214 



COLLEGE ZOOLOGY 



water; the maxillipeds are likewise kept mov- 

 ing; and the antennules and antennae are 

 in continual motion exploring the surround- 

 ings. 



Crayfishes are more active between dusk 

 and dawn than during the daytime. At this 

 time they venture out of their hiding places 

 in search of food. 



Locomotion 



Locomotion is accomplished by either 

 walking or swimming. Crayfishes are able to 

 walk in any direction, forward usually, but 

 also sidewise, obliquely, or backward. In 

 walking, the fourth pair of legs is most 

 effective and bears nearly the entire weight 

 of the animal; the fifth pair serves as props 

 and to push the body forward; the second 

 and third pairs are less efficient for walking 

 since they are modified to serve as grasping 

 organs and as toilet implements. Swimming 

 is not resorted to under ordinary conditions, 

 but only when the animal is frightened or 

 shocked. In such a case the crayfish extends 

 the abdomen, spreads out the uropods and 

 telson, and, by sudden contractions of the 

 flexor abdominal muscles, bends the abdo- 

 men and darts backward. The swimming 

 reaction apparently is not voluntary, but is 

 almost entirely reflex. 



Equilibrium 



The crayfish, either at rest or in motion, 

 is in a state of unstable equilibrium and 

 must maintain its body in the normal posi- 

 tion by its own efforts. The force of gravity 

 tends to turn the body over. From a large 

 number of experiments, it has been proved 

 that the statocysts are the organs of equi- 

 librium. The structure of these organs is 

 described on page 211. The contact of the 

 statoliths with the sensor)' hairs furnishes 

 the stimulus which causes the animal to 

 maintain an upright position. 



When placed on its back, the crayfish has 

 some difficulty in righting itself. Two 

 methods of regaining its normal position are 

 employed. The usual method is that of rais- 



ing itself on one side and allowing the body 

 to tip over by the force of gravity. The sec- 

 ond method is that of contracting the flexor 

 abdominal muscles, which causes a quick 

 backward flop, bringing the body right side 

 up. In general, the animals right themselves 

 by the easiest method when placed on their 

 backs; and this is found to depend usually 

 upon the relative weight of the two sides 

 of the body. When placed upon a surface 

 which is not level, they take advantage, after 

 a few experiences, of the inclination by turn- 

 ing toward the lower side. 



Senses and their location 



The crayfish has more highly developed 

 sense organs than the annelids. The sense of 

 touch in crayfishes is perhaps the most 

 valuable, since it aids them in finding food, 

 avoiding obstacles, and in many other ways. 

 Touch organs are located in specialized 

 hairlike bristles or setae (Fig. 112) on vari- 

 ous parts of the body. These are especially 

 abundant on the mouth parts, chelae, cheli- 

 peds, and edge of telson. Vision in crayfishes 

 is undoubtedly of real value to the animal 

 in detecting moving objects. No reactions 

 to sound have ever been observed in cray- 

 fishes, and apparently they do not hear. The 

 reactions formerly attributed to hearing are 

 probably due to touch reflexes. In aquatic 

 animals it is so difficult to distinguish be- 

 tween reactions of taste and smell that 

 these senses are both included in the term 

 chemical sense. The end organs of this 

 sense are found in hairs located on the an- 

 tennules, tip of antennae, mouth parts, and 

 other places. 



Reactions to stimuli 



Contact. Positive reactions to contact are 

 exhibited to a marked degree by crayfishes; 

 the animals seek to place their bodies in 

 contact with a solid object if possible. The 

 normal position of the crayfish when at rest 

 under a stone is such as to bring its side or 

 dorsal surface in contact with the walls of 

 its hiding place. This, no doubt, is of dis- 



