624 THE INVERTEBRATA 



the body is prolonged as arms in the direction of the radii, and the 

 ambulacral and abambulacral surfaces are subequal in extent. On the 

 other hand, in the spherical or cushion-shaped Echinoidea and the 

 sausage-shaped Holothuroidea, the body is compact, and the am- 

 bulacral surface extends over most of it, leaving only in the Echinoidea 

 a small, and in the Holothuroidea a minute, aboral area opposite to 



I'igr- 433- Asterias ruhe?ts. A, Part of an oral view: in one of the arms shown 

 the adambulacral spines have closed over the ambulacral groove ; in the others, 

 the radial nerve can be seen. B, An aboral view of the disc, showing the 

 madreporite. C, The tip of an adambulacral spine, showing pedicellariae. 



the mouth (Fig. 434). Externally and internally the symmetry is 

 never quite perfect. At best the presence of the madreporite (see 

 below), or of the anus, or of a genital opening, differentiates one of the 

 interradii, and in some echinoids and holothurians a new and con- 

 spicuous bilateral symmetry has developed, and affects a number of 

 organs. 



