THE ANIMAL BODY 133 



which are homologous and to study the modifications which 

 . are associated with change of function. (See p. 353.) 



We have considered the principle of specialization and 

 fusion of the segments of the higher Arthropods in so far 

 as it affects external structures, but profound modifications 

 of the internal organs also occur. In the first place, the 

 partitions between the various segments which are present 

 in the Earthworm have disappeared in the Crayfish. Again, 

 the alimentary canal of the Earthworm is a nearly straight 

 tube extending through the coelom, with relatively slight 

 modifications in certain segments for the elaboration of the 

 food material as it passes along from mouth to anus ; while in 

 the Crayfish we see the accentuation of such modified regions, 

 and the development of large outpocketings which are spe- 

 cialized for the formation of chemical substances to DIGEST 

 the food material. That is, to change the food into a soluble 

 form so that it can pass through the cellular membrane which 

 lines the digestive tract and thus actually pass to the circula- 

 tory system for distribution to the tissues of the animal. 



As a final illustration we may take the nervous system. 

 In the Earthworm this consists of a nerve cord which runs 

 along the body in the mid-ventral line below the digestive 

 tract. At the anterior end, it bifurcates into commissures 

 which encircle the digestive tract and unite above in a rela- 

 tively large body of nervous tissue which constitutes the 

 cerebral ganglion, or BRAIN. In each segment the nerve cord 

 also is somewhat enlarged to form masses of nerve tissue 

 (GANGLIA) from which nerves pass to the organs in the vicinity. 

 The nervous system of the Crayfish exhibits the same general 

 plan as that of the Earthworm, but certain modifications have 

 been brought about by the coalescence of segments in the 

 region of the head and thorax. This process has resulted in 

 the union of the segmental ganglia in this region into larger 



