

PHYLUM ECHINODERMATA 



387 



ossicles, usually beset along their edges with longer or shorter- 

 spines; sometimes irregular calcareous granules take the place 

 of plates. Hook-like organs of adhesion are present only in the 

 Euryalida. Each of the arms is supported by a row of internally 

 situated ambulacral ossicles. Tube-feet are present and are pro- 

 truded at the sides of the arms between the lateral plate-like 

 ossicles; but they have no sucking-discs and no ampulla?, and 

 locomotion is effected in the majority of the Ophiuroids by active 

 flexions and extensions of the arms. In one genus there is a pair 



FIG. 307. Astrophy ton arborescens, dorsal view. (After Ludwig.) 



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

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

 ventral surface, and not on the dorsal 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, v 

 or flattened and disc-like ; dorsal and ventral surfaces are always 

 distinctly recognisable. The exoskeleton is in the form of a rigidly 

 articulated system of calcareous plates, fitting in closely together 



c c 2 



