LAR VAL DE I 'EL OPME.\ ' 7 '. 



137 



primary slits. This new slit constitutes the definitive first 

 slit of the secondary series (Fig. 77). 



The larva shown in Fig. 77 presents a very different 

 aspect from that shown in Fig. 74; the transition from one 

 stage to the other is, of course, gradual, and all intermediate 

 steps can be observed. In the stage which we are now 

 considering (Fig. 77), the atrial cavity has become com- 

 pletely closed up in front, so that now none of the gill-slits 

 open directly to the exterior. 



None of the primary slits now lie entirely on the right 

 side, but they have become bent under the pharynx, and 



Fig. 77. Anterior portion of larva, in which the secondary slits have become 

 perforated, and the primary slits are passing round to the left side. From the right 

 side. (After WlLLEY.) 



i.o. Sense-organ of preeoral pit. v. Right half of velum, e. Endostyle, grow- 

 ing beyond the club-shaped gland gl. p.s . First primary slit, much reduced in 

 size. s.s ' . First secondary slit. /.J 12 . Twelfth primary slit, behind which is to be 

 seen a vestige of the thirteenth slit. 



thus extend round to the left side. This bodily migration 

 of the primary slits from one side to the other occurs in 

 correlation with the increase in size of the secondary slits, 

 which, as they continue to grow, push, as it were, the 

 primary slits before them, and so cause the latter to bend 

 under the pharynx in the way described. The peculiar 

 growth by which the primary gill-slits are gradually carried 

 from the right to the left side, may be described as a trans- 

 verse or rotatory growth affecting the pharynx in toto in 



