THE ECHINODEEMATA. 



43 



which soon become restricted to, and, at the same time, largely 

 developed upon, one, two, or more bands, which are disposed 

 either transversely, or more or less obliquely to the longitudinal 

 axis of the body, but which are, in any case, bilaterally sym- 

 metrical (Fig. 18). 



Fig. 18. 



Fig. 18. Diagram exhibiting the general plan of the development of the Echinoderms 

 (after Miiller). A. Common form whence the Holothurid (B, B') and Ophiurid or 

 Echinid (C, 0') larvae are derived. D, D'. Younger and more advanced stages of the 

 Asterid (Bipinnarid) larvae, a. Mouth. 6. Stomach, c. Intestine, d. Anus. 

 e. Ciliated band. e'. Second or anterior ciliated circlet. 



The parts of the body which carry the ciliated band, or 

 bands, often become developed into processes, which correspond 

 upon each side of the body, and thus render its bilateral sym- 

 metry more marked (Fig. 18, C', D'). And, in the larvaB of 

 some Echinidea and Ophiuridea, other bilaterally symmetrical 

 processes are developed from parts of the body which do not lie 

 in the course of the ciliated bands. 



The larva) of Asteridea and Holothuridea are devoid of any 



