254 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



form which characterize each of the genera and species, 

 would appear in the place of the names of the former, or 

 of the circles which represent the latter. All these 

 figures would represent abstractions — mental images 

 which have no existence outside the mind. Actual facts 

 would begin with drawings of individual animals, which 

 we may suppose to occupy the place of the dots above 

 the upper line in the diagram. 



That all crayfish es may be regarded as_ modifications, of 

 the common plan A, is not an hypothesis, but a generali- 

 zation obtained by comparing together the observations 

 made upon the structure of individual crayfishes. It is 

 simply a graphic method of representing the facts which 

 are commonly stated in the form of a definition of the 

 tribe of crayfishes, or Astacina. 



This definition runs as follows : — 



Multicellular animals provided with an alimentary 

 canal and with a chitinous cuticular exoskeleton ; with 

 a ganglionated central nervous system traversed by the 

 oesophagus ; possessing a heart and branchial respiratory 

 organs. 



The body is bilaterally symmetrical, and consists of 

 twenty metameres (or somites and their appendages), of 

 which six are associated into a head, eight into a thorax, 

 and six into an abdomen. A telson is attached to the 

 last abdominal somite. 



The somites of the abdominal region are all free, those 

 of the head and thorax, except the hindermost, which is 



