Page:Popular Science Monthly Volume 17.djvu/626

608 more or less distinctly. But in many articulates, as some worms, myriapods, 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, gizzard, 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 notommata, one of the rotifers, never has digestive organs; it lives its brief life upon the nourishment 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 indigestible 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 gizzard, like birds, and are said to swallow stones to assist the trituration of food. They approach birds also in possessing a mesentery, a membrane which supports the food-tract and fastens it to the walls of the body. In all lower animals the canal lies loosely in the body cavity. The food-tract of birds varies 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 membrane. 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,