24 



ON THE EMBRYOLOGY OF ECHINODERMS. 



way through the envelope, I. When compressed, the mode of branching of the 



young tentacles is clearly seen. 



Incomplete as these observations are, they are of considerable value in a zoological 

 point of view. It is evident that those Holothurians which want the ambulacral 

 tentacles,* and have only a limited number of tentacles round the mouth, such 

 as Caudina, Synapta, Fistularia (the Apodes), in which there are but very few 

 deposits of limestone particles, stand lower than those Holothurians in which, as 

 in Cuvieria and Psolus, some of the ambulacra are provided with suckers, forming 

 a kind of sole, and in which the limestone particles perform an important part in 

 covering the body. We should therefore place higher still those Holothurians in 

 which, as in the preceding group, the tentacles round the mouth have a highly 

 ramified character, and which have besides ambulacral suckers equally developed on 

 the different ambulacra. It is evident from this that several of the forms observed 

 by Forbes,f such as Psolinus, etc., will prove to be only the young of Pentacta, 

 since he has distinguished genera principally from the degree of development of 

 the gills and of the ambulacral system. 



General Plan of Development. 



The figures of Miiller show without doubt that in Holothurians as well as Echi- 

 noids the young Echinoderm is in its earliest stages an open spiral star. An exam- 

 ination of the figures of Plates III. figs. 6-9, V. figs. 5-8, and Plate VI. first Memoir, 

 and of Plates VI. fig. 14, VII. figs. 4, 6, 7, 9, and IX. figs. 3, 4, fourth Memoir, will 

 satisfy any one that his figures cannot be made to agree with one another on any other 

 supposition than that of an open pentagonal spiral surface which develops the actinal 

 system of Echinoids on the surface of one water-tube, and of a similar spiral surface on 

 the other water-tube which develops the abactinal area. In the same way Miiller's 

 figures of Auricularia, particularly those of Plate I. fourth Memoir, and Plate III. 

 sixth Memoir, show that the young Holothurian also commences by having an open 

 spiral actinal tentacular system. The figures of young Auricularians which are given 

 on Plate I. fourth Memoir also show without doubt that the tube which leads to b, in 

 fig. 36 of this Memoir, is the tube leading from the water-tubes to the dorsal pore, and 

 that b is really the madreporic body. Muller also considers it as such in his fi^ 

 and it is certainly very remarkable that, with the admirable figures he has eWen of 



gliica, 



the young Ophiuran and Auricularian, he should not have noticed the intim 



of the 



water-tubes and of the young Echinoderms. It is natural that the 



* L. Agassiz. Methods of Study. 1863. t Forbes, E. A History of the British Starfishes. 



