STRUCTURE OF THE VERTEBRATES 343 



enclosed by the lateral growth of the flat nerve cord and left 

 as the neurocoel of the vertebrate. (2) As it was necessary to 

 get rid of the numerous appendages along the body, and as they 

 were found to secrete small amounts of digestive enzymes, he 

 conceived of them fusing along the bottom and thus forming 

 the intestinal tract. 



With all the evidence stated these theories do not appear as 

 fanciful as a brief summary' would indicate. However, when the 

 three diagnostic characters of the chordates are found in such 

 primitive condition in living chordates, it seems to the author 

 unreasonable to search for the ancestral line among specialized 

 phyla which have no embryological similarity with the verte- 

 brates. As stated previously, these early ancestors would hardly 

 be preserved as fossils; but, like other worms of the Cambrian 

 Period, would be known to us only as burrows in the primeval 

 sands. 



B. Adapted Radiations of the Early Chordates 



It is believed that the chordate stock had reached an Amphi- 

 oxus-like form in the Cambrian Period. This would give three 

 main evolutionary lines: (1) the most primitive which has sur- 

 vived as the two widely different groups of Hemichordates; (2) 

 the ancient, but now highly specialized, Urochordates (Tuni- 

 cates) ; and (3) the line with a complete notochord which has 

 survived as Amphioxus and the vertebrates. The last apparently 

 gave rise in the Cambrian to two distinct lines: (A) the cyclo- 

 stomes and the armored ostracoderms, the latter showing more 

 relationships with the cyclostomes than with the other verte- 

 brates, and both being more closely similar to Amphioxus than to 

 the fish; and (B) the Elasmobranchs. There is also much ev- 

 idence that the bony fish had arisen before the end of the 

 Cambrian. 



The earliest sharks known have a reduced number of gill arches, 

 and the jaws are already formed from the first pair of branchial 

 cartilages. Cladoselache, which has been described, supplies in- 

 valuable evidence as to the fin-fold theory. The most generalized 

 sharks still living have seven gill slits, and others six. These 

 groups show the beginnings of the heterocercal (upturned) tail. 



