TNE 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 ceca, 
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 asoluble 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 
F2 
