The alimentary canal consists of a single digestive cavity
passing through the whole body, as in Worms, the anterior part of which
is surrounded by a large liver. What is true of the Lobsters is true
also, so far as class-characters are concerned, of all the Crustacea.
Highest in this type are the Insects, and among these I include Spiders
and Centipedes as well as Winged Insects. It is true that the Centipedes
have a long uniform body like Worms, and the Spiders have the body
divided into two regions like the Crustacea, while the body in true
Insects has three distinct regions, head, chest, and hind body; but
notwithstanding this difference, both the former share in the peculiar
class-character that places them with the Winged Insects in a separate
group, distinct from all the other Articulates. We have seen that in the
Worms the respiratory organs are mere vesicles, while in the Crustacea
they are more highly organized gills; but in Centipedes, Spiders,
and Winged Insects, the breathing-apparatus is aerial, consisting of
air-holes on the sides of the body, connected with a system of tubes and
vessels extending into the body and admitting air to all parts of it. In
the Winged Insects this system is very elaborate, filling the body with
air to such a degree as to render it exceedingly light and adapted to
easy and rapid flight. The general arrangement of parts is the same in
this class as in the two others, the typical character being alike in
all.
Pages:
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206