THE ARTHROPODS 363 



If a drop of blood be taken from the pericardial chamber of a 

 recently-killed crayfish, it will be found to be a colorless fluid with 

 white cells of amoeboid form. These cells are often filled with gran- 

 ules, which have been " eaten" (as an amoeba " eats ") by the blood- 

 cells. These are similar to the white blood-cells of vertebrates, 

 which also have red cells in their blood. 



Digestive System. In front of the heart lies the stomach (bluish 

 color in fresh specimens), and to the stomach a short esophagus leads 

 from the mouth, which lies between the jaws. The stomach has a 

 constriction which marks off an anterior and a posterior portion, 



FIG. 117. Internal organs of a crayfish, b, " brain" ; k, kidney; m, mouth ; 

 s, stomach ; I, digestive gland or liver ; r, reproductive gland (spermary) ; 

 A, heart; i, intestine ; n, ventral nerve-cord ; a, anus. (From McMurrich.) 



and by cutting open the stomach a peculiar food-grinding apparatus 

 (gastric mill) may be seen in the anterior portion. Food is crushed 

 and shredded before passing into the hinder part of the stomach, and 

 passed through a peculiar strainer before entering the intestine. 



On either side of the stomach lies a large digestive gland (greenish 

 yellow in fresh specimens), whose ducts open into the stomach near 

 the intestine. Its primary function is secretion of a digestive fluid 

 which is able to prepare foods for absorption. This fluid is similar 

 in function to the pancreatic secretion in higher animals. 



The intestine extends from the hinder end of the stomach, ventral 

 to the heart, and near the dorsal side of the abdomen, to the last 

 segment where its opening (anus) is on the ventral surface. Diges- 

 tion and absorption of food which begin in the posterior part of the 

 stomach continue in the intestine. 



Reproductive Organs. The male and female crayfishes are 

 easily distinguished because in the female all the appendages on the 



