CLASS ECHINOIDEA 
SEA-URCHINS, CAKE-URCHINS, HEART-URCHINS 
ORDER DESMOSTICHA 
SEA-URCHINS 
HE shell of a sea-urchin consists of many calcareous plates, 
or ossicles, fitted closely together and forming a contin- 
uous exoskeleton. The plates are so united that externally the 
marks of juncture are not perceptible, but on the interior the 
shape of these plates is well defined. In examining such a shell, 
or test (they abound on the beach), it will be seen that its surface 
is covered with numerous hemispherical projections or knobs, 
which are grouped in double rows and run in meridional lines 
from one pole to the other of the more or less spherical body, 
separating it into ten divisions. Five of these divisions have 
perforations, or small pores in the plates of the shell, and are 
called the ambulacral zones or areas, because through these pores 
pass the small tubes, in the living animal, which connect the 
tube-feet, or ambulacra, with the radial water-canals and the am- 
pulle (see page 206). The wide spaces between these double 
rows of pores are called the interambulacral zones or areas., The 
ten spaces diverge from the peristome, or soft part around the 
mouth, in the center of the lower surface, and converge in 
the small area at the top or aboral side. In the center of this 
small circular dorsal space is the excretory opening, and sur- 
rounding it are ten plates, five of which have openings into the 
egg-sacs. One of them is larger than the others, and is modified 
to form the madreporic plate. The other five plates have eye- 
specks. The ambulacral zones terminate at these ocular plates. 
The numerous spines which cover the animal are of three 
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