216 ARTHUR WILLEY. 



rotation (which, as above indicated, need only be a virtual 

 phenomenon) may only have afifected at first the most anterior 

 portion of the pharynx, namely, the portion containing the 

 endostyle, club-shaped gland, and first gill-slit. 



In the ventral view shown in fig. 10 it will be noticed how 

 the primary slits curve round posteriorly to the ventral middle 

 line. It may be that formerly this tendency of the hinder 

 slits to lie in the direction of the left side, though now only 

 reaching the middle line, was more accentuated, and extended 

 to the more anterior slits. The position of the primary slits 

 which succeed the first on the right side has thus to be accounted 

 for, and this is done by assigning the cause of it to the further 

 adaptation which led to the huge size of the lateral mouth. 



Having got on to the left side in the way above described, 

 it was found advantageous to have as large a mouth as circum- 

 stances would permit, and accordingly the capacious mouth of 

 the larva was established by the action of natural selection. 

 A glance at the figures should carry with it the conviction that 

 where the mouth is, there the gill-slits could not be. 



Consequently the gill-slits which succeed the first develop, 

 like it, on the right side, though, as we have seen, behind the 

 region of the mouth they continue to retain a ventral position. 



The lateral position of the mouth, then, is correlated with the 

 forward extension of the notochord, and the unilateral position 

 of the primary gill-slits with the lateral position of the mouth. 



The rotation of the original left side gill-slits on to the right 

 side has led, as might have been expected, to the temporary 

 obliteration of the slits which properly belong to the right 

 side, but it is not so much an obliteration as a retardation in 

 development that has been efi"ected. Thus, in a larva with 

 apparently only a single series of gill-slits, there is potentially 

 a double series ; but, owing to the force of conditions which 

 have been secondarily induced, one of the two rows is some- 

 what late in putting itself in evidence. 



That we have to do here with a retardation in the develop- 

 ment of a whole series of gill-slits is certainly proved by the 

 fact that when the secondary slits do appear, they form mainly 



