TISSUE-DIFFERENTIATION 109 



nervous centers reach a large size and a very high 

 degree of complexity. They are then called the brain. 

 But most animals are more or less elongate, and this 

 central condensation of the nervous system is also 

 distributed along the longitudinal axis. In verte- 

 brates this forms the spinal cord, which is a thick- 

 walled hollow tube. In invertebrates the central 



FIG. 39. Diagrammatic comparison between the vertebrate type 

 (A) and the invertebrate type (B), with respect to the relation of the 

 nervous system to the alimentary canal. 



nervous system is solid and often double, or in the 

 form of a ladder. In vertebrates the central nervous 

 system and brain is constantly dorsal 1 to the alimen- 

 tary tube ; in invertebrates, with few exceptions, the 

 central nervous system is ventral to the alimentary 

 canal, although the brain is always dorsal. Accord- 

 ingly the connections between the brain and the rest 

 of the nerve cord in invertebrates have to go around 



1 " Dorsal " and " ventral " are here used in their ordinary though 

 somewhat inaccurate meanings. 



