IX PHYLUM ECHINODERMA.TA 421 



of fin-like appendages, supported by slender spines, on each joint 

 of the arms. The madreporite is situated inter-radially on the 

 oral, and not on the aboral surface as in the Asteroidea. In 

 the Euryalida there are five madreporites and five madreporic 

 canals. 



In the Echinoidea the body is either globular, or heart-shaped, 

 or flattened and disc-like. The evoskeleton is in the form of a 



Fig. 337.— Astropbirton arborescens, aboral surface. (After Ludwig.)- 



rigidly articulated system of calcareous plates, fitting closely 

 together by sutures, so as to form a continuous shell or corona. 

 Asthenosoma and allies, deep-sea forms, differ from all the rest in 

 having a corona possessing a certain degree of flexibility and 

 performing movements which are brought about hj the contrac- 

 tions of five longitudinal bands of muscle running along the 

 ambulacral areas on the inner surface. 



In the globular forms, or regular Sea-urchins, the mouth is situ- 

 ated at the oral pole of the globe, the anus at the aboral, and 

 the plates of the corona are in twenty regular meridional rows, 



