172 
ARCHICELOMATA. 
Fig, 98.—A BritrLesrar (Natural size). 
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View of oral surface showing mouth and genital pores. Notice the jointed arms. 
Ciass III.—2£chinotdea (Sea-urchins).—These are spherical or oval 
in shape, and the calcareous skeleton forms a continuous mass of 
plates bearing spines. The anus opens at one pole and is sur- 
rounded by five gemztal plates which are inter-radial. One forms 
the madreporite and a genital opening is situated oneach. Between 
these there lie the smaller ocz/ars. They bear the simple eyes and 
are radials. From these ten plates there run down orally ten 
double rows of plates. Those below the oculars are called the 
radials or ambulacrals as they bear rows of tube-feet. Those 
between them are zx¢er-radza/s or antambulacrals. 
The mouth opens in the middle of a buccal membrane and is 
armed with five teeth. These are borne by a beautiful calcareous 
structure often called Avrdstotle’s lantern. 
