5 i8 HISTORY OF THE HUMAN BODY 



and its oesophagus, furnished by fourth ventricle and neuro- 

 pore, there is also the hypophysis, or rather, that portion of it 

 that is pushed up from the alimentary canal, for which there is 

 yet no satisfactory explanation. Its origin from the alimentary 

 canal, its constant appearance in all vertebrates, its relationship 

 to the nervous system and its position, all suggest that it also 

 is a remnant of an early oesophagus. 



The formation of a new, ventrally placed anus is due to a 

 procedure similar to that which forms the new mouth, although 

 there is here no suggestion of a gilt-slit. The vertebrate anus 

 arises as a mid-ventral inpushing of the ectoderm at some dis- 

 tance from the end of the tail, and thus reaches the primary 

 intestine along its course, leaving beyond it a piece of consid- 

 erable length, the post-anal gut, which soon atrophies. This 



HI 



FIG. 140. 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, stomatodaeum; pr, proctodaeum; 

 nt, notochord. 



phenomenon, inexplicable by other means, is easily explained 

 by the postulate of an annelid ancestor, for in these animals the 

 anus is at the posterior extremity of the body, and the forma- 

 tion of a new anus in the vertebrate position would actually 

 leave just such a piece as the one in question. 



The gills of aquatic vertebrates receive also an adequate ex- 

 planation through the annelid hypothesis. The annelid gills 

 are external duplicatures of the integument, and occur upon 

 the sides of every somite, attached to the parapodia, or short 

 locomotor organs. In simple forms they are plates, but when 

 specialized they become fringed or dendritic and somewhat re- 

 semble the external gill-bushes of amphibians. Although pri- 

 marily distributed along the entire body, in certain specialized 



