THE DIGESTION OF FOOD, 63 
importance to recollect this distinction by-and-by, when 
the development of the alimentary canal is considered. 
If the treatment to which the food is subjected in 
the alimentary apparatus were of a purely mechanical 
nature, there would be nothing more to describe in this 
part of the crayfish’s mechanism. But, in order that 
the nutritive matters may be turned to account, and 
undergo the chemical metamorphoses, which eventually 
change them into substances of a totally different cha- 
racter, they must pass out of the alimentary canal into 
the blood. And they can do this only by making their 
way through the walls of the alimentary canal; to which 
end they must either be in a state of extremely fine 
division, or they must be reduced to the fluid condition. 
Tn the case of the fatty matters, minute subdivision may 
suffice ; but the amylaceous substances and the insoluble 
protein compounds, such as the fibrin of flesh, must be 
brought into a state of solution. Therefore some sub- 
stances must be poured into the alimentary canal, which, 
when mixed with the crushed food, will play the part 
of a chemical agent, dissolving out the insoluble proteids, 
changing the amyloids into soluble sugar, and convert- 
ing all the proteids into those diffusible forms of protein 
matter, which are known as peptones. 
The details of the processes here indicated, which 
may be included under the general name of digestion, have 
only quite recently been carefully investigated in the 
‘erayfish ; and we have probably still much to learn about 
