HOW ANIMALS DIGEST. 



607 



of digested and undigested matter. This perfect stomach is realized so 

 gradually, or by such slight degrees, in a large number of lower ani- 

 mals, that it is not easy to say positively which animal has the honor 

 of its first possession. To the little sea-urchin, which is the first pos- 

 sessor of true teeth, is generally given the credit. The ctenophore, a 

 lower animal, has indeed two or more apertures to its food-cavity, but 

 the mouth is still the main excretory orifice, and this cavity freely 

 communicates with a system of body canals. The starfish, on the con- 

 trary, has the stomach distinct from the other cavities of the body, 

 with, however, in most cases only a single opening. 



In the sea-urchin we also find an intestine and indications of the 

 several parts which are so distinct in higher animals. The sea-cucum- 



Fig. 8. Digestive System op the Common Fowi, (after Owen): 0, gullet; c, crop; p, proven- 

 triculus ; g, gizzard ; sm, small intestine ; ft, intestinal caeca ; I, large intestine ; cl, cloaca. 



ber, a near relative of the former but with a better digestive canal, is 

 almost as highly favored regarding its stomach as the low amoeba. 

 If its stomach becomes troublesome from indigestion or other cause, 

 it simply ejects it through the mouth, along with its other internal 

 organs. Then it quietly awaits the growth of a new set certainly 

 a very happy and efficient method. Many human dyspeptics would 

 rejoice in the same power. This animal is said to reject its viscera 

 when it is injured or alarmed. This is interesting, as showing in the 

 low animals that which is well known in the highest, the immediate 

 effect of fear and pain upon the internal organs, or the close depen- 

 dence of the nutritive organs upon the nervous system. 



Among articulates and mollusks we find a great diversity in the 

 character of the digestive canal. Its main divisions are always shown 



