. Animal biology. Biology; Zoology; Physiology. 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
. Animal biology. Biology; Zoology; Physiology. 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
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