WORMS TO VERTEBRATES 59 



much in length and retain the advantage of the protec- 

 tion of the shell ; and the shell was the dominating 

 structure. It had entered upon a defensive campaign. 

 Motion, slow at the outset,became more difficult, and the 

 protection of the shell therefore all the more necessary. 

 The shell increased in size and weight and motion be- 

 came almost impossible. The snail represents the 

 average result of the experiment. It can crawl, but 

 that is about all; it is neither swift nor energetic. 

 Even the earthworm can outcrawl it. It has feel- 

 ers and eyes, and is thus better provided with sense- 

 organs than almost any worm. It has a supra-ceso- 

 phageal ganglion of fair size. 



The clams and oysters show even more clearly what 

 we might call the logical results of molluscan structure. 

 They increased the shell until it formed two heavy 

 " valves ' hanging down on each side of the body 

 and completely enclosing it. They became almost 

 sessile, living generally buried in the mud and gaining 

 their food, consisting mostly of minute particles of 

 organic matter, by means of currents created by cilia 

 covering the large curtain-like gills. Their muscular 

 system disappeared except in the ploughshare-shaped 

 "foot " used mostly for burrowing, and in the muscles 

 for closing the shell. That portion of the body which 

 corresponds to the head of the snail practically 

 aborted with nearly all the sense-organs. The nervous 

 system degenerated and became reduced to a rudi- 

 ment. They had given up locomotion, had withdrawn, 

 so to speak, from the world ; all the sense they needed 

 was just enough to distinguish the particles of food as 

 they swept past the mouth in the current of water. 

 They have an abundance of food, and " wax fat." 



