The primitive Tracheates, delivered from the increasing
carnivores of the waters, grew into a large and varied family, as
all such new types do in favourable surroundings. From them in
the course of time were evolved the three great classes of the
Myriapods (millipedes and centipedes), the Arachnids (scorpions,
spiders, and mites), and the Insects. I will not enter into the
much-disputed and Obscure question of their nearer relationship.
Some derive the Insects from the Myriapods, some the Myriapods
from the Insects, and some think they evolved independently;
while the rise of the spiders and scorpions is even more obscure.
But how can we see any trace of an Annelid ancestor in the vastly
different frames of these animals which are said to descend from
it? It is not so difficult as it seems to be at first sight. In
the Myriapod we still have the elongated body and successive
pairs of legs. In the Arachnid the legs are reduced in number and
lengthened, while the various segments of the body are fused in
two distinct body-halves, the thorax and the abdomen. In the
Insect we have a similar concentration of the primitive long
body. The abdomen is composed of a large number (usually nine or
ten) of segments which have lost their legs and fused together.
Pages:
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210