LATER LARVAL DEVELOPMENT OP AMPHIOXUS. 201 



end into the cavity of the mouth. (See also 8, pi. xxx, 

 figs. 2—4.) 



It requires, and would probably repay, further study. From 

 the present stage onwards I could not recognise this so-called 

 nephridium in the larvse. The structure marked '' x" in the 

 figures is probably part of the ciliated tract of the prseoral pit, 

 seen through the oral hood. 



In fig. 19 a view of the endostyle is given. It does not 

 difi'er markedly from its condition in Stage Y, extending about 

 the same distance backwards between the anterior secondary 

 and primary slits, but the absence of the club-shaped gland (of 

 which feeble remnants are still shown) has to be noticed. 



The number of secondary slits in this stage varies again 

 from seven to nine. 



Stage VII.— Figs. 13, 13«, and 14. 



The slits — primary and secondary — have now begun to elon- 

 gate in a direction at right angles to the long axis of the body. 



A view of the left side of a larva is given in fig. 13. The 

 first slit on this side (i. e. the original second primary slit) is 

 simple and remains simple in the adult, while all the others 

 become doubled by the formation of the tongue-bars. 



The ninth slit of this side is very small and lies at a rather 

 deeper focus than the other slits, so that in this case it would 

 in all probability close. Behind the ninth (i. e. the tenth 

 primary) is seen a rudimentary indication of the eleventh 

 primary slit. 



A separate view of the velum with the four velar tentacles 

 is given in fig. 13a. Fresh tentacles arise between the four 

 primary ones at a much later stage, when the gill-slits are 

 about three times as long as they are now, and the endostyle 

 twice as long. The final number of velar tentacles is twelve. 



The right wall of the velum is not quite opposite to the left 

 wall even yet : it is slightly in front of the left wall, and the 

 extent to which it is so corresponds to what has been hitherto 

 spoken of as the apparent length of the aperture of the mouth. 



On the right side (fig. 14) it is usual to find eight slits. The 



