108 



ANIMAL BIOLOGY 



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 seg- 

 ments 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 body, 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 modi- 

 fied regions, and the development of 

 large outpocketings, or glands, which 

 are specialized 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 ac- 

 tually pass to the circulatory system 

 for distribution to the tissues of the 

 animal. 

 Fig. 66. - - Diagram of the As a final illustration we may take 



anterior portion of the central the nervous system. In the Earthworm 



nervous system of an Earth- ,, • • , n „ „ u* u 



worm (A) and a Crayfish (B). thlS COnSlsts ° f a NERVE CORD which 



a, brain (cerebral, or supra- runs along the body in the mid-ven- 



esophageal, ganglion); b, nerve tral line below the digestive tract. At 



commissures, encircling the .-, , • i •. j« -i • . . „ n 



pharynx (shown in section); the anteri0r end ' ll dlVldeS int ° tWO 



c, subesophageal ganglion; d, branches which encircle the digestive 



ganglia of the ventral nerve tract and unite above in a relatively 

 cord, with nerves emerging. ^^ body of nervous tigsue 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 modifica- 

 tions have been brought about by the uniting of segments in the 

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

 fusion of the segmental ganglia in this region into larger ganglionic 

 masses. The brain of the Crayfish, for example, comprises the 



