Glass 3. Amphibia. 397 



The olfactory organs are two canals whicli lead from the 

 outer side of the head into the mouth, and open there behind the 

 edge of the jaw ; the external nares can be opened and closed. 

 Eyelids are wanting in the larv^, in the Perennibranchiata, and in 

 the Gymnophiona, which have rudimentary eyes; where they are 

 present only the lower one is movable, it is often semi-transparent, 

 and like a nictitating membrane. Lacrymal glands are absent, 

 although a lacrymal canal occurs in the adult ; a Harderian gland is, 

 on the other hand, present. 



Auditory apparatus. In most Anura, a short, wide canal; 

 the tympanic cavity extends from the posterior region of the 

 mouth behind the first gill-bar towards the exterior, it is not open 

 at the surface, but is closed in by a thin membrane, the tympanum. 

 The canal traverses that region of the skull which encloses the 

 membranous labyrinth, and is perforated in the region of the sacculus 

 by a foramen {fenestra ovalis). The fenestra is covered by a small 

 cartilaginous plate, the expanded end of a partially ossified rod, 

 the ear-bone [columella awris), whose other end is attached to 

 the tympanum (Fig. 325). In other Amphibia (some Anura, 

 e.g., the Toad; all Urodela and Gymnophiona), the tympanic cavity 

 and membrane are wanting ; but all possess the fenestra ovalis 

 and the columella. 



Alimentary canal. Teeth may be present on maxilla, 

 premaxillse, mandibles, vomers, and pterygoids, exceptionally also on 

 the parasphenoid ; in living Amphibia they are always small and 

 simple in form. The tongue is better developed than in Pisces ; it is 

 attached by its under surface to the floor of the mouth, in such a 

 way, however, that the edge is free. It is characteristic of the 

 Anura that the posterior tip, which is free and sometimes bifid, 

 is especially well developed ; whilst the anterior edge, which is 

 insignificant, is attached in front, so that the tongue can be flicked 

 out of the mouth from behind forwards. In some Urodela it can 

 be stretched out upon a kind of shaft, projecting from its ventral 

 surface. The tongue is absent from Pipa and an allied genus. The 

 oesophagus is short and wide, the intestine short. 



The respiratory organs of Amphibia are gills or 

 lungs, the former will be considered first. 



In the urodelan larva there are, on either side, four gill-slits, 

 the first between the hyoid and the first gill-bar, the last between the 

 third and fourth gill-bars. Bach bar bears on its outer edge a thin 

 membranous plate, and the series is covered by a thick membranous 

 fold without ossifications, which corresponds to the operculum of 

 Pish. The plates correspond to the septa between the gill-clefts in 

 Pish, but bear no gill lamellse. At the dorsal end of each of the first 

 three pairs of clefts there is, however, a gill, not covered by the oper- 



