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ARCHICCELOMA TA. 



Fig. 98. — A Brittlestar (Natural size). 



View of oral surface showing mouth and genital pores. Notice the jointed arms. 



Class III. — EchUioidea (Sea-urchins). — Tliese 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 genital plates which are inter-radial. One forms 

 the madreporite and a genital opening is situated on each. Between 

 these there lie the smaller ocular's. 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 inter-radials 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 Aristotle's lantern. 



