6 CHORDATA. 



and that its present position outside it and on the ventral surface 

 is a secondary one, due to shifting and to atrophy of the part of 

 the nerve rudiment in front of it. There are a number of 

 features of vertebrate morphology of the highest interest in 

 connection with this point : such are the cranial flexure, the 

 close relation of the infundibulum, which there is good reason 

 to believe is the real front end of the nerve axis, to the anterior 

 end of the mouth, the slit-like form * (Fig. 2), which at first 

 characterizes the buccal opening, and its extension into the rudi- 

 ment of the pituitary body ; but there is no actually ascertained 

 fact which tends to show that the mouth is a derivate of the 

 blastopore, as it must be conceded to be in most other coelomate 

 phyla, t 



The last chordate character to be considered is the posses- 

 sion of lateral pharyngeal apertures. These are often used 

 for respiration and are in consequence generally termed gill- 

 slits. They are not however always respiratory in function 

 indeed in the majority of the Vertebrata in which they 

 form a very conspicuous feature they are not respiratory at 

 all, but are entirely functionless, being found only in the 

 embryo. 



An attempt has been made in some quarters to refer the 

 chordate mouth to a modification of a pair of these structures. 

 We can see no fact in favour of such a view, and we are not 

 prepared to give up the homology of the chordate mouth with 

 that of other Coelomata. We have already stated the case with 

 regard to its relation to the blastopore, and we have seen that 

 there is no good embryological evidence in favour of its being 

 so related, but we do not consider that this absence of evidence 

 is sufficient to put out of court the view that it is the homologue 

 of the mouth of other Coelomata. In many of these, too, no 

 relation can be shown between the mouth and the blastopore in 

 development, but yet we well know that in them the mouth 

 is homologous with the mouth of forms in which it is directly 

 derived from a part of the blastopore. 



Finally there is one point in the morphology of the Chordata, 



* Sedgwick, "Notes on Elasmobranch Development," Q.J.M.S., 33, 

 1892, p. 559. 



f For a fuller discussion of these questions, the reader is referred to the 

 article "Embryology" in the recently issued supplement of the Encyclo- 

 paedia Britannica. 



