GILLS OR BRANCHI^ 257 



the oxygen needed by the blood. As a rule this water is drawn into 

 the mouth by the enlargement of the oral cavity, and by its contrac- 

 tion it is forced out through the clefts, passing over the gills on its 

 way. In the myxinoids the oesophageo-cutaneous duct is supposed to 

 act as the incurrent passage when these animals have the front of the 

 head immersed in the flesh of a fish. In the lampreys the water is 

 said to be taken and forced out through the gill clefts when these 

 animals are attached to some object. In many of the elasmobranchs 

 the spiracle serves as an incurrent opening, and it is provided with a 

 valve developed from the anterior wall, which closes and prevents any 

 backflow. Both sturgeons and Polypterus have spiracles through- 

 out life. 



In the amphibia the gill pouches are formed in the same way as in 

 the fishes, but the first and the fifth never break through, and all 

 clefts are usually closed in the adult, the exceptions being the per- 

 ennibranchs and the derotremes, where from one to three external 

 openings persist through life. In caecilians and urodeles there is a 



Fig. 277. — External gills of young Amphiuma, partially covered by opercular fold. 



reduced operculum which never is prominent, being merely a fold of 

 integument in front of the gill area. In the larval anuran the oper- 

 cular fold is well developed, though it never has any skeletal support. 

 Before the time of metamorphosis this fold grows backward over the 

 gill clefts, the gills, and the fore limbs, and fuses with the sides and 

 the floor of the body behind the latter. In this way these parts are 

 enclosed in an atrial chamber, the chambers of the two sides being in 

 communication below the pharynx. During larval life the atrial 

 chambers open to the exterior in most anura by a single excurrent 

 pore, usually on the left side, but in the larval aglossate amphibia 

 {Pipa, Xenodactylus, etc.) right and left excurrent openings occur. 



The gills of the amphibia are certainly of ectodermal origin. In 

 the early larva usually three pairs of external gills are formed before 

 the clefts break through. These are covered with ciliated epidermis. 

 They arise on the outer surface of the third, fourth and fifth arches 

 and are supplied by the corresponding aortic arches of the blood 

 system. They have no skeletal support and vary considerably in 

 17 



