328 GENERAL ZOOLOGY 



smooth skin without scales, abundantly supplied with mucus- 

 glands. There are four appendages, the limbs, the anterior 

 of which are divisible into upper arm, forearm, and hand; the 

 posterior, into thigh, lower leg, and foot. The hand ends in 

 four short fingers; the foot, in five toes, joined by a web. Both 

 fingers and toes are often spoken of as digits. The eyes are 

 situated prominently on the top of the head, and possess, 

 in addition to an upper eyelid, a thin fold of skin called 

 the nictitating membrane, which can be drawn across the eye- 

 ball from below. There is no true lower eyelid. In front of 

 the eyes, near the anterior end of the head, are the external 

 openings of the nostrils; posterior to the eyes are smooth, 

 round spots, the tympanic membranes, the outer portions of 

 the frog's ears. 



The Digestive System. The wide mouth-cavity (Fig. 165, 1) 

 narrows into the short, straight oesophagus (Fig. 165, 6). 

 Conical teeth are placed along the edge of the upper jaw 

 (Fig. 165, 2) and on the roof of the mouth. A thick, fleshy 

 tongue (Fig. 165, 3), notched posteriorly, is attached by its 

 anterior margin to the ventral surface of the mouth-cavity. 

 The tongue can be thrust out suddenly for quite a distance 

 to capture food, which consists largely of insects, worms, and 

 mollusks. Two openings lead from the mouth to the nostrils 

 (Fig. 165, 4), and two openings (the Eustachian tubes, Fig. 

 165, 5) communicate with the ears. The stomach (Fig. 165, 7) 

 is a thick-walled sac tapering gradually into the coiled small 

 intestine (Fig. 165, 8). Beyond the small intestine the ali- 

 mentary canal suddenly increases in diameter, forming the 

 large intestine or rectum (Fig. 165, 9), which passes without 

 change of diameter into the terminal cloaca (Fig. 165, 10), 

 communicating with the surface at the cloacal opening (Fig. 

 165, 11). Between the lobes of the liver (Fig. 165, 13) lies a 

 large gall-bladder (Fig. 165, 14). A pancreas (Fig. 165, 15) 

 and a spleen (Fig. 165, 16) are present. 



