PHYLUM ECIIINODERMATA 



203 



plates (7), so called because they are penetrated by tube-feet (8) 

 and five pairs of columns of inter -ambulacral plates (6). On the 

 inside of the test around the peristome in many sea-urchins are 

 five arches, often incomplete, called auricles. Most of the plates 

 bear spines which are attached by muscles and move freely on 

 little knob-like elevations called tubercles (9). The pedicellaria, 

 are more specialized than those of the starfish; they commonly 



Fig. 142. — Dried test of a sea-urchin, Echinus esculentus. i, anus; 2, peri- 

 proct; 3, madreporite; 4, a genital plate; 5, an ocular plate; 6, an interambu- 

 lacral plate ; 7, an ambulacral plate ; 8, pores for protrusion of tube-feet ; 

 q, tubercles of primary spines. (From the Cambridge Natural History.) 



have three jaws. The mouth is provided with five white teeth; 

 these are part of a complicated structure known as " Aristotle's 

 Lantern " (Fig. 143, comp., eph.). 



Nutrition. — The food of the sea-urchin consists of marine 

 vegetable and animal matter which is ingested by means of 

 " Aristotle's Lantern." The intestine (Fig. 143, int) is very 

 long; it takes one turn around the inside of the body and then 

 bends upon itself and takes a turn in the opposite direction. A 

 small tube, the siphon (Fig. 143, siph.), accompanies the in- 



