CRAYFISHES AND THEIR RELATIVES 157 



where limestone is found, since lime is needed in making 

 their hard outer covering. During the day they hide under 

 stones, in the crevices of rocks, in the mud, and sometimes 

 in specially constructed burrows along the banks. Since 

 the animal backs into these hiding places, its big claws are 

 ready for business if an enemy attacks it. 



Then, too, the colors of crayfishes aid somewhat in pro- 

 tecting them since these colors are usually similar to the color 

 of the bottoms of the streams in which they live. Lastly, 

 the wide range of vision, which the stalked eyes afford must 

 serve to warn the animal of the approach of danger. Never- 

 theless they do not always escape since crayfish are often 

 captured by certain birds and fishes. In fact, crayfishes are 

 often used by man as a bait for catching fishes. 



114. Food, food getting, and digestion. At night cray- 

 fishes crawl about in search of food, concerning which 

 they are not at all fastidious, since dead fish and other 

 dead animals seem to be fully as acceptable as when 

 alive. In fact, they are natural scavengers. Crayfish 

 seize their food with their large claws and with the 

 aid of the small pincers on the front walking legs and 

 with the mouth parts, especially the mandibles, reduce 

 the food to pieces small enough to be eaten. We have seen 

 in plants (P. B., 63, 70, 71) that digestion many take place in 

 any living cell where food is stored or manufactured. Hence, 

 plants have no special part devoted to digestion. In cray- 

 fishes, however, it is quite different, since a part of the body, 

 known as a digestive system, is devoted* to preparing the 

 food for absorption and use. This digestive system consists 

 of a food tube known as the alimentary canal and certain 

 masses of cells known as digestive glands. 



After the food is digested, it can pass into the blood by 



