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 oesophagus, 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 



