ECHINODERMA TA 



241 



between the most dorsal ambulacral and the radial plates that 

 new ambulacral pieces are intercalated. One of the ambulacra 



Fig. 142. — A portion of the shell of 

 Echinus graaUis, Alter Agassiz. 



It. Ambulacral plates. 



b. Poriferous zone. 



c. Interambulacral plates. 



is regarded as anterior, and an interradius is posterior ; in those 

 forms in which the anus is not central, it lies in this posterior 

 interradius. Adopting this orientation, the madreporic pores 

 are usually found on the right anterior genital plate. 



Both the ambulacral or radial and the interambulacral or 

 interradial areas are composed of a double row of pentagonal 

 plates, firmly united with all the contiguous plates. Each of 

 the ambulacral plates is formed by the fusion of several small 

 plates, the pore-plates ; these latter are pierced by two holes. 



Pig. 143. — Spine of an Echinid. After Leuckart. 



1. Spine. 



2. Basal knob. 



3. Circular muscle of spine. 



4. Ligament. 



•:;.:2 ■/••■• 



through which two processes from the water-vascular system 

 pass and fuse to form one tube-foot. Both the radial and 

 interradial plates bear calcareous knobs, upon which long 

 spines are articulated; these are moved by certain muscles 

 attached to their base, and form important locomotor organs. 

 Pedicellariae, with usually three jaws, are also present. Some 



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