CHORDATA , 217 



theory. The higher phyla of Invertebrates have practically all 

 been studied by investigators with the idea that affinities with the 

 vertebrates could be determined. 



Amphioxus Theory. — The Amphioxus theory as developed by 

 various workers and set forth by Willey is as follows. The ancestor 

 of the vertebrates was a free swimming animal intermediate between 

 the tadpole of the Tunicate and Amphioxus. It had the ventral 

 mouth, pituitary and notochord (limited) of the Tunicate; and the 

 myotomes, coelomic epithelium, and straight alimentary canal of 

 the Amphioxus. The chief factor in the evolution of the verte- 

 brates has been the concentration of the central nervous system 

 along the dorsal side of the body and its conversion to a hollow tube. 

 The hypophysis may have become evolved in connection with a 

 functional neuropore. Adam Sedgewick and Van Wijhe suggested 

 that the neural canal had as its original function the promotion of 

 oxygenation of the tissue of the Central Nervous System, the water 

 entering by the neuropore and leaving through the posterior neu- 

 renteric canal. Harmer and Brooks suggested independently that 

 the gill slits arose at first to carry away the bulk of water con- 

 stantly entering the mouth with the food, obviating the necessity 

 of the flow of water through the alimentary canal. Later they aided 

 in performing the function of respiration. (Cephalodiscus has 

 luxuriant branchial plumes, sufficient for respiration, and the pair 

 of gill slits allow the water to pass from the pharynx.) The noto- 

 chord occurs in Balanoglossus {Enteropneusta) in the proboscis; 

 and in the tail of larval Tunicates. In Balanoglossus it may be a 

 divergent structure. The notochord may have arisen as a solidi- 

 fication of the endoderm, continued into the caudal portion of the 

 body to afford axial support for a locomotor tail. Endoderm as a 

 stiffening substance has been developed in some medusae and hy- 

 droid polyps as there is skeletal tissue in their tentacles in the form 

 of a solid endodermal axis. 



The Amphioxus theory supposes that Amphioxus was derived 

 from a Tunicate (Ascidian) and that the Tunicate arose from a form 

 possibly hke Balanoglossus. Balanoglossus may or may not have 

 arisen from the Echinodermata. The ciliated Tornaria larva of 

 Balanoglossus is similar to the larvae of the Echinoderms in that it 

 possesses bilaterality, ciliated bands, pelagic life and is small and 

 quite transparent. 



That the earlier fishes were similar to Amphioxus and that the 



