228 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



oxidation, and repairing themselves by taking into their 

 substance the matters which serve them for food ; hke 

 the crayfish, they shape themselves according to a defi- 

 nite pattern of external form and internal structure ; like 

 the crayfish, the}' give off germs which grow and develojje 

 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 anj' such process of reproduc- 

 tion. 



Again, common observation early leads to the discri- 

 mination of living things into two great divisions. No- 

 bod}' 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 animalit}', but they all, 

 like it, possess a complete alimentary canal; special appa- 



