There is an evident superiority in this class over the preceding one, in
the greater prominence of the anterior extremity, where there are two or
more feelers, with which eyes more or less developed are connected; and
though there is nothing that can be properly called a head, yet there
can be no hesitation as to the distinction between the front and hind
ends of the body.
[Illustration: Limpet, Patella, cut transversely: _a_, foot; _b_, gills;
_c_, mantle; _d_, shell; _e_, heart; _f_, main cavity, with intestines.]
The third and highest class of Mollusks has been called Cephalopoda, in
reference again to a special feature of their structure. They have long
arms or feelers around the head, serving as organs of locomotion, by
which they propel themselves through the water with a velocity that is
quite extraordinary, when compared with the sluggishness of the other
Mollusks. In these animals the head is distinctly marked,--being
separated, by a contraction or depression behind it, from the rest of
the body. The feelers, so prominent on the anterior extremity of
the Gasteropoda, are suppressed in Cephalopoda, and the eyes are
consequently brought immediately on the side of the head, and are very
large in proportion to the size of the animal. A skin corresponding
to the mantle envelops the body, and the gills are on either side of
it;--the stomach with its winding canal, the liver, and heart occupy the
centre of the body, as in the two other classes.
Pages:
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199