or four days the two new tentacles were scarcely distinguish- 

 able from the primitive five. 



One or two general observations are suggested by the dif- 

 ferent steps in this process of development. 



As explained above, the first stages in the development of 

 the embryo from the egg w^ere unfortunately lost ; still, the 

 water in the tank had been, up to a few days before the 

 appearance of the young, most carefully watched for a swim- 

 ming stage. The young, when first observed, were all 

 crowded together at one spot, as if they had just emerged 

 from a batch of eggs. They were very small, and showed no 

 traces of the provisional alimentary system of anAuricularia, or 

 of the ciliated bands of a " pupa.""^ They presented, in fact, all 

 the appearance of a very early stage, in the differentiation of 

 the organs of the perfect form from an oval sarcode embryo. 



The great mass of the later observations on Echinoderm 

 development seem to tend to the conclusion that the peculiar 

 mode of development with the intervention of a highly 

 organised " pseudembryo,^' first detected by Sars in the Stel- 

 leridse, and afterwards established for all the recent orders by 

 the wonderful researches of Johannes Miiller, is confined to 

 comparatively exceptional cases, whose frequency has pro- 

 bably been exaggerated by Professor Miiller's mode of ob- 

 servation. In almost every instance in which the develop- 

 ment of an Echinoderm has been traced from the e^g, it has 

 been found to present no such extreme anomaly, although 

 it has usually shown a certain deviation from the ordinary 

 mode in the same direction, in the formation of deciduous 

 lobes or processes, provisional appendages, apparently, of the 

 ambulacral vascular system. The observations of Derbes on 

 Echinus, form however, a marked exception to this rule. 

 Information on the development of the Holothuridse is very 

 meagre. I shall give a short abstract of the few careful 

 observations as yet on record. 



