644 



COMPARATIVE ANATOMY 



and over the epithelial wall of the cephalic stomach. (See Fig. 524.) 

 The presence of neuromeres in the brains of embryonic arthropods and 

 vertebrates is taken as another point of similarity between the two phyla. 

 According to Gaskell, the great enlargement of the brain in arthropods 

 made necessary a new mouth. Just before vertebrates appeared, the 

 arthropods were in a "terrible dilemma." They were compelled to 

 choose between the capacity for taking in food without intelligence 

 to capture it, or intelligence sufRcient to capture food and no power to 

 consume it. The enlargement of the brain, through which the esophagus 



DORSAL 



CORPUS STRIATUM' 



VENTRICLES 

 OF BRAIN. 

 > INFRA-INFUNDIBULAR BRAIN. 

 ^■'iNFUNDIBULUM 

 'CRURA CEREBRI 



^^SPINAL 

 CORD. 

 SEGMENTAL NERVES 



VENTRAL . 



OGRSAL 



SUPRA-ESOPHAGEAL 

 GANGLION 



•ESOPHAGUS- 

 \CIRCUM-ES0PHAGEAL 

 , COMMISSURE. 



VENTRAL. 



Fig. 524. — Gaskell's diagrams (redrawn) illustrating how the alimentary canal 

 and the nerve cord of an arthropod (B) may be — in his opinion has been — converted 

 into the central nervous system of a vertebrate (A) without assuming a reversal of 

 dorsal and ventral surfaces. The arthropod stomach becomes the ventricles of the 

 vertebrate brain. The process obviously leaves the animal with the necessity of invent- 

 ing a new alimentary canal. How this is effected is illustrated in Fig. 527. 



passed, meant the strangulation of the esophagus, while the atrophy 

 of the brain meant degeneration. Only a new intestine would solve 

 the problem. But since the skin cells of arthropods and cyclostomes 

 have been shown to have digestive powers, there is to Gaskell no more 

 difficulty in getting a new digestive system from the skin than there is 

 for an animal to gain a new respiratory or circulatory system. All of 

 this sounds teleological, but is probably not intended to imply conscious 

 purpose on the part of the animal. 



As the result of the changes which occurred, the anterior ganglia 

 of the arthropod formed the cyclostome brain and their cephalic stomach 

 became the ventricles and ependymal lining of the brain, while the 

 intestine and nerve cord became the tubular spinal cord. 



Gaskell believes that the assumption that cyclostomes are degenerate 

 forms is not substantiated by the evidence. The transformation of the 



