544 INVERTEBBATA CHAP. 



consisted of plates movable on one another. This view is strengthened 

 by the complete armour of plates in which the young Cucumaria is 

 enveloped, and which gradually become separated from one another as 

 the animal grows older, owing to the more rapid growth of the inter- 

 stitial tissue ; the manner in which the intestine grows after meta- 

 morphosis is complete also resembles what occurs in the Echiuoid. 



But if Echinoidea and Holothuroidea have diverged from a 

 common stock, then the Echinoid larva has been much modified 

 since that time in its external appearance ; whilst in the Holothuroid 

 larva the internal asymmetry in the development of the coelom has 

 been pushed back to a very early period of development. 



CRINOIDEA 



The group of the Crinoidea is perhaps the most interesting- 

 division of Echinoderniata because it is the dying remnant of the 

 class Pelrnatozoa, a class which once was represented by an enormous 

 number of forms, and by such crowds of individuals that their 

 skeletons form mountains of limestone in certain localities. 



The development of only a single species, viz. Antedon rosacea, is 

 known, and this species has yolky eggs, which only develop into a free 

 swimming larva on the fifth day. This larva has a very brief free 

 life, during which the mouth does not communicate with the stomach, 

 and, consequently, no food is taken in. We cannot, therefore, 

 compare this larva directly with such larvae as the Bipinnaria, 

 Ophiopluteus, Echinopluteus, and Auricularia, but rather with the 

 larva which emerges from such yolky eggs as those of Solaster and 

 Cribrella. 



Although Antedon rosacea is confined to the Western Atlantic 

 and to the Mediterranean, other species of Antedon occur all over the 

 world, and there is no reason to think that the development of any 

 of these differs in any important respect from that of Antedon rosace a. 



Leaving aside the earlier authors, such as Carpenter (1866), who 

 described the external features of the development, we find that we 

 have only to take account of the work of Bury (1888) and of Seeliger 

 (1892), who investigated the development by means of modern 

 methods. These two workers used practically the same methods, and 

 their results happily confirm one another in all but unimportant 

 details. Perrier (1880) deals, strictly speaking, with the post-larval 

 development. As his work is earlier than that of Bury and Seeliger, 

 and as, moreover, his methods are not so perfect and his starting 

 point is later, it is quite obvious that he has not been so successful 

 in unravelling the complex relations of the organs in the just meta- 

 morphosed larva, as they have been. To Bury belongs the credit of 

 being the first to determine with accuracy the real course of Crinoid 

 development. Seeliger, who came after him, did little but confirm his 

 results. Both Bury and Seeliger obtained their material by the 

 natural fertilization of the eggs. 



