154 



DIGESTIOX IX THE LOWER ANIMALS 



urinary tubes. The anus liy which this cloaca opens in this 

 animal, therefore, is the place of discharge of other matter 

 besides indigestible food. The advance of this animal 

 over tlie fish is seen in tlie increase of cU2,■csti^•e surface 



FlG.r>r — Section of fr-'S; ?, tongue ; /.month; .<;)', ?ton\:ioh ; j). pancri/as : ./. duo- 

 dennnl ; ,s;, small intestine ; r, rectum, anil b. bladdei', opening into a common 

 canity, the cloaca; (/r, ureter, and ord, iividnot, also opening into the cloaca; 

 A', kidney ; or, ovary ; /;r, liver. I>avisi>n, y?oo/.7r/)/. 



])}' coihng, the placing of the teeth in relation to the jaws, 

 ami the greater tendency to regional arrangement. 



Adaptations in warm-blooded animals. — In these animals 

 (with the exce].)tion of liinls) are found for the first 

 time, true teeth in sockets. All sucli teeth may be 

 divided into fnur classi's (incisors, canines, premolars, 

 ami molars) as in man, and are {daced uiiifoi'inlv upon 

 the jaws. The kiiul wiiich ]ired(iiiiiiiales in any animal 

 i,- iletermiiied Ii}- the kind of Inoil which it eats. Thus, 

 si[uii'rels and rals ha\'e tlie iucisoi-s gi-eatly developeil for 

 gnawing, while the flesh eaters are marked liy the i)redomi- 

 nauce of canines, and the grass eaters bv the molars. 



