Metazoa Rana. 265 



form is so much specialised as to render it a less suitable 

 subject for the preliminary study of that system. We will 

 therefore adopt the frog as the subject for consideration, 

 save. in the case of the skeletal system, where the skeleton of 

 the salamander will be taken as our type. 



External characters. A very cursory examination of 

 the external characters of the frog enables us to distinguish 

 a head, neck, trunk, and fore and hind limbs (fig. 128). The 

 wide mouth, nostrils, eyes are easily seen on the head ; and 

 the ears may be identified as two small rounded patches 

 a short distance behind the eyes. There is no external ear, 

 but the drum, or tympanic membrane, is itself exposed. 

 Posteriorly only one opening can be made out, the opening 

 of the cloaca, a small chamber into which the ducts of the 

 renal and reproductive glands open together with the ali- 

 mentary canal. 



The animal is covered externally by a soft, moist, pig- 

 mented skin, which is very loosely attached to the body 

 proper. The underlying space is occupied by a colourless 

 fluid, called lymph, which will call for a more detailed de- 

 scription presently. 



The alimentary system. We have already seen that 

 the alimentary system in the animal consists essentially of a 

 tube of variable length, running from mouth to anus, into 

 which the food is introduced, and in which it is subjected 

 to the action of certain fluids or secretions which render it 

 capable of being absorbed through the walls of the ali- 

 mentary canal, directly or indirectly into the terminal vessels 

 (capillaries) of the circulatory system. In Lumbricus and in 

 Amphioxus that canal is straight and simple, but in the frog 

 it has become much modified. In the first place it is much 

 longer than the body of the animal, and consequently is 

 coiled up inside the body-cavity. In the second place, it 

 has become differentiated into several distinct regions : a 

 buccal cavity, where (though not in the frog) food is mas- 

 ticated ; an oesophagus, or tube for the carriage of the 



