574 



COMPARISON OF ECHINODERM LARV^ii. 



forms. We are therefore justified in concluding that in the 

 viviparous forms the development is abbreviated and modified. 



All the free forms are nearly alike in their earliest stage after 

 the formation of the archenteron. The surface between the 

 anus and the future mouth becomes flattened, and (except in 

 Antedon, Cucumaria, Psolinus, etc. which practically have an 

 abbreviated development like that of the viviparous forms) a 

 ridge of cilia becomes established in front of the mouth, and a 

 second ridge between the mouth and the anus. This larval 

 form, which is shewn in fig. 264 A, is the type from which the 

 various forms of Echinoderm larvae start. 



In all cases, except in Bipinnaria, the two ciliated ridges 

 soon become united, and constitute a single longitudinal post- 

 oral ciliated ring. 



The larvae in their further growth undergo various changes, 

 and in the later stages they may be divided into two groups : 



(1) The Pluteus larva of Echinoids and Ophiuroids. 



(2) The Auricularia (Holothuroids) and Bipinnaria (Aster- 

 oids) type. 



The first group is characterized by the growth of a number 

 of arms more or less surrounding the mouth, and supported 

 by calcareous rods. The ciliated band retains its primitive 

 condition as a simple longitudinal band throughout larval life. 

 There is a very small prae-oral lobe, while an anal lobe is very 

 largely developed. 



The Auricularia and Bi- A. B 



pinnaria resemble each other 

 in shape, in the development 

 of a large prae-oral lobe, and 

 in the absence of provisional 

 calcareous rods ; but differ in 

 the fact that the ciliated band 

 is single in Auricularia (fig 

 271 A), and is double in Bi- 

 pinnaria (fig. 271 B). 



The Bipinnaria larva shews 

 a great tendency to develop 

 soft arms; while in the Auri- 

 cularia the longitudinal ciliat- 



sf. 



FIG. 271. A. THE LARVA OF A HOLO- 

 THUROID. B. THE LARVA OF AN ASTE- 

 RIAS. 



m. mouth; st. stomach; a. anus; I.e. 

 primitive longitudinal ciliated band; pr.c. 

 pne-oral ciliated band. 



