THE ANCESTRY OF THE VERTEBRATES 



649 



brates was evolved with its culminating organism — man — whose massive 

 brain with all its possibilities could never have been evolved if he had 

 still been compelled to pass the whole of his food through the narrow 

 esophageal tube, still existent in him as the infundibular tube." 



^NOTOCHORDAL GROOVE . 



NOTOCHORD. 



AUMENTARY CANAL, 



HEART 



FAT BODY. 



NERVOUS SYSTEM 



MESONEPHROS 



NOTOCHORD 



-MYOTOME 



APPENDAGE 



ATRIAL CHAMBER 



METACOELE 



PLEURON . 



c 



Fig. 527. — A diagram illustrating how a trilobite-like animal (.4) might evolve into 

 a vertebrate (D). B and C are hypothetical stages transitional between A and D. 

 All figures represent cross sections. Since Gaskell's theory of the origin of vertebrates 

 assumes that the arthropod intestine has become the lining of the brain and spinal cord 

 of vertebrates, a new alimentary canal and a notochord have to be invented. These, 

 according to Gaskell, arise by infolding of the skin along the mid-ventral line, first the 

 notochord and later the alimentary canal. Germ layers mean nothing to Gaskell. 

 Pleural appendages unite to form the body-wall. (Redrawn after Gaskell.) 



As further evidence of the genetic relationships of arthropods and 

 vertebrates, Gaskell points out that eurypterids and limulus, like ostra- 

 coderms, have both median and lateral eyes. Furthermore, arachnids, 

 like vertebrates, have an inverted retina, while other invertebrates have 



