ORGANS AND THEIR FUNCTIONS. 9 
Such is the nucleus ; so that Amoeba is a genuine organism. 
Ascending to the flagellate Infusoria (Fig. 1), we have the 
flagella developedj as external, permanent organs of locomo- 
tion. In the Hlydra (Fig. 36) the tentacles are organs 
whose functions are generalized. Inthe worms we have or- 
gans arranged in pairs on each side of the body, and in gen- 
eral among the higher invertebrates, especially the crusta- 
ceans and insects, and markedly in the vertebrates, we have 
the bilateral symmetry of the body still farther emphasized 
in the nature and distribution of the appendages. 
Of the internal organs of the body, the most important is 
the digestive cavity, which is at first simple and primitive in 
the gastrula or embryo of all many-celled animals, and as we 
ascend in the animal series we witness its gradual special- 
ization, the digestive tract being differentiated into dis- 
tinct portions (7.e., the cesophagus, stomach, and intestine), 
each with separate functions while the organs of respiration, 
digestion, secretion, and excretion originate as offshoots or 
outgrowths from the main alimentary tract. In like man- 
ner the skeleton is at first simple and afterward is extended 
into the different organs, the various parts of the ap- 
pendicular skeleton corresponding to the increased flexi- 
bility and diversified leverage power ; so that limbs become 
subdivided into joints, and these joints still further subdi- 
vided as we go from the points of attachment to the peri- 
phery or extremities, as seen in the tendency to an irrelative 
repetition of joints in the limbs and feelers of crustaceans 
and insects, and the digits of the lower vertebrates. 
Correlation of Organs.—Cuvier established this princi- 
ple, showing that there is a close relation between the forms 
of the hard and soft parts of the body, together with the 
functions they perform, and the habits of the animal. For 
example, in a cat, sharp teeth for eating flesh, sharp curved 
claws for seizing smaller animals, and great muscular activ- 
ity coexist with a stomach fitted for the digestion of animal 
rather than vegetable food. Soin the ox, broad grinding 
teeth for triturating grass, cloven hoofs that give a broad 
support in soft ground, and a several-chambered stomach 
coexist with the habits and instincts of a ruminant. Thus 
