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 amlmlacral 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 ampullae, 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. As trophy ton arborescens, dorsal view. (After Ludwig.) 



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

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

 vcntrcd 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, 

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

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

 articulated system of calcareous plates, fitting in closely together 



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