146 ANIMAL ACTIVITIES. 
Summary of Drawings. (a) A living sea-urchin, 
natural size. 
(4) A single spine showing mode of attachment to 
the shell. 
(c) Several ambulacral and interambulacral plates. 
(d@) External form of a sea-cucumber. 
Fic. 118.—The Structure of a Sea-urchin. A, interior of shell; B, 
teeth. 
Activities of the Sea-urchin. The activities of the 
sea-urchin resemble those of the starfish. One pecul- 
iarity deserves mention. The mouth of the sea-urchin 
Fic. 119.—A Sea-cucum- 
ber. 
is provided with a complicated 
arrangement of teeth, five in 
number, uniting at a point. 
This whole apparatus is called 
Aristotle’s lantern in honor of 
the philosopher who first de- 
scribed it. With these teeth and 
possibly by the aid of oral secre- 
tions the sea-urchin is enabled 
to burrow into solid rock. 
Other Echinoderms.  Sea- 
cucumbers of many kinds, the 
worm-like Synapta with its 
anchor-shaped plates, the great 
variety of starfishes, sand-dol- 
lars, Crinoids both fossil and present, with other less 
common forms all bear a striking resemblance to the 
two forms studied. 
