THE ECHINODEKMATA. 



43 



ment of which has been examined, the impregnated egg gives 

 rise to a free-swimming, ovoid, ciliated embryo, the cilia of 

 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. 17). 



Ficr. 17. 



Fig. 17. — Diagram exhibiting the general plan of the development of the Echinoderms 

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

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

 Asterid (Bipinnarici) larvae, a'. Mouth, b. Stomach, c, Intestine. cL Anus. 

 e. Ciliated band. <?'. 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. 17, C, J)'). And, in the larvse of some 

 Echinidea and Ojihiuridea, other bilaterally symmetrical processes 

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

 course of the ciliated bands. 



