BREATHING MECHANISM OF THE LAMPREYS. 15 



cranial walls (Fig. 8). These fibers form arches convex toward 

 the external opening and end on a narrow raphe on the dorsal 

 and ventral walls of the pouch. The action of these muscles is 

 to compress the whole muscular pouch and to cause its lateral 

 end to be drawn toward the water tube thus greatly shortening 

 its long axis and that of the gill sac. 



On the inner surface of the gill pouch, at about the junction of 

 the middle and lateral thirds of its major axis, are found strong 

 bands of muscular fibers, the deep compressors of the gill pouches, 

 lying beneath the fibers of the pouch proper. They arch in the 

 same general direction as those already described and end on the 

 dorsal and ventral borders of the pouch as do the muscle fibers 

 of the pouch itself. In the center of the arch, however, some of 

 the fibers runout diagonally toward the external gill opening and 

 are attached to the surface of the gill sac at about the junction 

 of the fourth and lateral fifths of its longer axis. They function 

 in compressing the gill pouch and in fastening the gill sac to the 

 pouch. These bands are found on both sides of the pouch, but 

 are much larger on the cranial side. This may be due to the 

 fact that the gill sac is so placed in its pouch that a greater space 

 is found between the pouch and the sac on the cranial side 

 than on the caudal side. On the caudal side of the internal open- 

 ing may be seen many blood vessels (branches of the afferent 

 branchial artery entering the gill sac) and these also serve to 

 fasten the sac to the pouch at this point. 



If a gill sac be turned inside out, the gill lamella; may be seen 

 projecting from the inner wall and lying close together like the 

 leaves of a book (Figs. 9 and I i). The lamellae are found 

 mostly upon the flattened cranial and caudal surfaces of the sac. 

 The lateral or distal end of the sac is but slightly encroached 

 upon by them. The medial end shows a smooth lenticular space 

 around the internal gill opening with its long axis dorso-ventrad. 

 From the edge of this space the lamellae extend toward the ex- 

 ternal gill opening. Those at the middle of the caudal and 

 cranial walls of the gill sac (/. e., those lying in a direct line be- 

 tween the gill openings) are the longest. Thence they diminish 

 in length dorsally and ventrally on both sides until those near 

 the dorsal and ventral borders of the gill sac are only about 5 



