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 Mol- 
lusks, Crabs, and offal. 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 Clypeaster , are flat, and the anal 
orifice is near the margin. 
Fig. 214.—Under-surface of a Sea-urchin (Echinus earn - 
lentm), 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* 
