550 The Animal Kingdom 



THE CLASS ECHINOIDEA 



Of all the echinoderms, none lives up to the name "spiny skin" to 

 the extent that some of the members of this class do. Anyone who has 

 had the misfortune to encounter a sea urchin's spines will attest to the 

 truth of this statement. To this class belong not only the spinose sea 

 urchins and heart urchins, but also the flattened sand dollars. 



Typically a sea urchin of the genus Arbacea (Fig. 183,/) has a 

 hemispherical test covered with numerous heavy spines and no arms. 

 The test is formed of rows of plates, which, however, are arranged in 

 such a manner as to make the body quite inflexible. The tube feet 

 are arranged in five pairs of rows which correspond to the arms of 

 the starfish. These tube feet are very long and can reach out beyond 

 the ends of the spines for attachment. Numerous pedicellariae are lo- 

 cated between the spines. These function not only to keep the test 

 of the animal free from debris and sessile organisms but also to capture 

 small animals and to carry them to the mouth. 



Fig. 181. — Larvae of echinoderms. A, Bipinnaria of starfish; B, pluteus of 

 sea urchin; C, auricularia of sea cucumber. 



On the oral surface, the mouth occupies a central position and is 

 surrounded by five strong teeth. These teeth possess a complicated 

 framework of calcareous plates and muscles which is known as "Aris- 

 totle's lantern." These serve to crush and grind the food of the sea 

 urchin. This food consists of small crustaceans, sponges, and other 

 small animals. 



The sexes are separate, and the gonads develop along the inter- 

 ambulacral areas inside the test. Fertilization is external and a larval 

 form, the pluteus (Fig. 181,5), develops. Unlike the starfish and ser- 

 pent stars, the sea urchin has only limited ability to regenerate parts. 



