THE LOWER \ EUTEBRATA. 139 



the same in all fundamental respects as that of the rabbit. 

 We have dorsally a vertebral column enclosing a spinal 

 cord and surrounded by muscles; ventrally, a cojlouj, with 

 alimentary canal and other viscera. But there is no 

 diaphragm, and the coelom forms an undivided space, 

 excepting 'only for the narrow pericardia! chamber. As in 

 the rabbit, the coelom stops short about the level of the 

 heart and extends no farther forwards. Beneath the skin 

 \vc find an almost continuous cavity, which makes the skin 

 strikingly loose : this cavity (or series of cavities) contains 

 lymph and connects with the lymphatics. It is called the 

 subcutaneous lymph-space. 



3. The alimentary canal (fig. 68) is more nearly a 

 simple tube in the frog than it is in the rabbit. Teeth are 

 found only along the upper jaw (ma*infl.ry teeth) and in 

 tw~o patched (5n~TEe roof of the mouth (vomerine teeth). 

 They are numerous, small, pointed, and slightly hooked : 

 they cannot be classed as incisors, canines, etc., and have no 

 masticatory function they simply serve to prevent insects 

 from slipping out. Instead of the two sets of teeth (milk- 

 and permanent), we find a continual replacement of worn- 

 out ones by new ones developed beneath. The tongue is 

 attached to the floor of the mouth by its anterior end, and 

 the posterior end is free and can be flicked out with great 

 rapidity and suddenness, and aimed with precision at the 

 small insects which form the frog's staple diet. In ac- 

 cordance with the absence of mastication, there are no 

 salivary glands. The internal nares open into the mouth 

 iit its front end, just beneath the external nares : thus the 

 narial passage is absent, and there is nothing corresponding 

 to the rabbit's palate which separates the buccal and narial 

 cavities. The roof of the frog's mouth answers to the roof 

 of the rabbit's narial passage. The glottis is far removed 

 from the nares : it is a longitudinal slit on the floor of the 

 throat, and is coin pletely _clpsed while food is being swallowed. 

 Near it, in a dorso-lateral position, are a pair of pits, wliich 

 lead out to the inner side of the tympanic membrane: these 

 obviously answer to Eustachian tube plus tympanum (middle 

 car), and are called the tympano-Eustachian recesses. 



