WORMS TO VERTEBRATES 61 



lessly beaten by the vertebrates. They struggled 

 hard, but too late. 



The history of mollusks is full of interest. They 

 show clearly how intimately nervous development is 

 connected with the use of the locomotive organs. The 

 snail crept, and slightly increased its nervous system 

 and sense-organs. The clam almost lost them in con- 

 nection with its stationary life. The cephalopods were 

 exceedingly active, developed, therefore, keen sense- 

 organs and a very large and complicated supra- 

 cesophagal ganglion, which we might almost call a 

 brain. 



The articulate series consists of two groups of 

 animals. The higher group includes the crabs, spiders, 

 thousand-legs, and finally the insects, and forms the 

 kingdom of arthropoda. The lower members are still 

 usually reckoned as worms, and are included under the 

 annelids. Of these our common earthworm is a good 

 example, and near them belong the leeches. But the 

 marine annelids, of which nereis, or a clam- worm, is a 

 good example, are more typical. They are often quite 

 large, a foot or even more in length. They are com- 

 posed of many, often several hundred, rings or seg- 

 ments. Between these the body-wall is thin, so that 

 the segments move easily upon each other, and thus 

 the animal can creep or writhe. 



These segments are very much alike except the first 

 two and the last. If we examine one from the middle 

 of the body we shall find its structure very much like 

 that of our schematic worm. Outside we find a very 

 thin, horny cuticle, secreted by the layer of cells just 

 beneath it, the hypodermis. Beneath the skin we find 

 a thin layer of transverse muscles, and then four heavy 



