AMPHIBIA. 



135 



There can be but little doubt that the hyomandibular diverticulum gives 

 rise, as in the Amniota, to the Eustachian tube and tympanic cavity, except 

 when these are absent (i.e. Bombinatoridye). Gotte holds however that 

 these parts are derived from the hyobranchial cleft, but his statements on 

 this head, which would involve us in great morphological difficulties, stand 

 in direct contradiction to the careful researches of Parker. 



Shortly after hatching, there grows out from the hyoid arch 

 on each side an opercular fold of skin, which gradually covers 

 over the posterior branchial arches and the external gills (fig. 

 80 d}. It fuses with the skin at the upper part of the gill arches, 

 and also with that of the pericardial wall below them ; but is 

 free in the middle, and so assists in forming a cavity, known 

 as the branchial cavity, in which the gills are placed. Each 

 branchial cavity at first opens by a separate widish pore behind 



A. 



FIG. 80. TADPOLES WITH EXTERNAL BRANCHIAE. (From Huxley; after Ecker.) 



A. Lateral view of a young tadpole. 



B. Ventral view of a somewhat older tadpole. 



kb. external branchiae; m. mouth; n. nasal sack; a. eye; o. auditory vesicle; 

 z. horny jaws ; s. ventral sucker; d. opercular fold. 



C. More advanced larva, in which the opercular fold has nearly covered the 

 branchiae. 



s. ventral sucker ; ks. external branchiae ; y. rudiment of hind limb. 



(fig. 80), and in Dactylethra both branchial apertures are preserved 

 (Huxley). In the larva of Bombinator, and it would seem also 

 that of Alytes and Pelodytes, the original widish openings of the 

 two branchial chambers meet together in the ventral line, and 



