ECHINODERMATA OF NEW ENGLAND. 61 



rows of protrusible legs. No line of demarcation be- 

 tween body and arms. 



Ophiuroidea. 



Body stellate with a central disk and peripheral arms 

 sharply marked off from each other. 



Echinoidea. 



No peripheral arms, body spherical or discoidal, spinif- 

 erous, inflexible. 



Holothurioidea . 



No peripheral arms, body columnar, flexible, tegu- 

 mentary, partially squamous or leathery. Not spinifer- 

 ous. 



ASTEROIDEA. 



Body stellate, with no separation between disk and arms. 

 Abactinal region large, flexible, with embedded calcareous 

 deposits. Spines on the abactinal region small. With suck- 

 ers and ampullae arranged in two or four rows in each 

 arm. Ambulacral plates not covered. Nervous system 

 and water system of the arms naked. Eye-spots- at ex- 

 tremity of the arms. Ovarian openings in the angle of the 

 arms. Madreporic body conspicuous on aboral surface. 

 Stomach and hepatic cceca in both arms and disk. Polian 

 and racemose vesicles. Young a free brachiolarian, or 

 viviparous. Pedicellariae sessile, bipaited. 



A. Body stellate, five or six rays. 



B. Body stellate, with numerous rays. 

 (7. Body markedly pentagonal. 



Asterias. Body star-shaped with normally five arms 

 (six? in one species). Rays with marginal spines and 

 plates, and with four rows of ambulacra! plates. Arms 

 long, inflated. 



a. Five rays (in normal specimens). 



