6 Prof. Miiller on the Anatomy and Development 



body in an oblique manner. It is continued forwards on one 

 side and back on the other, upon the processes of the calcareous 

 rods, and thereby attains a great length and complicated appear- 

 ance, but fundamentally its relations are such as have been de- 

 scribed. In some of these larvae Prof. 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 larvae 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-larvae (fig. 2) these do not exist, but four 

 little prominences, richly ciliated, are developed on the hemi- 

 spherical portion just where the long processes leave it. These 

 are the " epaulettes " of Miiller. 



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

 of cilia (fig. 1). 



2. The second form of larvae has no internal calcareous skeleton. 

 It falls into two subdivisions : (a.) the form of the Holothuriadae, 

 and (b.) the form of the Asteridae. 



a. These larvae, the Auricularia of Miiller (fig. 6 8c 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 larvae. The anus opens on the ventral sur- 

 face 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 ante- 

 rior margin of the fissure in which the mouth lies. 



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

 corresponding parts of the fringe are thrown into numerous late- 

 ral processes which give it a scolloped appearance. 



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

 kind of larvae. 



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 

 commoner 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 Plutcus from Heligoland, but not in other larvae. 



