THE CRAYFISH 329 



backward with sudden strokes of the fin-hke uropods, which are 

 pressed strongly against the water by contractions of the muscles 

 that fold the posterior half of the abdomen in a ventral and 

 anterior direction. One such powerful stroke of the tail-fin formed 

 by the telson and uropods will carry the animal several feet 

 backward and usually out of danger. As the crayfish thus shoots 

 through the water, the folded abdomen offers little resistance; it 

 expands gently as the animal comes to rest upon the bottom, 

 ready for another stroke if the occasion demands. The anten- 

 nules function as tactile organs, but the antennae are more useful 

 for this purpose, since they can be moved in all directions: for- 

 ward, side wise, and even backward along the abdomen. In the 

 dark crevices under stones and in the burrows where little light 

 penetrates, they must be one of the most important means the 

 animal possesses of exploring its surroundings. 



Although the crayfish is primarily an aquatic animal, there 

 are times when it leaves the water. Observations made at night 

 with a lantern often show crayfish in the shallow water near shore 

 and upon the bank at short distances from the water. In a 

 laboratory aquarium, they thrive better if they can spend part of 

 the time, including some of the daytime, upon stones or other 

 objects that may project from the water. In nature, they seem 

 occasionally to make short nocturnal expeditions on the land in 

 search of food, if there is sufficient moisture in the atmosphere; 

 and they do this regularly in a laboratorj^ vivarium having a 

 sloping bank. 



In spite of its means of defense and escape, the crayfish suffers 

 from many enemies, both internal and external. It has a variety 

 of parasites. Fresh-water oHgochaete worms, of the genera 

 Branchiobdella and BdeUodrilus, and certain nematodes parasitize 

 the gills. ]\Iany fish, such as the black bass, hunt for craj^fish 

 and devour them greedily. Toads, frogs, salamanders, water 

 snakes, and turtles prey upon them. Even birds, such as the eagle 

 and kingfisher, have been observed with crayfish in their claws or 

 the remains of crayfish found in their nests. ]\Ian, however, is the 

 most serious enemy in well-populated countries, since he not only 

 consumes many crayfish as food but may also destroy large areas 

 of the natural habitat by drainage and similar projects. Wherever 

 fairly normal conditions present themselves, however, the crayfish 

 usually thrives even in the face of attempts at extermination. 



