LAR VAL DE VEL OPMENT. 



157 



gill-slits and the prseoral pit. As described in the fore- 

 going pages the asymmetry of the prasoral pit is a second- 

 ary occurrence, since it arises at first as one of a pair of 

 symmetrically disposed head-cavities, or anterior intestinal 

 diverticula, while the unilateral asymmetry of the gill-slits 

 is ontogenetically primary. The unilateral gonads of the 

 species of Amphioxus from the Bahamas and Torres 

 Straits also belong to this category. 



Although, on account of their essentially azygous nature, 

 the mouth and endostyle have been separated from the 

 gill-slits in the above classification, it is obvious that their 

 asymmetrical position in the larva must be ascribed to one 

 and the same cause. In the succeeding pages we shall 

 endeavour to demonstrate what this cause was. 



Explanation of Asymmetry of Mouth and Gill-slits. 



It is quite evident that the primary gill-slits which 

 appear on the right side of the larva belong primitively, 

 or ancestrally, to the left side, to which, in fact, they are 

 eventually transferred. Meanwhile, the left side of the 

 larval pharyngeal region is largely occupied by the huge 

 oral aperture. 



We may figure to ourselves the primitively left-side gill- 

 slits being carried over to the right side by a semi-rotation 

 from left to right of the pharyngeal wall. The primitive 

 right side of the pharynx would thus be crowded out, so to 

 speak, and the right-side gill-slits would be temporarily 

 obliterated owing to lack of room, while the original mid- 

 ventral line would be carried high up on the right side, 

 where, in point of fact, it is plainly indicated by the bran- 

 chial artery, which lies actually above the primary gill-slits 

 in the larva (Fig. 74, etc.). 



