6o8 



THE POPULAR SCIENCE MONTHLY. 



more or less distinctly. But in many articulates, as some worms, myr- 

 iapods, larvae of insects, and crustaceans, the tube is quite straight ; 

 while in others it is highly convoluted. Biting insects have all the 

 parts ever found in the food-tract, namely, pharynx, gullet, crop, giz- 

 zard, stomach, small and large intestines. In some members of the 

 spider family, the short and straight food-canal sends off branches into 

 the limbs and other members. The absurdity of a creature carrying 

 its stomach in its legs ! 



Many snails have crop and gizzard, as also has the nautilus. In 

 snails the intestine passes through the liver, and in clams through the 



heart. Many butterflies take no food, and 

 the digestive organs are entirely absent. In 

 this case the eating and storing of nutriment 

 was performed in the earlier larval state 

 with excellent organs. But the male notom- 

 mata, one of the rotifers, never has digestive 

 organs ; it lives its brief life upon the nour- 

 ishment of the egg from which it was derived. 

 Fishes have a short and comparatively 

 simple alimentary tube with generally a wide 

 gullet, and seem commonly to disgorge indi- 

 gestible substances. Reptiles usually have 

 , the parts more strongly marked. Tadpoles 

 have a very long and greatly convoluted 

 canal, but the vegetable-eating turtles have 

 the longest. Crocodiles have a powerful giz- 

 zard, like birds, and are said to swallow stones 

 to assist the trituration of food. They ap- 

 proach birds also in possessing a mesentery, 

 a membrane which supports the food-tract 

 Fig. 9.-Diagram op the Diges- an( j fastens it to the walls of the body. In 



tive System of a Mammal: m \ 



g, gullet; s, stomach; sm, small a n lower animals the canal lies loosely in the 



intestine; Im, large intestine; , . * 



r, rectum, terminating in the body cavity. The food-tract of birds varies 



aperture of the anus. " ^ , ... 1 i i 



in length and character according to the kind 

 of food. The crop and gizzard have already been described, the latter 

 in a former article. 



In mammals the great body cavity is divided into two chambers, 

 thorax and abdomen, by a transverse partition called the diaphragm. 

 The gullet passes through and the stomach lies just beneath this mem- 

 brane. The parts of the food-tract are always clearly marked, and 

 the stomach is frequently divided. In all animals digestion is more 

 prolonged in proportion as the food is unlike animal substance. With 

 carnivorous mammals the process is simple. The whole length of the 

 food-tract in members of the cat tribe is only about three times the 

 length of the body. Man employs a mixed diet, and has the canal six 

 times the length of the body. No food is less like flesh than herbage, 



