14 AN INTRODUCTION TO ZOOLOGY 



of as the external characters. You will readily observe that the 

 frog as a whole is divisible into head, trunk and limbs. The head, 

 unlike our own, is not joined to the body by means of a flexible neck, 

 but the two are continuous with one another, a condition that we 

 frequently meet with in water-dwelling animals. For the purposes 

 of description we speak of the back as the dorsal surface, the under- 

 neath as the ventral surface, the end that is foremost in moving, 

 i.e. the head, as anterior, and the after part of the body as posterior. 

 A little closer observation will show that the skin is quite smooth, 

 and has no structures resembling scales, feathers, hairs or nails upon 

 it. Such structures, when present, as in many animals, we term 

 collectively the exoskeleton. 



Turning first to the head, we find it possesses a bluntly rounded 

 snout, and that the mouth is a long slit running the greater part of 

 the way round its edge. Two small openings, the nostrils or external 

 nares, are situated on the top of the front end of the snout. If the 

 living animal be watched carefully it will be noted that the neck is 

 constantly falling and rising, and at the same time a little flap or 

 valve just inside the external nares is opening and shutting. These 

 movements are concerned with breathing or respiration, and the 

 air is not drawn into the lungs by expanding the chest, as in our- 

 selves, but pumped in by the action of the throat. A short distance 

 behind the external nares are situated the large protruding eyes, 

 whose prominence in some degree compensates for the absence of a 

 neck by allowing a wide field of vision. In each we can make out, 

 as in ourselves, an eyeball, in the middle of which is a circular black 

 space, the pupil, through which the light enters, surrounded by a 

 wide coloured band, the iris. .The upper eye lid is fairly well marked 

 and slightly movable, but the lower one is represented by a very 

 transparent skin, the nictitating membrane, which can be drawn 

 right up over the eye. An external ear or pinna, such as our own. 

 is absent, but just behind the eye and above and behind the mouth 

 is a circular patch of black, thin, lightly stretched skin. This is 

 the ear drum or tympanic membrane. 



The trunk is not marked by any striking characters, save that in 

 the sitting position a hump appears in the middle of the back ; the 

 meaning of this will be made clear when the internal structure is 

 examined. It ends bluntly without any tail. At the posterior end 

 of the body, between the hind legs and slightly on the dorsal side, 

 is an opening, the cloaca, through which the waste matters and 

 reproductive products are expelled from the body. Attached to the 

 trunk are the limbs or appendages, consisting of a pair of fore limbs 

 or arms, and a pair of hind limbs or legs. The arm is composed, as 

 in ourselves, of an upper arm or brachium, a fore-arm or ante- 



