14 JEAN DAWSON. 



gill sac (Fig. 8, r), arise on the medial one fifth of the dorsal and 

 ventral borders of the muscular pouch. From this origin the 

 fibers spread over the caudal and cranial surfaces of the gill 

 sac not covered by the external compressor. They form arches 

 whose concavities are directed toward the median plane. These 

 muscles function to compress the gill sac while shortening its 

 major axis. 



The muscular pouches are placed side by side so that the 

 cranial wall of each and the caudal wall of the next in front of 

 it are in contact. So close do they lie that there appears to be 

 but a single wall separating the contents of the successive pouches 

 from one another (Fig. 5). Most writers indeed speak of the 

 septa between the gill sacs without recognizing their double 

 nature and the resulting muscular pouches. The double nature 

 of these walls can be detected only by the microscope. They 

 are very thin toward the center but become much thicker 

 toward their borders. In preserved specimens a great abundance 

 of coagulated lymph is found between the gill sac and its pouch. 

 The muscular pouches are supported at their external openings 

 by the small rings of cartilage already mentioned as lying close 

 to the last seven vertical bars of the branchial basket (Fig. 14, b}. 

 These vertical and longitudinal bars of this basket lend support 

 to the pouch on its lateral, dorsal and ventral walls. The 

 medial wall is supported near its center by the wall of the water 

 tube with which it is continuous and by cartilage at its extreme 

 dorsal and ventral ends. These muscular pouches are placed in 

 the cartilaginous basket very obliquely with the medial ends of 

 the major axis craniad of the lateral ends. This added to the 

 obliquity of each gill sac in its pouch causes the gill sacs to over- 

 lap each other like shingles on a roof. Nowhere can a cross 

 section be made through the gill region without cutting two gill 

 sacs. A line connecting the internal and external gill openings 

 of any gill sac thus makes an angle of 45 with the long axis of 

 the body. 



The fibers of the muscular gill pouch are very difficult to 

 follow on the outside of the pouch on account of the pigment 

 found deposited there, but if the pouches be turned inside out, 

 the fibers may be plainly seen crossing the flattened caudal and 



