128 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



for the belief that they must needs do so. The analog}' of 

 a machine that, sooner or later, must be brought to a 

 standstill by the wear and tear of its parts, does not 

 hold, inasmuch as the animal mechanism is continually 

 renewed and repaired ; and, though it is true that indi- 

 vidual components of the body are constantly d3dng, yet 

 their places are taken by vigorous successors. A city 

 remains, notwithstanding the constant death-rate of its 

 inhabitants ; and such an organism as a crayfish is only 

 a corporate unity, made up of innumerable partially 

 independent individualities. 



Whatever might be the longevity of crayfishes under 

 imaginable perfect conditions, the fact that, notwithstand- 

 ing the great number of eggs they produce, their number 

 remains pretty much the same in a given district, if 

 we take the average of a period of j^ears, shows that 

 about as many die as are born ; and that, without the 

 process of reproduction, the species would soon come to 

 an end. 



There are many examples among members of the group 

 oi Crustacea to which the crayfish belongs, of animals which 

 produce 3'oung from internall}^ developed germs, as some 

 plants throw off bulbs which are capable of reproducing 

 the parent stock ; such is the case, for example, with the 

 common water flea {DapJinia). But nothing of this kind 

 has been observed in the crayfish ; in which, as in the 

 higher animals, the reproduction of the species is de- 

 pendent upon the combination of two kinds of living 



