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THE AMERICAN NATURALIST [Vol. XL 



The great difficulty in the way of accepting any of the well de- 

 veloped jointed invertebrates as the ancestral vertebrate type lies 

 in the necessity of a revolution of the body through an angle of 

 180 degrees, whereby the dorsal surface of the invertebrate be- 

 comes the ventral surface of the vertebrate; and also in the con- 

 comitant necessity of the formation of a new mouth and the total 

 disappearance of any trace of the old mouth. While there is, at 

 the present time, abundant evidence to show that the functional 

 mouth of the vertebrate of to-day is a neomorph, and that the origi- 

 nal mouth was situated at a point anterior and dorsal to the pres- 

 ent location, it does not follow and the evidence does not tend to 

 show, that the old mouth corresponds in any way to the mouth, 

 e. g., of the arthropod. 



This translocation of the mouth from the invertebrate ventral 

 to the vertebrate ventral surface must have been connected with 

 the reduction of the circumoesophageal nerve ring, and with the 

 total disappearance of that section of the stomodseum which con- 

 nects the mesenteron with the mouth by preforating the nervous 

 system through the territory of the circumoesophageal nerve ring. 



The fact that the stomodseum no longer perforates the nerve 

 ring is a fact which must be satisfactorily explained by in some 

 way discovering the stages through which the transformation has 

 passed from the invertebrate to the vertebrate condition, or the 

 genealogy of the vertebrate stock with the arthropod as the an- 

 cestral form cannot be satisfactorily explained. 



There is great doubt that the vertebrates are derived from a 

 highly organized annulate invertebrate. They are more probably 

 a distinct branch split off from the unsegmented worms, and de- 

 veloped independently. Many are the theories which have been 

 offered to harmonize the annulate and the arthropod conditions 

 with the vertebrate, but none of them have accounted for a suffi- 

 cient number of facts to warrant their general acceptance, and, as 

 above stated, the main difficulty has consisted in the inability to 

 picture the revolution of the invertebrate body in such a w^ay as 

 to make it physiologically possible in living forms. 



It matters not what position we take with reference to the origin 

 of the vertebrate stock, when we arrive at the stage of development 

 represented by amphioxus we are compelled to admit, in the light 



