128 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



for the belief that they must needs do so. The analogy 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 djing, 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 prett}^ much the same in a given district, if 

 we take the average of a period of years, 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 craj^fish belongs, of animals which 

 produce young from internally 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 {Daphnia). 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 



