GILLS 



359 



to the exterior : they are covered over by an opercular-like fold of 

 the skin, thus leaving only a single aperture externally. In the 

 larvae, 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 ; these extend backwards, pro- 

 jecting freely to the exterior, and are unsupported by cartilage. 

 Each consists of a main axis with secondary branches, so that the 

 gills have the form of tufts or delicately branched structures 



FIG. 26.5. LARV.JJ OF (a) Protopttru* annectem (17 DAYS AFTER HATCHING) 

 AND (b) Polypterux lapradei (1 IN. LONG, x ABOUT 4). (After J. S. 

 Budgett.) 



BF, pectoral fin ; HO, sucker, or cement organ ; KB, external gills (single in 



Polypterus). 



showing the most varied arrangements for increasing the respira- 

 tory suiface (cf. Fig. 266). These external gills 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. 



The gills are lost at metamorphosis in the Derotremata and 

 Myctodera. In larval Myctodera, as mentioned above, there are 

 five open clefts, all of which disappear at metamorphosis. In the 



