LATER LARVAL DKVBLOPMBNT OF AMPHTOXUS. 233 



Feeble traces of the first primary slit can just be detected iu front. Tlie 

 twelfth primary slit is again seen to be closing. Notice the series of primary 

 slits curving round to the mid-ventral line behind. The tongue-bars are 

 complete in most of the secondary slits, but in none of the primary slits. 



Fig. 11. — Stage VI. Left side. Nine (primary) slits, the first being of 

 course originally the second primary ; but it has come to be the first in position 

 in consequence of the atrophy of the true first. Behind there are traces of the 

 eleventh and twelfth primary slits. Only the left half of the velum can now 

 be seen from this side. I do not know what has become of the nephridium 

 of Hatschek; x is probably part of the ciliated tract of the prgeoral pit. 

 Notice the left half of the endostyle which extends under three slits ; the right 

 half is out of focus. In this larva there were eight secondary slits on the 

 right side. 



Fig. 12. — Stage VI. Left side. This shows the twelfth and thirteenth 

 primary slits in course of closure. The condition of the gill-slits is not quite 

 so far advanced as in the preceding figure, but the buccal cirri are farther 

 advanced. There were seven (secondary) slits on the right side. 



Fig. 13. — Stage VII. Left side. In this larva the ninth slit (= tenth 

 primary) would probably have closed. The eleventh primary slit is represented 

 by a rudiment. Cgecum is commencing to bulge in the region of the twentieth 

 myotome. Notice the length of the endostyle. The buccal skeleton is 

 beginning to grow into the upper portion of the oral hood. 



Fig. 13 «. — Separate view of the velum of the same larva, showing the 

 four primary velar tentacles. The right and left halves of the velum are seen 

 to be not quite opposite to one another. 



Fig. 14. — Stage VII. Right side. The slits are commencing to elongate in 

 a direction at right angles to long axis of body. There are eight (secondary) 

 slits on this side. On the left side of this larva there were eight (primary) 

 slits, and there was also a median rudimentary slit on the point of closure, 

 viz. the tenth primary, which sometimes persists, as explained above, as the 

 ninth slit of the left side. The view of the endostyle is very important, 

 showing its double nature — upper and lower halves. It has grown consi- 

 derably in length. The right metapleur is omitted. The floor of the atrium 

 consists of the expanded subatrial ridges. 



Fig. 15. — Stage VIII. Right side. In this larva there were eight slits on 

 each side and no rudimentary one behind, so that this is typical of the critical 

 period. The view of the endostyle is again very important. The two halves 

 have now assumed a definite right and left position. Its double origin can be 

 detected here for the last time. Notice in this and the preceding figures the 

 hyperpharyngeal baud. Right metapleur is omitted. 



Fig. 16. — Stage VIII. Left side. Nine gill-slits ; the ninth has not fully 

 come round to this side, but bends under the pharynx. There were eight slits 



