RESPIRATION. 233 



integument or mucous membranes, which contain an abundant 

 supply of bloodvessels, and which hang out freely into the sur- 

 rounding water. In many kinds of aquatic lizards, as, for exam- 

 ple, in menobranchu-s (Fig. 67), 

 there are upon each side of the 

 neck three delicate feathery 

 tufts of thread-like prolonga- 

 tions from the mucous mem- 

 brane of the pharynx, which 

 pass out through fissures in 

 the side of the neck. Each 

 tuft is composed of a prin- 

 cipal stem, upon which^the HEAD AXD GILLS OF MEXOBBAircHi;- . 

 filaments are arranged in a 



pinnated form, like the plume upon the shaft of a feather. Each 

 filament, when examined by itself, is seen to consist of a thin, rib- 

 bon-shaped fold of mucous membrane, in the interior of which 

 there is a plentiful network of minute bloodvessels. The dark 

 blood, as it comes into the filament from the branchial artery, is 

 exposed to the influence of the water in which the filament is 

 bathed, and as it circulates through the capillary network of the 

 gills is reconverted into arterial blood. It is then carried away by 

 the branchial vein, and passes into the general current of the cir- 

 culation. The apparatus is further supplied with a cartilaginous 

 framework, and a set of muscles by which the gills are gently waved 

 about in the surrounding water, and constantly brought into con- 

 tact with fresh portions of the aerated fluid. 



Most of the aquatic animals breathe by gills similar in all their 

 essential characters to those described above. In terrestrial and 

 air-breathing animals, however, the respiratory apparatus is situated 

 internally. In them, the air is made to penetrate into the interior 

 of the body, into certain cavities or sacs called the lungs, which 

 are lined with a vascular mucous membrane. In the salamanders, 

 for example, which, though aquatic in their habits, are air-breathing 

 animals, the lungs are two long cylindrical sacs, running nearly the 

 entire length of the body, commencing anteriorly by a communi- 

 cation with the pharynx, and terminating by rounded extremities 

 at the posterior part of the abdomen. These lungs, or air-sacs, 

 have a smooth internal surface ; and the blood which circulates 

 through their vessels is arterialized by exposure to the air contained 

 in their cavities. The air is forced into the lungs by a kind of 



