402 ZOOLOGY SECT. 



of grooves or areas radiating out from the mouth, and containing 

 a double series of tubular appendages of the ambulacral system, 

 the tube-feet, usually employed in locomotion, and in the majority 

 of cases provided with terminal suckers : the anus usually aboral ; 

 the mouth on the surface that is habitually directed downwards, or 

 at the end habitually directed forwards in locomotion. 



CLASS I. ASTEROIDEA. 



Free Echinoderms with star-shaped or pentagonal body, in 

 which a central disc and usually five arms are more or less readily 

 distinguishable, the arms being hollow, and each containing a 

 prolongation of the coelome and of its contained organs. There 

 are distinct oral and aboral surfaces, on the former of which the 

 anus and the madreporite are situated, and on the latter the 

 mouth and five narrow ambulacral grooves lodging the tube-feet. 

 The larva has the form either of a bipinnaria or of a brachiolaria. 

 This class includes the Starfishes. 



ORDER 1. SPINULOSA. 



Asteroidea with only two rows of tube-feet in the ambulacral 

 grooves ; the dorsal surface covered with small close-set plates 

 bearing spines. 



ORDER 2. VELATA. 



Asteroidea having the dorsal surface covered with a single mem- 

 brane or velum supported by sheaves of spines, or with a number of 

 such structures. 



ORDER 3. PAXILLOSA. 



Asteroidea in which the dorsal surface is covered with paxilla}. 

 The pedicellarise are never forcipulate. 



ORDER 4. VALVATA. 



Asteroidea with granular surface without prominent spines ; 

 valvate pedicellarise. 



ORDER 5. FORCIPULATA. 

 Asteroidea with forcipulate pedicellariae. 



CLASS II.-OPHIUROIDEA. 



Star-shaped free Echinoderms, with a central disc and usually 

 five arms, which are more sharply marked of? from the disc than in 

 the Asteroidea and which contain no spacious prolongations of the 

 coslome. There are distinct oral and aboral surfaces. The anus 

 is absent ; the mouth, as well as the madreporite, on the oral 



