158 THE HAUNTS OF LIFE 
parts of which are elastic, so that they can be 
stretched out further orshortened down. Thus 
some animals have a very long embryo period 
(like Peripatus), others a very long larval pe- 
riod (like May-flies), others a long childhood 
(like kittens), others a long maturity (like 
horses). Whenitis necessary, a part of the life- 
curve can be, as it were, stretched out in the 
course of generations—man is stretching out 
his youthful period—and another part can be 
shortened down. Many fresh-water animals 
have shortened down the riskful juvenile 
period. 
A clear example of what we mean may be 
found in the fresh-water crayfish. It is as high 
up the genealogical tree as the lobster, and 
almost as high up as the crab. But while the 
shore-crab has a long life-history, sketched in 
our study of the seashore, with one larval stage 
after another, the young of the fresh-water cray- 
fish is hatched as a miniature of its parent. It 
is practically identical with its parents, except 
that the tips of its claws are bent in, the better 
for gripping the empty egg-shells which are 
glued to the swimmerets of the mother. It has 
no larval stages to pass through; it remains in 
shelter under its mother’s tail until it is able to 
