CHAPTER I. 
THE PHYSIOLOGY OF THE CRAYFISH. THE MECHANISM BY 
WHICH THE PARTS OF THE LIVING ENGINE ARE SUPPLIED 
WITH THE MATERIALS NECESSARY FOR THEIR MAIN-~ 
TENANCE AND GROWTH. 
Aw analysis of such a sketch of the “ Natural History 
of the Crayfish” as is given in the preceding chapter, 
shows that it provides brief and general answers to three 
questions. First, what is the form and structure of the 
animal, not only when adult, but at different stages of 
its growth? Secondly, what are the various actions of 
which it is capable? Thirdly, where is it found? If we 
carry our investigations further, in such a manner as to 
give the fullest attainable answers to these questions, 
the knowledge thus acquired, in the case of the first 
question, is termed the Morphology of the crayfish; 
in the case of the second question, it constitutes the 
Physiology of the animal; while the answer to the third 
question would represent what we know of its Distribu- 
tion or Chorology. There remains a fourth problem, 
which can hardly be regarded as seriously under dis- 
cussion, so long as knowledge has advanced no further 
than the Natural History stage; the question, namely, 
