362 



THE FROG. PHYLUM CHORDATA 



Fig 27b— (From left to right) Successive postures of a toad during crawling.— 

 ■ ' ■ After Gray. 



ALIMENTARY SYSTEM 



The food of the frog is received and digested by a winding 

 tube, known as the gut or ahmentary canal, which runs from 

 mouth to cloacal opening and has a muscular wall Hned by a 

 soft, glandular mucous membrane 

 (p. 509). The gape of the mouth 

 Ues between two jaws, of which 

 the upper is not movable, but 

 the lower is hinged to the upper. 

 There are no teeth in the lower 

 jaw, but the upper bears a row of 

 maxillary teeth, and a patch of 

 vomerine teeth is found on each 

 side of the roof of the mouth. The 

 teeth are small, sharp-pointed struc- 

 tures (Fig. 277) consisting of a base 



m.- 



FiG. 277. — Two of the 

 maxillary teeth of 

 a frog, seen from 

 the outside of the 

 jaw. 



b.. Base of the tooth ; cr., 

 crown ; m., edge of the 

 maxilla. 



Fig. 278. — A vertical section 

 through a tooth and part of 

 the maxilla of a frog. 



b., Base of the tooth, composed of bone; 

 d., dentine ; e., enamel ; m., maxilla ; 

 o.p.c, opening of the pulp cavity. 



and a crown. The greater part of the crown is composed of ivory 

 or dentine, but the base is formed of bone, and the crown is 

 covered by a cap of very hard substance known as enamel, and 



