CLASS ECHINOIDEA 



SEA-URCHINS, CAKE-URCHINS, HEART-URCHINS 



ORDER DESMOSTICHA 



SEA-URCHINS 



THE 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 ambulacra} 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- 

 pullce (see page 206). The wide spaces between these double 

 rows of pores are called the interambnlacral 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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