THE DIGESTION OF FOOD. 67 



the yellow fluid accumulating in the ducts passes into 

 the mid-gut. The yellow colour is due to the globules of 

 fat. In the young cells, at the summit of the caeca, 

 these are either absent, or very small, whence the part 

 appears colourless. But, lower down, small yellow 

 granules appear in the cells, and these become bigger 

 and more numerous in the middle and lower parts. In 

 fact, few glands are better fitted for the study of the 

 manner in which secretion is effected than the crayfish's 

 liver. 



We may now consider the alimentary machinery, the 

 general structure of which has been explained, in 

 action. 



The food, already torn and crushed by the jaws, is 

 passed through the gullet into the cardiac sac, and there 

 reduced to a still more pulpy state by the gastric mill. 

 By degrees, such parts as are sufficiently fluid are 

 drained off into the intestine, through the pyloric strainer, 

 while the coarser parts of the useless matters are probably 

 rejected by the mouth, as a hawk or an owl rejects his 

 casts. There is reason to believe, though it is not certainly 

 known, that fluids from the intestine mix with the food 

 while it is undergoing trituration, and effect the transforma- 

 tion of the starchy and the insoluble protein compounds 

 into a soluble state. At any rate, as soon as the strained-off 

 fluid passes into the mid-gut it must be mixed with the 

 secretion of the liver, the action of which is probably 



