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, Pl. 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 punctulata, 
Pl. 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, Pl. 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 
—pedicellarie—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* 
