228 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
oxidation, and repairing themselves by taking into their 
substance the matters which serve them for food; like 
the crayfish, they shape themselves according to a defi- 
nite pattern of external form and internal structure ; like 
the crayfish, they give off germs which grow and develope 
into the shapes characteristic of the adult. No mi- 
neral matter is maintained in this fashion; nor grows in 
the same way; nor undergoes this kind of development ; 
nor multiplies its kind by any such process of reproduc- 
tion. 
Again, common observation early leads to the discri- 
mination of living things into two great divisions. No- 
body confounds ordinary animals with ordinary plants, 
nor doubts that the crayfish belongs to the former cate- 
gory and the waterweed to the latter. If a living thing 
moves and possesses a digestive receptacle, it is held to 
be an animal; if it is motionless and draws its nourish- 
ment directly from the substances which are in contact 
with its outer surface, it is held to be a plant. We need 
not inquire, at present, how far this rough definition of 
the differences which separate animals from plants holds 
good. Accepting it for the moment, it is obvious that 
the crayfish is unquestionably an animal,—as much an 
animal as the vole, the perch, and the pond-snail, which 
inhabit the same waters. Moreover, the crayfish has, in 
common with these animals, not merely the motor and 
digestive powers characteristic of animality, but they all, 
like it, possess a complete alimentary canal; special appa- 
