CHAPTER IX. 
COMPARATIVE ANATOMY OF ORGANS. 
Havine 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 (Figs. 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 
haveasolid framework of teeth, the food consists of minute 
forms of life, protozoans and higher soft-bodied animals, 
