70 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



Aniedon. This may then be regarded as holding good for the Comatulid 

 larvse as a whole. The fact is very noticeable that the larva of Notocrinus 

 has the nervous system exceptionally well developed, in spite of its having 

 no vibratile bands and that it remains inclosed in a marsupium, giving it 

 no space to move. It is hard to see why this larva should have the nervous 

 system so well developed. 



9. THE SHAPE OF THE LARVA; VIBRATILE BANDS; VESTIBULUM. 



All the Crinoid larvae thus far kno^^^l have the same shape as the Antedon 

 larva, being simple and barrel-shaped, and provided with a number of ciliated 

 bands. Thus this larva corresponds closely to the Dipleurula, the hypothetic 

 bilateral ancestor of Echinoderms. The great question, then, is whether 

 this larval shape is a primary feature, the larva thus representing the original 

 type of Echinoderm larvse, or whether it is a secondarily acquired form. 

 The interesting observations of Casw^ell Grave,^^ that a similar arrangement 

 of the ciliated band in rings may occur in both Ophiurids and Echinoids in 

 the metamorphosis stage, a sort of pupa-stage, comparable with that known 

 in the Holothurians already from Joh. Miiller's researches, have an important 

 bearing on this question. Such barrel-shaped larvse, provided with ciliated 

 rings only, occur likewise in Cucumaria (and very probably in Dendrochirote 

 Holothurians upon the whole) and in some Ophiurids (besides Ophioderma 

 brevispinum in Ophionercis^^ squamulosa Koehler, and evidently also in the 

 "wurmformige Astcrien Larve" of Joh. MLiller).^'' These facts, combined, 

 would seem to point in the direction of this simple larval type being the primary, 

 and that the other types of Echinoderm larvae {Auricularia, Bipinnaria, 

 Ophiopluteus, and Echinopluteus) are secondary adaptations to the pelagic 

 life, as Caswell Grave maintains. This will not mean, of course, that we can 

 not expect to find true pelagic larvae among Crinoids, corresponding to the 

 other types of pelagic Echinoderm larvae. As I have said above (page 3), 

 I expect that such will be found, especially among the stalked Crinoids." 



The number of ciliated bands is either 4 or 5. In the development of the 

 anterior one of these bands there is some variation. In Antedon adriatica 

 and A. mediterranea there are 5 distinct bands (Bury, Seeliger), while in 

 Antedon bifida there are only 4, according to Wyville Thomson. Whether 

 the latter statement is correct remains to be ascertained, but it is certain 

 that in Tropiometra there are only 4 bands, the anterior one lacking com- 

 pletely. In Compsometra the anterior band is fairly distinct on the dorsal 



'* Caswell Grave. On the oeciirrenee among Echinoderms of larvte with cilia arranged in transverse 

 rings, with a suggestion as to their significance. Biol. Bulletin, 1903, p. 169. 



" Th. Mortensen. On the development of some West Indian Echinoderms. Year Book No. 15 of the 

 Carnegie Institution of Washington, p. 193. 1916. 



'' Joh. Miiller. Abhandlungen iiber die Larven und die Metamorphose der Echinodermen. Ill, p. 26; 

 IV, p. 40; VI, taf. i, fig. 15-16 (Abh. d. Akad. d. Wiss. Berlin a. d. Jahren 1848-18.53). 



" MacBride (Text-book of Embryology, I, 1914, p. 560) also e.xpects that true pelagic larvae will be 

 found in such Crinoids as have small yolkless eggs. 



