262 



COMPARATIVE ZOOLOGY. 



passing from pole to pole, like the ribs of a melon, alter- 

 nate with five other double rows. In one set, called the 



ambulacra, the 

 plates are perfo- 

 rated for the pro- 

 trusion of tubular 

 feet, or suckers, as 

 in the Star - fish. 

 So that altogether 

 ^ there are twenty 

 series of plates- 

 ten ambulacral, 

 andteninterambu- 

 lacral. The shell 

 is not cast, but 

 grows by the en- 

 largement of each 

 individual plate, 

 and the addition of new ones around the mouth and the 

 opposite pole. Every part of an Echinus, even sections 

 of the spines, show the principle of radiation. If the up- 

 per surface of a Star -fish should shrink so as to bring 

 the points of the arms to meet above the mouth, we 

 should have a close imitation of a Sea-urchin. Echini live 

 near the shore, in rocky holes or under sea-weed. They 

 are less active than Star-fishes ; but, like them, feed on 

 Mollusks and Crabs. They reproduce by minute red eggs. 



Regular Echini, as the common Cidaris, are nearly 

 globular, and the oral and anal openings are opposite. 

 Irregular Echini, as the Glypeaster, are flat, and the anal 

 orifice is near the margin. 



Fig. 214. — Under-surface of a Sea-urchin {Echinus escu> 

 lentus), showing rows of suckers among the spines, 

 British seas. 



Class IV. — Holothuroidea. 



These worm-like " Sea-slugs," as they are called, have a 

 soft, elongated body, with a tough, contractile skin contain- 



