CHAPTER IX. 



COMPARATIVE ANATOMY OP ORGANS. ' 



Having studied the morphology of animals in a system- 

 atic way, it will be well for the student to make a brief re- 

 view of those facts stated in the foregoing chapters bearing 

 on the origin and successive degrees of complication of the 

 most important organs. 



Organs of Digestion— The Mouth and Teeth. — The most 

 important organs in the animal system are those relating to 

 digestion, as an animal may respire solely through its body- 

 walls, or do without a circulatory or nervous system, but 

 must eat in order to live and grow. The opening by which 

 the food is taken into the alimentary canal is called the 

 mouth, whether reference is made to the "mouth" of a 

 hydra or of a vertebrate ; although the structure of the edges 

 may differ radically, still in all Metazoa the mouth is due to 

 an inpushing of the ectoderm, however differently the 

 edge of the mouth may be supported and elaborated. The 

 edges of the mouth are usually called the lips, but true lips 

 for the first time appear in the Mammalia. The trituration 

 or mastication of the food is accomplished among the in- 

 vertebrates in a variety of ways, and by organs not always 

 truly homologous. 



Hard bodies serving as teeth occur for the first time in the 

 animal series in the sea-urchins, where a definite set of cal- 

 careous dental processes or teeth (Pigs. 78 and 79), with solid 

 supports and a complicated muscular apparatus, serves for 

 the comminution of the food, which consists of decaying an- 

 imals and sea- weeds. In those Echinoderms which do not 

 have a solid framework of teeth, the food consists of minute 

 forms of life, protozoans and higher soft-bodied animals, 



