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. 

 In 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 

 crayfish ; and we have probably still much to learn about 



