ECHINODERMATA 241 
between the most dorsal ambulacral and the radial plates that 
new ambulacral pieces are intercalated. One of the ambulacra 
Fic. 142.—A portion of the shell of 
Echinus gracilis. After Agassiz. 
a. 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, 
Fic, 143.—Spine of an Echinid. After Leuckart. 
1. Spine. 
2. Basal knob. 
3. Circular muscle of spine. 
rs 
. Ligament. 
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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