The digestive and reproductive systems have thus nearly attained
their final form. From the higher worms upward the digestive system
will improve greatly. Its lining will fold and flex and vastly
increase the digestive and absorptive surfaces. The layer of cells
which now secrete the digestive fluids will in part be replaced by
massive glands. Far better means of grasping food than the horny
teeth of annelids will yet appear. But all these changes are
inconsiderable compared with the vast advance made by the muscular
and nervous systems. Reproduction and digestion are losing their
supremacy in the animal body. Their advance and improvement will
require but little further attention.
In the annelid especially, and to some extent in the schematic worm,
the supra-oesophageal ganglion is relieved in part of the direct
control of the muscular fibrils and has become an organ of
perception and the seat of government of lower nervous centres. In
all higher forms it innervates directly only the principal
sense-organs of the head. And at this stage the light-perceiving
directive eye has developed into a form-perceiving, eidoscopic
organ. The eye was short of range and its images were perhaps rude
and imperfect, but it was a visual eye and had vast possibilities.
The animal is taking cognizance of ever more subtle elements in its
environment.
Pages:
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148