CHAPTEE II. 



THE PHYSIOLOGY OF THE CKAYFISH. THE MECHANISM BY 

 WHICH THE PARTS OF THE LIVING ENGINE ARE SUPPLIED 

 WITH THE MATERIALS NECESSARY FOR THEIR MAIN- 

 TENANCE AND GROWTH. 



An analj'sis 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 tlje 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, 



