LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 



193 



when all the secondary slits^ are established^ the one that stands 

 first in position is not the first formed, but arises rather later 

 than the six slits which follow it. The usually late appearance 

 of the first secondary slit is possibly correlated in some way 

 with the complicated growths which are taking place in the 

 anterior region, and is not of any special significance ; while, on 

 the other hand, the simultaneous appearance of the greater 

 number of the secondary slits is of considerable importance, as 

 will appear later. 



To bring the complicated nature of the transformation which 

 is being effected vividly before the mind, it may be noted that, 

 while the mouth is moving forwards, the endostyle is growing 

 backwards, and the primary gill-slits are crossing bodily over to 

 the left side. 



The oral aspect of a larva at this stage (fig. 25) presents no 

 particularly new features, but merely an extension of the pro- 

 cesses described in the preceding stage as being in operation ; 

 thus the anterior portion of the mouth is seen at a still 

 deeper focus, the apparent length of the aperture of the mouth 

 when seen from the side has decreased, and the buccal 

 skeleton has further advanced in development. 



The following table comprises a few observations chosen out 

 from a number to show some of the variations that are met 

 with in this stage : — 



' The primary slits form, as has been said, to the maximum number of 

 fifteen. The secondary slits never normally exceed the number of nine. By 

 far the greater number of the gill-slits of an adult Amphioxus must, there- 

 fore, be distinguished by the name of tertiary slits. The reason of this will 

 become clear as we proceed. 



