49 



come to separate barij or lilainonts attaciied ut both ends 

 (fig. 2G). And aa the fold, when it appears, is attached 

 at the point of insertion of the mantle on the sides of 

 the body, and the ventral edfje fuses v/ith the upper part 

 of the foot the gill-bars or filaments lie almost hori- 

 zontally in the mantle chamber. 



This mode of formation of gill-slits in a fold which 

 ±\ises continuously, at first with the sides of the body 

 and visceral mass (fig. 3) and later, v/ith its fellow of 

 the opposite side as well (fig. 6) is kept up during lil'e. 

 However, beginning with stages still loss than 1 mm. long, 

 the process is modified as follo'.vs:- In specimens less 

 than 1 mm. long (fig. 3) the gill of either side consists 

 of a membrane with a series of gill slits which decrease 

 in size from before backwards. When, however, there are 

 about fifteen slits in the series, a perforation in tlie 

 gill-fold or membrane appears opposite and internal to the 

 tenth (usually, rarely ninth or eleventh) slit of the 

 first series. Mew ones are added in succession posterior 

 to it, so that a second series of slits comes to be formed 



internal to the first (fig. 6). At the posterior end the 



la^-s 

 intern'-il always slightly behind the extei-nal in its devel- 

 opment. As shown in I'ig. 6, there are no slits in the in- 



