222 THE CRAYFISH 



The Crayfish. Adaptations for Protection. An animal which well 

 illustrates adaptation for life in the water is the fresh-water crayfish 

 or the salt-water lobster, both members of a large group of animals known 

 as crustaceans. The body of such an animal is seen to be covered more or 

 less completely with a hard covering, which is jointed in the posterior region. 

 This exoskeleton (outside skeleton) is composed largely of lime, as may be 

 proved by testing with acid. The exoskeleton fits over the anterior part 

 of the animal, forming an unjointed carapace, or armor. This armor is 

 clearly protective and is thus an adaptation. If the crayfish is watched 

 in a balanced aquarium, the colors, too, are seen to blend remarkably 

 with the stones and water weeds of the bottom. The animal is protec- 

 tively colored. The under s : de of the animal is seen to be less well pro- 



Crayfish: A., antennae; E., stalked eye; C.P., cephalothorax ; Ab., abdomen; 

 C.F., caudal fin; M., mouth; Ch., chelipeds. From photograph. 



tected than the upper, and the joints of the abdomen, or posterior region, 

 are seen to extend completely around the body. The animal is thus seen 

 to be segmented, the abdomen showing this plainly. The seven segments 

 in the abdomen are constant for every crayfish. 



Locomotion. Those of us who have caught crayfish in fresh-water 

 streams or lakes know that it takes skill and quickness. They dart 

 backwards through the water with great rapidity, or they may move 

 forward by crawling on the bottom. Examination of a crayfish shows us 

 five pairs of walking legs attached to the under side of the cephalothorax 

 (head + thorax), the anterior part of the body. These legs are jointed, 

 the first three bearing pinchers. The large pincher claw is used partly for 

 food-catching, and for locomotion as well. Try to find out, in a living 

 specimen, exactly what part it plays. 



Under the abdomen, one to each segment except the last, are found 

 jointed appendages, made up of three parts, a base and two branches. 

 These are called swimmerets, though they are not used for swimming. 

 Now look at the broad pair of appendages that, together with the last 



