LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 203 



as the fate of the tenth primary slit has been decided. In 

 fig. 16 the ninth slit has not perfectly assumed its position 

 on the left side, and so its permanence cannot be considered 

 as being beyond question, although its size and appear- 

 ance would lead one to think that it would persist and not 

 close. 



The length or vertical height of the gill-slits has further 

 increased, and has been taken as the distinguishing feature of 

 this stage. 



Fig. 17 represents the last (seventh) pair of slits of a larva, 

 in which not only had the tenth primary slit closed, but the 

 ninth was also on the point of closing. The seventh slit of the 

 right side is seen with a surface focus, the ninth rudimentary 

 primary slit is seen with a median focus, and the seventh slit 

 of the left side, which is doubling in, is seen with a deep focus. 



The mouth or velum (fig. 16) is now anterior and median in 

 position; the right and left walls of it are quite opposite to one 

 another, and thus the apparent length of the mouth in side 

 view is now reduced to zero. 



The oral hood and buccal cirri have not yet, however, attained 

 their final condition. 



The endostyle (fig. 16) has greatly increased in length, 

 extending to the fifth gill-slit on this side. 



The right side of another larva of this stage is shown in 

 fig. 15, chiefly for the sake of the endostyle. This is the last 

 stage at which its double origin can be recognised. The right 

 half of the endostyle is now seen to lie at the side of, and not 

 above, the left half. 



The ciliated hyperpharyngeal band appears to die out behind 

 the pharynx. On this side there are eight (secondary) slits : 

 on the left side of this larva there were also eight (primary) 

 slits, and no indication of any more, so that it is a typical 

 example of a larva which has entered upon the critical 

 period. 



It is important to emphasise the fact that the critical period, 

 in which the larva has usually eight pairs of slits, has a con- 

 siderable duration, in the course of which the only changes 



