STRUCTURE OF THE VERTEBRATES 341 



The Hemichordata and Urochordata of the present day are 

 highly specialized, but they show the three diagnostic characters 

 of the vertebrates (dorsal hollow nerve cord, notochord, and 

 pharyngeal gill slits) in a simple, yet progressive, condition. The 

 theory does not, however, account for their evolution from the 

 invertebrates; and it is admitted that no clear evidence exists as 

 to the parental stem of the phylum. 



The greatest difficulty to be overcome in deriving vertebrates 

 from invertebrates is the position of the body and the internal 

 organs. The invertebrate has a ventral, solid nerve cord, and the 

 currents of the large blood vessels are exactly opposite those 

 of the vertebrate. Also, the retinal cell layers of the vertebrate 

 are inverted, while those of the invertebrate have the sensory 

 cells toward the source of light — except in the Cephalopods 

 (Phylum ]Mollusca). The chordate theory presupposes that the 

 first chordates came from a race in which definite eyes were not 

 well developed, or were lacking, and that the vertebrate eye is 

 a new development. The adherents of this theory have not been 

 entirely fair in stating that other theories derive a vertebrate 

 ''by turning an invertebrate inside out and upside down." 



The oldest theory of vertebrate origins traced their ancestry 

 to the Annelids (earthworm group), and found many points 

 of comparison. (1) "Dorsal" and ''ventral" in any given animal 

 are arbitrary terms, the mouth being considered ventral, and 

 an annelid might easily reverse his position. The nerve cord 

 and blood vessels would then be in the proper position; and (2) 

 a new mouth is believed to have developed, leaving the old 

 mouth as a vestigial structure — the neuropore found in Amphi- 

 oxus. (3) A new anus developed anterior to the old, which is 

 supported by the fact that a post-anal gut is found in the caudal 

 end of many vertebrate embryos. (4) The notochord evolved 

 from strands of connective tissue which are found along the 

 ventral aorta of the annelid, a point which is not corroborated 

 by the endodermal origin of the chordate structure. (5) The gill 

 slits are homologized with the external openings of the anterior 

 nephridia, and the fact that no nephridia develop in the pharyn- 

 geal region of the vertebrates is used as evidence. However, with 

 all the apparent evidence, most embryologists and comparative 



