XI 



AMPHIBIA 



407 



numerous epidermal glands. The superficial horny layer of the epidermis 

 is better developed than in fishes and is shed at intervals, being re- 

 placed from beneath by a new horny layer. In one or two cases (e.g. 

 in the aquatic toad, Xenopus, of South and West Africa) certain of 

 the toes (I, II and III of the hind-foot) have their tips ensheathed in 

 hard, dark-coloured, hollow cones formed by local exaggeration of the 

 horny layer. These are the first representatives of the claws which 

 become so conspicuous in the higher tetrapods. 



Fig. 181. 



Diagrammatic transverse section tlirougli one side of the head of a frog, passing through the 

 tympanic cavity, a. Auditory nerve; a.c, auditory capsule; t:, columella; E, Eustachian tube; 

 m, medulla oblongata ; ot, otocyst ; p, cavity of pharynx ; t.c, tympanic cavity ; t.-m, tympanic 

 membrane ; l.r, skeletal ring within which the tympanic membrane is stretched. 



The pharynx develops gill-clefts just as in the fishes. Of these the 

 post-spiracular clefts remain functional during the aquatic larval stage 

 but as a rule become completely obliterated later, being covered in 

 by an opercular flap the free edge of which becomes completely fused 

 with the body. The spiracle shows in the Anura among existing Am- 

 phibians a remarkable modification in connexion with the sense of hearing 

 which has persisted during the evolution of the higher vertebrates. The 

 gill-pouch becomes dilated at its outer end to form a wide cavity — the 

 tympanic cavity (Fig. 181, t.c.) — closed in from the exterior by a thin 

 tensely stretched membrane — the_ tympanic membrane or ear-drum (t.m). 

 This membrane is thrown into vibration by sound waves and its vibra- 

 tions are carried across the cavity by a stiff rod {c) — the columella — 



