BREATHING MECHANISM OF THE LAMPREYS. 



that which passes through the internal openings of the same pair 

 of sacs. The principal axis of each gill sac, the axis connecting 

 the branchipores, is thus oblique to the long axis of the body 

 and is directed from its medial end, caudo laterad (Fig. $,f). 

 The gill sac also crosses its muscular pouch obliquely. Its in- 

 ternal opening lies near the cephalic margin of the medial end of 

 the pouch, while its external 

 opening lies near the caudal 

 margin of the lateral end of the 

 pouch. The muscular pouches 

 are larger than the gill sacs, so 

 that the two lie in contact only 

 at their openings, where they are 

 united (Figs. 5 and 8). 



When the gill sacs are exam- 

 ined from the outside, the lines 

 of attachment of the gill lamellae 

 are seen (Fig. 8). While the 

 walls of the gill sacs are thus 

 transparent, they are neverthe 

 less covered with a layer of mus- 

 cle fibers (Fig. 8). These fibers 

 are very delicate and form a thin 

 layer which spreads over the gill 

 sac like a spider's web and might 

 easily escape notice. There are P ouch - The P ouch is cut near its lateral 



- , , end and reflected from the sac. a, gill 



two of these muscles on each , .,, , , , , 



sac; b, gill pouch; c, ental muscle; a, 



side of the Sac. They have not lines of the gill lamellre showing through 

 hitherto been described. One the gill sac ;*, internal compressor muscle 



, , 7 of the gill sac ; /, deep compressor mus- 



muscle, tlie external compressor 



cle of the gill pouch ; g, external com- 



of the gill sac (Fig. ,/), is a pressor of the gill sac . 

 narrow band near the external 



opening of the sac. Its fibers arise from the lateral one fifth of 

 the dorsal and ventral border of the gill sac and form over its 

 surfaces arches with their concavities toward the external open- 

 ing. The muscle functions in compressing the external one fifth 

 of the gill sac. 



The fibers of the larger muscle, the internal compressor of the 



FIG. 8. Gill sac with its muscular 



