24 MANUAL OF ELEMENTARY ZOOLOGY 



In examining the body of a frog, we are struck, first by 

 the fact that its mottled green and yellow skin 

 Features. * s s0 ^ anc * slimy and without the covering of 

 hairs, or scales, or feathers which we find in 

 other animals, and next by its consisting only of head, trunk, 

 and two pairs of limbs. There is no neck or tail. The 

 trunk is flattened and bears the head at one end and the 

 limbs of each pair opposite to one another on the narrow 

 sides. In consequence of this symmetrical arrangement 

 we may distinguish a back or dorsal surface, a lower or 

 ventral surface, right and left sides, and fore and hind ends. 

 Such a symmetry is called bilateral, and we shall see that 

 in the frog it extends to the arrangement of nearly all the 

 organs of the body. The fore or anterior end is that which 

 is foremost when the animal moves, and is thus the first 

 part to come into relation with objects in the world around 

 it. At this end is placed the head, a distinct region of the 

 body, smaller than the trunk, which bears the mouth with 

 which food is taken and the three pairs of principal sense 

 organs by which the animal becomes aware of the nature 

 of its surroundings. The eyes are large, and have stout, 

 almost immovable upper lids and thin, translucent, movable 

 lower lids. 1 The nostrils or external nares are a pair of 

 small openings on the top of the head in front of the eyes. 

 Each of them leads into a chamber which communicates 

 with the mouth. There is no flap to the ear, but the 

 drum shows upon the surface at the side of the head 

 behind and somewhat below the eye. If the drum be 

 pierced, a bristle passed through it will be found to reach 

 the mouth. On the lower side of the trunk there may be dis- 

 tinguished two regions — the large, soft-walled belly or abdomen 

 behind, and the smaller stout-walled breast region in front. 



The limbs of each pair resemble one another, and those 

 of the two pairs correspond roughly in shape, each con- 

 sisting of three successive parts, the first two slender and 

 the third broad and adapted to be applied to the ground. 

 In the fore limb or arm the segment nearest the body is 



1 These do not represent the lower lids of man, which are wanting in 

 the frog. The lower lid of the frog is the third eyelid or nictitating 

 membrane found in many other animals (pp. 398, 419). All three 

 eyelids are well developed in birds. 



