548 THE INVERTEBRATA 



epineural canal over the nerve cord. In the Asteroidea, Ophiuroidea 

 and Crinoidea 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 com- 

 pact, and the ambulacral surface extends over most of it, leaving only 



Fig. 408. Asterias riihens. 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. 



in the Echinoidea a small, and in the Holothuroidea a minute, aboral 

 area opposite to the mouth (Fig. 409). Externally and internally the 

 symmetry is never quite perfect. At best the presence of the madre- 

 porite (see below), or of the anus, or of a genital opening, differenti- 

 ates one of the interradii, and in some echinoids and holothurians a 

 new and conspicuous bilateral symmetry has developed, and affects 

 a number of organs. 



