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 crayfishes 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 
cesophagus ; 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 
