EVOLUTION 131 
from the water around them, and the consequent 
development of the gill in response to this demand. 
The fact, for instance, that Anodonta has developed 
such a complicated gill-structure becomes intelligible 
when it is borne in mind that it lives mostly in ponds 
or sluggish water, and has, moreover, for six or eight 
months out of the twelve to shelter within its gill- 
chamber hundreds of young, all like itself consuming 
oxygen from the same limited supply. 
In the nervous system of the Mollusca (Plate III., 
Fig. 3) a definite progressive development is trace 
able. In the earlier and more archaic Gastropoda 
the nervous system is diffuse, the nerve ganglions 
are comparatively widely separated, and the con- 
nectives and commissures that unite them are long. 
Passing to higher and higher representatives, the 
nerve centres tend to become more and more con- 
centrated; at first the sensory and motor nerve 
centres, and then all the others, till they form a ring 
round the anterior part of the cesophagus, and finally 
are intimately united and localized on the dorsal 
surface of the latter, as in Pleurobranchus, or the 
ventral side, as in the thecosomatous Pteropods. 
This progressive advance is observable also in the 
Cephalopoda, and to a lesser degree in the Pele- 
cypoda, and even the Amphineura. 
Of the sensory organs, only the eyes call for 
mention. With the exception of the Cephalopods, 
and possibly the Heteropods, the vision of the 
