196 AKTHDR WILLED. 



alteration in form, the actual size of the mouth does not 

 materially change ; though in the adult, of course, the relative 

 size of the mouth or velum is much less than in the larva. 



The variations in the condition of the secondary slits at this 

 stage are not sufficiently striking to make it necessary to 

 tabulate them. Their number, as a rule, varies between seven 

 and eight ; and almost invariably the first secondary slit 

 appears at this stage between the second and third primary 

 slits (fig. 5). 



Stage v.— Figs. 7—10 and 18. 



There is a considerable difference between the condition of 

 a larva at the beginning of this stage and at the end of it, as 

 will be seen by comparing fig. 7 with fig. 10. The constant 

 peculiarity for the stage is, that although the primary slits 

 have not quite attained their final position on the left side, 

 yet the tongue-bars have commenced to grow down from the 

 dorsal borders of the slits, but do not meet the ventral borders 

 during this stage. There is also a feature which is more 

 obvious in the preceding stages, and which we see for the last 

 time in the present stage. I refer to the fact that the long 

 axis of the secondary slits is at this and the foregoing stages 

 parallel to the long axis of the body, and at right angles to 

 the long axis of the several primary slits. This is the case 

 even in fig. 10, where, however, one would probably fail to see 

 it at a glance. It is important in that it is characteristic of 

 the earlier stages in this period of the development. 



In fig. 7 a view is given of the right side of a larva which 

 had just entered on this stage. There are eight secondary 

 gill-slits, the first, as one would expect, being between the 

 second and third primary slits. The tongue-bars, as we have 

 already seen, are in course of formation, and that of the third 

 secondary slit has actually fused with the ventral border of 

 the slit. Unfortunately in the larva here figured (fig. 7) the 

 endostyle is not typical for this stage, being in the condition 

 described in Stage IV. A characteristic endostyle is, however, 

 shown in ficr. 18. It has become much more horizontal in 



