230 THE COMPARATIVE MORPHOLOGY OP 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__morp_hology. 

 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 

 hi 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 " ecre- 

 visse a pieds routes " of France ; not to the English crayfish, which is 



