LARVAL DEVELOPMENT. 141 



The next stage to be considered is characterised above 

 all by the simultaneous atrophy, closure, and disappearance 

 of the club-shaped gland, and the first primary gill-slit 

 (Fig. 79). At this stage the increase in size of the 

 secondary slits has progressed to such an extent that the 

 primary slits have been displaced entirely from their 

 original position, and are no longer to be seen from the 



.5.0 



Fig. 79. Anterior portion of larva from right side after the disappearance of 

 the club-shaped gland. (After WlLLEY.) 



s.o. Sense-organ, e. Endostyle. p.b. Peripharyngeal band. s.s'. First secondary 



slit. 



right side, except in the case of the hindermost slits of 

 the series, which remain, as mentioned above, in a median 

 ventral position until their disappearance. 



A larva seen from below, so as to show the relative 

 positions of the gill-slits and endostyle, etc., at this stage, 

 is represented in Fig. 80. 



It is obvious, from what has been said above, that in the 

 passage of the primary slits from their original position on 

 the right side of the body to their final position on the left 

 side, their dorsal and ventral margins are reversed. What 

 was at first the dorsal edge of a primary slit becomes its 

 ventral edge, and vice versa. In other words, what is 

 actually the dorsal border of the primary slits in Fig. 74 

 is morphologically the ventral border ; and conversely, what 

 is actually the latter is morphologically the former ; and it is 



