146 
ANIMAL ACTIVITIES. 
Summary of Drawings, (a) A living sea-urchin, 
natural size. 
($) A single spine showing mode of attachment to 
the shell. 
(c) Several ambulacral and interambulacral plates. 
( d ) External form of a sea-cucumber. 
A B 
Fig. 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 
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 i t s 
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. 
