THE ANCESTRY OF THE VERTEBRATES 643 



acquiring a new mouth has, in more recent years, been espoused by two 

 distinguished biologists, Gaskell (1908) and W. Patten (191 2), who have 

 followed the comparison of arthropods and vertebrates into greater 

 detail than has been done by other students of vertebrate phylogenesis. 

 Both lean heavily upon paleontological evidence. Both assume that 

 evolutionary change occurs through the transformation of one dominant 

 group into a higher dominant group. Both authors assume that the 

 most highly differentiated types of invertebrates such as the arachnids 

 are the progenitors of the vertebrates, contrary to prevalent opinion 

 which assumes that evolutionary change has affected chiefly generalized 

 types. 



It is significant that the two reach fundamentally different conclusions 

 as to the most primitive types of vertebrates. These, according to 

 Gaskell, are the cyclostomes, while Patten regards the ostracoderms as 

 the immediate descendents of the sea scorpions. Both succeed in finding 

 an amazing degree of resemblance between limulus and a fish, and present 

 a great mass of evidence to prove this similarity. This resemblance, 

 however, seems to be more for the details of anatomical structure than 

 for fundamental characteristics. 



Both authors agree that hemichordates, urochordates, and cephalo- 

 chordates are degenerate, not ancestral types. Both assert that the 

 comparative anatomy of the central nervous system affords the strongest 

 structural evidence of the similarity of arthropods, and more especially 

 arachnids, with vertebrates. 



Gaskell's Hypothesis 



According to Gaskell the neural tube of chordates is derived directly 

 from the alimentary canal of arthropod ancestors. The ventricles of 

 the vertebrate brain, Gaskell asserts, have evolved from the cavity 

 of the cephalic portion of the arachnid stomach. Their ependymal lining 

 corresponds to the mucous lining of the arthropod stomach. Differences 

 in their germ-layer origin do not disturb Gaskell, who is a physiologist! 



Gaskell finds the brain of both scorpions and vertebrates to consist 

 of three chief divisions, forebrain, midbrain, and hindbrain. The con- 

 strictions which separate these represent ontogenetically two nerve 

 commissures, which cross the cephalic stomach in the prevertebrate stage, 

 as the result of the mid-dorsal position of the pineal eyes and of the 

 insertion of the superior oblique muscle. The anterior of the two is 

 the commissure between the two supra-esophageal ganglia. The anterior 

 end of the cephalic stomach of the arthropod becomes the lamina termi- 

 nalis of the vertebrate brain. The evolution of the vertebrate brain 

 from that of the arthropod is the result of the growth and extension of 

 nervous material, which was originally ventral to the stomach, around 



