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-fisli. 

 So that altogether 

 there are twenty 

 series of plates — 

 ten ainbulacral, 

 andteuinterambn- 

 lacral. The shell 



is not cast, but 



grows by the en- 



FiG. 214.— TJiider-Biirface of a Sea-urchin {Echinus escu- i? i 



lentus), showing rows of Kucltei's among tlae spiues. largCmeilt OI eaCU 

 British seas. ..,..., i i . 



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 tiie arms to. meet above the month, we 

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

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

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

 lusks. Crabs, and offal. They reproduce by minute red eggs. 

 Regular Echini, as the common Oidaris, are nearly 

 globular, and tlie oral and anal openings are opposite. 

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

 orifice is near the margin. 



Class IV. — Holothuroidea. 

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

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



