234 



HOW DO WE CLASSIFY ANIMALS? 



probably does not have very distinct vision. A crayfish, however, 

 easily distinguishes moving objects and prefers darkness to light, 

 as has been proved by experiment. 



Food-getting. The food of the crayfish is obtained with the aid 

 of the pincer claws and shoved toward the mouth. It is pushed 

 on by three pairs of small appendages called foot jaws or maxil- 

 lipeds, and to some degree by two smaller paired maxillae just 

 under the maxillipeds. Ultimately the food reaches the true 

 jaws, or mandibles, and, after being ground between them, is 

 passed down the gullet into the stomach. 



Digestion. Food which has not been ground up previously into 

 pieces small enough for the purpose of digestion is still further 

 masticated by means of three strong projections or teeth called 

 the gastric mill, one placed on the mid-line and two on the side 

 walls of the stomach. 



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"^t ornac tt" ! r heccrt aorta intestine 



A crayfish, cut lengthwise to show the principal organs. 



The stomach is divided into two parts. The entrance to the 

 posterior part is lined with tiny projections which make it act 

 as a strainer for the food passing through. Thus the larger 

 particles of food are kept in the anterior end of the stomach. 

 Opening into the posterior end of the stomach are two large 

 digestive glands, whose juices further prepare the food for absorp- 

 tion by the walls of the stomach and intestine. Once in the blood, 



