GILLS 



279 



on the hyoid. In Protopterus and Lepidosiren a reduction of these 

 organs has taken place, gills being absent in the former genus on 

 the first and second branchial arches ; there is, however, in addition, 

 an anterior hemibranch on the fifth branchial arch. 



Amphibia. In the embryos of Urodeles, five gill-clefts can 

 usually be recognised, but the most anterior and posterior of these 

 do not become open to the exterior. In the larvaB, as well as in adult 

 Perennibranchiates, there are three external gill-tufts in connection 

 with the three anterior branchial arches, lying one over the other ; 



- ssa^ggggsss 



FIG. 224, A and B. LARVA OF Epicrium glvtinosum, WITH EXTERNAL GILLS. 



(After Sarasin.) 



these extend backwards, projecting freely to the exterior, and are 

 composed of connective-tissue, unsupported by cartilage. They 

 either have the form of tufts, or may be delicately branched, 

 showing the most varied arrangements for increasing the respira- 

 tory surface (comp. Fig. 224). These external gills are ectodermal 

 in origin, and must not be confused with the internal gills, which 

 are wanting in all Urodeles. They are acted on by a complicated 

 system of muscles, and are covered by ciliated epithelium, which 

 serves to keep up a continual current in the surrounding medium. 

 In the Axolotl and in larval Salamanders there are four, and in 

 Necturus (Menobranchus) and Proteus only two gill-clefts perfora- 



