THE AWSTRACTIONS, SPECIES AND GENUS. 249 
distinguish these seventeen-gilled crayfishes, as a whole, 
from the eighteen-gilled species; and this is effected by 
changing the generic name. They are no longer called 
Astacus, but Cambarus (fig. 68). 
All the individual crayfish referred to thus far, there- 
fore, have been sorted out, first into the groups termed 
species ; and then these species have been further sorted 
into two divisions, termed genera. Each genus is an 
abstraction, formed by summing up the common char- 
acters of the species which it includes, just as each 
species is an abstraction, composed of the common 
characters of the individuals which belong to it; and 
the one has no more existence in nature than the other. 
The definition of the genus is simply a statement of 
the plan of structure which is common to all the species 
included under that genus; just as the definition of the 
species is a statement of the common plan of structure 
which runs throughout the individuals which compose 
the species. 
Again, crayfishes are found in the fresh waters of the 
Southern hemisphere; and almost the whole of what 
has been said respecting the structure of the English cray- 
fish applies to these ;-in other words, their general plan is 
the same. But, in these southern crayfishes, the podo- 
branchiz have no distinct lamina, and the first somite of 
the abdomen is devoid of appendages in both sexes. The 
southern crayfishes, like those of the Northern hem1- 
sphere, are divisible into many species; and these species 
