342 STRUCTURE OF THE VERTEBRATES 



anatomists accept the chordate theory rather than the annelid 

 theory of vertebrate development. 



Another theory traces the vertebrates back to the Xemertines, 

 a group of worms somewhat related to the flatworms. In this 

 case it is not necessary to turn the animal over, for the nerve 

 cord has two lateral branches and a small dorsal one. The verte- 

 brate would be evolved by the great overgrowth of the dorsal 

 branch with the modification of the lateral branches into the 

 paired vagi. It has been shown that the lateral nerves of this 

 worm have two branches, and these are supposed to be homolo- 

 gous with the intestinal and lateral branches of the tenth cranial 

 nerve. Unfortunately the other organs do not show as much 

 similarity as does the nervous system. 



Patten and others have developed the theory that the verte- 

 brates have come from an Arachnid line (spiders, scorpions, 

 the extinct euripterids, etc.). The Horseshoe crab is the closest 

 living relative of the extinct line which is used as the basis for 

 the theory. The theory automatically discards the primitive 

 chordates, leaving Amphioxus and the cyclostomes as degenerate 

 animals of no phylogenetic importance; for the vertebrates are 

 supposed to have arisen as armored cyclostomes with appendages 

 directly from an arachnid line. The theory gains strength from 

 recent research on the Ostracoderms, or armored cyclostomes. 

 Casts of this fossil group show resemblances to the extinct 

 arachnids which cannot be easily dismissed. Patten derives the 

 vertebrate jaws from the anterior appendages of the arachnid. 

 The objections to the theory are numerous. There are no known 

 cases of a group so highly specialized giving rise to another 

 highly specialized one. Further objections are: (1) the reversal 

 of layers of the eyes; (2) the dorsal chitinous armor of the 

 arachnid remains dorsal when the animal turns over; (3) the 

 arachnid shell becomes bony without intermediate evolution; 

 (4) the arachnid appendages arise from a single somite, the 

 vertebrate appendages from several; and (5) the general em- 

 bryological history of the two groups is entirely different. 



Gaskell, in England, working on crustacean materials, at- 

 tempted to overcome the difficulties of turning the animal over, 

 and developed the theory that a new gut evolved. There 

 are two major points in his theory. (1) The primitive gut was 



