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 (algae). 

 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. 



