I08 ANATOMY AND DEVELOPMENT OF THE ECHINODERMS 



upon the processes of the calcareous rods, and thereby attains a great 

 length and complicated appearance, but fundamentally its relations 

 are such as have been described. In some of these larvae Professor 

 Miiller considers that he has detected, in front of and above the 

 mouth, a rudimentary nervous system, consisting of two little ganglia 

 connected by a commissure, whence branches proceed.^ 



We have described the structure common to all the larvse of this 

 division ; there are certain peculiarities in some, however, which are 

 deserving of notice. Thus in some Echinus-larvae three long processes 

 containing calcareous rods are developed from the convex posterior 

 extremity of the larva (fig. 3). 



In other Echinus-larvje (fig. 2) these do not exist, but four little 

 prominences, richly ciliated, are developed on the hemispherical 

 portion just where the long processes leave it. These are the 

 " epaulettes " of Miiller. 



In Ophiurid-larvjE the convex side of the larva bears a circlet of 

 cilia (fig. i). 



2. The second form of larva; has no internal calcareous skeleton. 

 It falls into two subdivisions : (a) the form of the Holothuriada;, and 

 (1^) the form of the Asterida;. 



a. These larvae, the Auricularia of Miiller (fig. 6 and 7), are at first 

 bean-shaped, convex on the dorsal side, concave on the ventral side. 

 An irregular transverse fissure answers to the hilum of the bean, and 

 in this the mouth is placed. The margins of the fissure are edged by 

 a ciliated fringe exactly similar to that of the former kind of larvx. 

 The anus opens on the ventral surface of the larva, behind the fringe, 

 the posterior portion of which runs between it and the mouth. The 

 fringe forms a continuous circle, the anterior part of which is bent 

 back to form the anterior margin of the fissure in which the mouth lies. 



In the course of its growth the margins of the larva and the cor- 

 responding parts of the fringe are thrown into numerous lateral 

 processes which give it a scolloped appearance. 



The disposition of the intestine, stomach, &c. is as in the first kind 

 of larvc'E. 



As the larva increases in size and becomes more elongated in form, 

 the primary fringe becomes replaced by a number of ciliated rings 

 which encircle the now cylindrical body of the larva (fig. 7). 



b. The Asterid-larvce. — The Bipinnaria (fig. 4), which is the com- 

 moner form of Asterid-larva, closely resembles Auricularia in its 

 young condition, except that there is a distinct ciliated circle 

 ■developed upon the surface of the larva in front of the mouth. 



' In the Pliiteus from Heligoland, but not in other larvte. 



