252 COMPARATIVE ZOOLOGY. 
Crass II].—-Echinoidea. 
The Sea-urchin is encased in a thin hollow shell cov- 
ered with spines, and varying in shape from a sphere to 
a disk.” The mouth is underneath, and contains a dental 
apparatus more complicated than that of any other creat- 
ure from the Sponge to Man. It leads to a digestive tube, 
which extends spirally to the summit of the body. The 
spines are for burrowing and locomotion, and are moved 
by small muscles, 
each being articu- 
lated by ball-and- 
socket joint to a 
distinct tubercle. 
When stripped of 
its spines, the shell 
(or “ test”) is seen 
to be formed of a 
multitude of pen- 
tagonal plates, fit- 
ted together like 
a mosaic.* Five 
double rows of 
TOF 
WG i 
Fre. 209.—Under-surface of a Sea-urchin (EHehinus escu- 
lentus), showing rows of suckers among the spines. plates, passing 
ery att from pole to pole, 
like the ribs of a melon, alternate with five other double 
rows. In one set, called the ambulacra, the plates are 
perforated for the protrusion of tubular feet, or suckers, 
as in the Star-fish. So that altogether there are twenty 
series of plates—ten ambulacral, and ten interambula- 
eral. The shell is not cast, but grows by the enlargement 
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 princi- 
ple of radiation. If the arms of a Star-fish were turned 
