EVOLUTION OF THE VERTEBRATES 147 



alimentary tract as in the vertebrate. Formation of a ventral 

 mouth would then cut off the anterior end of the alimentary tract, 

 which in the annelid passes through the C(>ntral nervous system. 

 The entire central nervous system woukl then lie on the same side 

 of the alimentary tract. In the annelid the blood flows forward in a 

 dorsal vessel, down through paired connectives, and back through 

 a ventral vessel; by reversal this results in a flow similar to that 

 of the vertebrates, forward from the hc^art through tlie ventral 

 aorta, dorsad through the paired aortic arches, and back through 



OI'raMHV 



Fig. 8.3. — Reversible diagram illustrating the annelid theory of vertebrate 

 origin. Index letters applying to both forms: S, brain; X, nerve cord; H, 

 alimentary canal, applying to annelid only; m, mouth; a, anus; applying to 

 vertebrate only; st, stomodaeum; pr, proctodaeum; nt, notochord. (From 

 Wilder 's History of the Human Bodij, with the permission of Mrs. H. H. 

 Wilder and Henry Holt and Company.) 



the dorsal aorta. The typically vertebrate notochord finds .its 

 counterpart in the Faserstrang, a fibrous structure with similar 

 anatomical relations which appears in the annelid. 



While the idea is no more than a theory, we cannot fail to see in 

 it the possibility that vertebrates may have arisen through a form 

 similar to Amphioxus from some invertebrate with annelid char- 

 acters. 



Origin of Vertebrate Classes. Given the existing classes of 

 vertebrates, there is much evidence to explain their common 

 sources. The cyclostomes are probably derived from remote and 

 unknown ancestors. The remaining vertebrates are known as the 

 Gnathostomata, or hinge-mouthed animals, and come undoubtedly 

 from a common source. The sequence from fishes to amphibia, 

 thence to reptiles, and from this class by different lines of descent 

 to the birds and mammals is well marked. Whether we trace 

 entire groups with all available evidence, or single organs and sys- 

 tems alone, the succession is in most cases clear and well sub- 

 stantiated. 



Emergence of Terrestrial Vertebrates. From the most primi- 

 tive vertebrates, the fishes, the initial step is one of the greatest 



