278 TEXTBOOK OF ZOOLOGY 



is "mosaic" in that there is registered only a single image by the 

 eye. Each ommatidium which is in focus on the object registers 

 an image of that part. As the object moves, new ommatidia are 

 stimulated and movement is indicated by the rate of stimulation of 

 successive ommatidia. The farther the object is from the eye, the 

 fewer ommatidia will be stimulated. The crayfish eye is often 

 termed a modified appendage because an antennalike structure will 

 regenerate in case an eye is mutilated. 



Metabolism 



The crayfish ingests principally flesh from bodies of fish, snails, 

 tadpoles, insects, and other animals, some caught alive and others 

 found dead. The maxillae and maxillipeds hold the morsels while 

 they are crushed by the mandibles. Mastication continues in the 

 cardiac chamber of the stomach and chemical digestion begins in 

 the pyloric portion. The digestive juices possess enzymes which 

 convert the food into soluble form, and as it passes along the in- 

 testine, it is absorbed by the blood and distributed to the tissues 

 over the body. This conversion of food material into protoplasm is 

 assimilation. The external phase of respiration has put oxygen in 

 the blood, and it is distributed throughout the protoplasm of the 

 cells. The energy stored in the food material is released or converted 

 to kinetic form by union Avith the oxygen (oxidation) in the proto- 

 plasm. From this union there is excess heat produced. Mechanical 

 and chemical activity is the result of the harnessing of this energy. 

 As a by-product of this cataholism, excretory materials, such as excess 

 water, urea, uric acid, and other substances are formed in solution 

 and are collected by the blood. The green glands relieve the blood 

 of these and deliver them to the exterior. Of course growth results 

 when excess food materials are built into the cells at times when 

 the rate of anabolism exceeds that of catabolism. 



Reproduction 



These animals are dioecious (sexes separate) and the mating 

 takes place either in the spring or fall or perhaps both. The spring 

 hatch become well developed before winter. The eggs produced 

 in the fall may not be laid before spring. 



