230 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
crayfish and the pond-snail, or the crayfish and the 
perch. 
The exact determination of the resemblances and 
differences of animal forms by the comparison of the 
structure and the development of one with those of 
another, is the business of comparative morphology. 
Morphological comparison, fully and thoroughly worked 
out, furnishes us with the means of estimating the 
position of any one animal in relation to all the 
rest; while it shews us with what forms that animal 
is nearly, and with what it is remotely, allied: ap- 
plied to all animals, it furnishes us with a kind of 
map, upon which animals are arranged in the order of 
their respective affinities; or a classification, in which 
they are grouped in that order. For the purpose of 
developing the results of comparative morphology in the 
case of the crayfish, it will be convenient to bring toge- 
ther, in a summary form, those points of form and struc- 
ture, many of which have already been referred to and 
which characterise it as a separate kind of animal. 
Full-grown English crayfishes usually measure about 
three inches and a half from the extremity of the rostrum 
in front to that of the telson behind. The largest 
specimen I have met with measured four inches.* The 
* The dimensions of crayfishes at successive ages given at p, 31, 
beginning at the words “By the end of the year,” refer to the “ écre- 
visse 4 pieds rouges” of France; not to the English crayfish, which is 
