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 developed as external, permanent organs of locomo- 

 tion. In the Hydra (Fig. 36) the tentacles are organs 

 whose functions are generalized. In the 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 (i.e., the oesophagus, 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. So in 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 



