STAR-FISHES, SEA-URCHINS, ETC. 77 



form the apical disk is also situated the madreporic 

 tubercle. The entire surface of the shell, seen to 

 better advantage in a second species of our coast, 

 the sea-egg (Strongylocentrotus, PI. 5, Fig. 4), 

 bristles with spines, between which rise the very 

 delicate tube-feet, swaying to and fro in search of 

 objects to be placed within their grasp. The sea- 

 urchins like to conceal themselves, and they will 

 thus frequently cover their habitations with sea- 

 weed, grass, pebbles, and shell-fragments. In this 

 condition they can be readily passed over without 

 being noticed by their enemies. A southern species 

 when placed in a bucket of gravel and shell will 

 almost immediately disappear beneath the surface, 

 lifting the material over its back. Progression is 

 effected almost exclusively by means of the tube- 

 feet. 



Two other species of sea-urchin occur on our 

 coast — one, the purple sea-urchin [Arbacia punctuhia, 

 PI. 5, Fig. 6), a convex form, readily distinguish- 

 able by its comparatively stout and purple spines ; 

 and the other, the sand-cake proper {Mellita quin- 

 quefora, PI. 5, Fig. 1), a flattened species much like 

 the sand-dollar, easily recognized by the five slits 

 in its shell. 



It would scarcely be proper to complete our 

 notice of these interesting animals without calling 

 attention to their peculiar five-toothed masticatory 

 apparatus and to the singular bird-beaked bodies 

 — pedicellarise — which rise up between the spines. 

 The full function of the latter has not yet been as- 

 certained, but that they in part assist in removing 



7* 



