TYPES OF LAEV.E. 



281 



a 



225) of different form. These processes are arranged with a strict 

 regard to bilateral symmetry, and their number, position, and size 

 essentially determine the special shape of the body. An anterior 

 and a posterior ventral region of the band of cilia can be distinguished 

 from the lateral parts which form the dorsal portions ; the latter curve 

 round and pass into the former at the anterior and posterior ends of 

 the body (fig. 225, b). The dorso-lateral parts may, however, unite 

 anteriorly with one another without passing into the anterior ventral 

 band ; in this case the anterior continuations of the latter pass 

 directly into one another so Us to form an independent praeoral ring, 

 while the dorso-lateral and posterior ventral portions of the origin- 

 ally continuous band form a longitudinally directed post-oral ring. 

 This arrangement is characteristic of the larvae of the Asteridea 

 (Bipinnaria, 

 Brachiolaria). 

 In all other 

 forms a single 

 longitudinal 

 band of cilia 

 only is pre- 

 sent. In the 

 larvae of Holo- 

 thurians, the 

 Auricularia 

 (fig. 225), the 

 processes re- 

 main short 

 and soft; they 

 are found on 

 the dorso- 

 lateral edges and on the posterior dorso-ventral arch of the band 

 of cilia ; they also appear on the posterior ventral (umbrella) and 

 the anterior ventral (oral shield) parts of the band. The processes 

 have a similar disposition in Bipinnaria, where, however, they 

 are often much longer, but are in this case also not provided 

 with calcareous rods. The Brachiolaria are distinguished from the 

 Bipinnaria by the possession of three anterior arms, which are placed 

 between the anterior portions of the two rings of cilia, and serve as a 

 fixing apparatus. The bilateral larvae of the Ophiurids and Sea- Urcldns, 

 the so-called Pluteus forms, are distinguished by their large rod- 

 shaped processes, which are supported bv a system of calcareous rods. 



FIG. 225. Auricularia larvce (after J. Hiiller). a, from the dorsnl side ; 

 b, from the ventral side. 0, mouth beneath the oral shield ; Oc, oeso- 

 phagus ; M, stomach ; D, intestine with anus (A) ; P, peritoneal sac ; 

 V, Water-vascular rosette with pore ; R, calcareous wheel-like bodies. 



