188 ARTHUR WILLEY. 



Ou this ridge are developed typically six oval thickeniugs or 

 enlargements, alternating witli the primary slits and appearing 

 simultaneously. They consist of a fusion of the pharyngeal 

 wall with the body-wall at six different points, the first point 

 lying above and between the third and fourth primary slits. 

 They are the forecast of the second row of slits. Between the 

 ridge with its nodal thickenings and the row of primary slits 

 a longitudinal blood-vessel is seen to hold its course; this 

 becomes eventually the ventral or subintestinal vessel which 

 lies beneath the endostyle (see Lankester, 5) . 



The first and last thickenings, as Kowalewsky also noticed, 

 are usually rather smaller than the intervening ones. 



Slight deviations from the above mode of procedure occur. 

 One of these is shown in fig. 2, where only five secondary 

 thickenings have appeared at once, the first of them occurring 

 between the fourth and fifth primary slits, while the one which 

 usually has the first place at this stage has been somewhat 

 retarded in its development. 



In other cases, in which there are also only five thickenings, 

 it may be the sixth that is late in appearing. 



In the larva represented in fig. 2 there were only twelve 

 primary slits. As mentioned above, the number of primary 

 unpaired slits which are formed in the larvae varies from twelve 

 to fifteen. The most usual number is perhaps fourteen, while 

 fifteen is exceptional. 



In front of the gill-slits is seen the club-shaped gland, 

 closely apposed to the remarkable patch of modified hypo- 

 blastic epithelium which, in the paper by Professor Lankester 

 and myself (8) describing the larva, was termed the glandular 

 tract. The nature of this organ has been enigmatical up to 

 the present time ; but I may as well at once call it the 

 " endostyle," since it certainly becomes that organ. 



The opening of the club-shaped gland into the cavity of the 

 mouth — i. e. the intra-buccal orifice — lies at this stage slightly 

 dorsal, and at a later stage quite dorsal, to the endostyle. 

 This position of the internal aperture of the gland is important, 

 as will appear in the sequel. 



