258 ANIMAL LIFE 
with the protection and sustenance of the young, and the 
care of the young is undoubtedly a chief factor in the de- 
velopment of communal life. 
144. The invertebrates (except spiders and insects). —_ 
Among the lower invertebrates parental aid to the young is 
confined almost exclusively to exhibitions of pure instinct. 
And this is true of many of the higher animals also. Eggs 
are deposited in sheltered places, and in such places and 
under such circumstances that the young on hatching will 
find themselves close to a supply of their natural food. The 
depositing of eggs in water by parents with terrestrial hab- 
its whose young are aquatic, is an example. The toad, 
which lives on land, feeding on insects, has young which 
live in water and feed on minute aquatic plants (alge). 
The dragon fly, that hawks over the pond or brook with 
glistening wings, has young that crawl in the slime and 
mud at the bottom of the pool. With most animals, after 
laying eggs, the parents show no further solicitude toward 
their offspring. The eggs are left to the vicissitudes of 
fortune, and the parents know nothing of their fate. In 
many cases the parent dies before the young are hatched. 
The mammals and birds are the only two great groups ex- 
cepted, although there are numerous specific exceptions 
scattered among the lower invertebrates, fishes, batrachians, 
and higher invertebrates, notably the insects. 
There are no instances of care of the young after hatch- 
ing among the sponges, polyps, worms, or star-fishes and 
sea-urchins, and but few among the crustaceans and mol- 
lusks. But there are in some of these groups a few cases 
of nest or home building in a crude and simple way. Cer- 
tain sea-urchins (Fig. 156) and worms and mollusks bore 
into stones, and remain in the shelter afforded by the cav- 
ity. A shell-fish (Lima hiams) cements together bits of 
coralline, shells, and sand to form a crude nest or hiding- 
place. The cray-fish digs a cylindrical burrow in the ground 
in which it can hide. 
