THE ORIGIN AND EVOLUTION OF THE VERTEBRATES 77 



bly some sort of primitive paired appendages. That such an ances- 

 tral form will ever actually be discovered is highly improbable, for 

 the annelids and arthropods , and probably also the vertebrates, were 

 old at the dawn of Cambrian times, and the pre-Cambrian creatures 

 are not preserved for our edification. It would, however, be unfair 

 to leave the discussion of the ancestry of the vertebrates from annelids 

 or arthropods without presenting some of the evidences that have 

 been advanced in support of these theories. 



THE ANNELID THEORY OF VERTEBRATE ANCESTRY 



The chief basis for this hypothesis lies in the unmistakably close 

 resemblance between the embryonic characters of the vertebrates and 

 some of the structural peculiarities of the annelid worms. It has al- 

 ready been shown that the vertebrate embryo is distinctly meta- 

 meric, especially in the ccelom and its derivatives. The nephridia of 

 the lower vertebrates are ccelomoducts with the same relations as 

 those of the annelids. The vascular system, the nervous chain of 

 paired ganglia, the relations of the intestine, nervous system, and 

 circulatory system, are so much alike that a diagram (Fig. 41) of 



St VERTEBRATE 



FIG. 41. Reversible diagram illustrating the Annelid Theory. Reversible 

 designations applying to both forms: S, brain; x, nerve cord; H, alimentary canal. 

 Designations applying to annelid only: m, mouth; a, anus. Designations applying 

 to vertebrate only: st, stomodseum; pr, proctodseum; nt, notochord. (From 

 Wilder.) 



these structures for the annelid serves, if inverted, as a diagram of a 

 vertebrate. A vertebrate is said to be merely an annelid turned over 

 on its back and with certain minor changes to adjust the animal to 

 the new position, such as the development of a new mouth and a new 

 anus. Such positional reversals are not without parallel, for the squid 

 is reversed as compared with other mollusks, and a king-crab (Lim- 

 ulus) swims upside down. The chief stumbling-block of the anne- 

 lid theory is the notochord of the vertebrate; even this is not un- 

 surmountable for there has been found in annelids a Faserstrang, 

 which is described by Wilder as " a bundle of fibres running along the 



