146 DEVELOPMENT OF AAIPHIOXUS. 



level than previously (Fig. 82). The posterior groove of 

 the prasoral pit which we described in the last stage, seems 

 to be preparing the way for the mouth to dip inwards 

 towards the right wall of the pharynx, which, in fact, it has 

 actually begun to do. 



At a still later stage, corresponding to that shown in 

 Fig- 77> tne shape of the mouth has become entirely altered 

 (Fig. 83). 



It has now the form of a triangle with the apex directed 

 backwards and the base standing vertically in front. But 

 the apex and the base are not in the same tangential plane, 



Fig. 82. Anterior portion of larva somewhat older than preceding, to show 

 commencing adjustment of the mouth. (After WlLLEY.) 



e. Endostyle seen through the mouth. Other letters as above. 



the former being on the left side of the body, and the latter 

 much deeper inwards ; in fact, just below the skin on the 

 right side of the body. (Cf. Fig. 77.) 



We see, therefore, that the longitudinal diameter of the 

 larval mouth is gradually shortening. It is eventually 

 reduced to zero when the right and left sides of the mouth 

 or velum come to lie opposite to one another, the velum 

 ultimately attaining a circular form and a median sub- 

 vertical position underneath the oral hood. When the 

 larva has reached the age to which Fig. 1 1 refers, the right 



