STAR-FISHES, SEA-URCHINS, ETC. 



Class III. SEA-URCHINS (Echinoided). 



General Characteristics. The egg-shaped skeleton (Fig. 

 35) is made up of about six hundred hard, six-sided plates 

 in double rows, containing perhaps 3,720 pores for the emis- 

 sion of the tube-like feet. The star-fishes a sucker, but 

 the Echinus a biter, having five long calcareous teeth that 

 meet at a point, m (Fig. 35), and are renewed as they wear 

 away. They are moved by a 

 complicated system of mus- 

 cles, and held in place by a 

 leathery skin. The body is 



FIG. 35. Skeleton of sea-urchin 

 without spines. m, mouth ; 

 f h, foot-holes ; zv/i, madre- 

 poric plate ; e, eyes ; s, sock- 

 ets of large spines. 



covered with about 4,000 

 spines, each of which is made 

 up of hollow tubes, and works 

 on a ball-and-socket plan, s. 



Among these spines are over 2,000 suckers, or feet, of three 

 different kinds : i, suckers proper ; 2, and most frequently 

 found near the mouth, pedicellariae (Fig. 36), whose calca- 

 reous jaws are continually opening and shutting in loco- 

 motion and defense ; and 3, stalked button-like bodies 

 called sphceridia, probably organs of taste. The same 



FIG. 36. Jaws of Pedicellaria. 

 Highly magnified. 



