CRUSTACEANS 



217 



moving objects at a little distance and then close to the animal. You 

 may also test to see if the animal can distinguish light from darkness. This 

 may be done by covering half of the tray in which the crayfish is con- 

 fined. Then place the animal in the light end of the tray to see if it will 

 travel toward the dark end of the tray. This may be repeated by placing 

 the animal m the dark end. Thus it will bo possible to discover the reaction 

 of the animal to the light. Notice the position of the stalked eyes. Touch 

 the eye with a pencil; is it freely movable? In what direction? Notice 

 how well the eye is protected from injury. The anterior end of the cara- 

 pace projects to form a spiny process; this, wn'th the socket in which the 

 eye rests and its position on the side of the head, forms ample protection 



Mouth parts of the crayfish; /.walking appendage, showing attachraent of gill; 2, the jaw, with 

 palp; 3, first maxilla (second maxilla not shown); 4, third maxilliped; 5, second maxil- 

 liped, showing baler; 6, first maxilliped, showing gill attached; 7, swimmeret; 5, uropod. 



to this important organ. The eyes of the crayfish, like those of an in- 

 sect, are compound. They differ from those of the insect in being borne 

 on stalks. If a small bit of the exoskeleton covering the eye is placed un- 

 der the compound microscope, it wall be found to be made up of a num- 

 ber of little rectangles; this shows the size and shape of a surface view of 

 the units composing the compound eye. 



If it is possible to have the aquarium holding the crayfish in the school- 

 room, the method of feeding may be watched. Notice that the pincher claws 

 (chelipeds) are used to hold and tear food, as well as for defense and offense. 

 Living food is obtained with the aid of the chelipeds. Food is shoved by 

 the chelipeds toward the mouth; it is assisted there by several small ap- 

 pendages called foot jaws (maxillipeds) and to a slight degree by two still 

 smaller paired maxillce just under the maxillipeds. Ultimately the food 

 reaches the hard jaws and, after being ground between them, is passed down 

 to the stomach. If you hold the crayfish in such a position that you can 

 pour a little beef juice or other edible fluid over the mouth parts, it will l>e 

 possible to observe the mouth parts work as they do in a state of nature. 



