PROTECTIVE ACTIVITIES 



371 



ocean and a constant current of 

 water brings oxygen and food, and 

 carries off wastes and reproductive 

 cells (see Fig. 44). 



When we come to the highest 

 animals (the insects) of the branch 

 arthropods and the highest animals 

 of the backboned branch (birds and 

 mammals), we find very complex 

 activities related to the making of 

 homes. The solitary wasp goes no 

 farther than burying a few insects 

 that later serve as food for the 

 young. The social wasps and hor- 

 nets, like the related bees and ants, 

 build very elaborate homes out of 

 "paper" (which they make from 

 wood pulp and other materials) and 

 out of wax and earth (see Fig. 187). 



Nest-building among the birds 

 involves complex instincts, and pos- 

 sibly in some cases a degree of real 

 intelligence. From the crude whisps 

 of the grouse, or the simple mud 

 heap of the flamingo, to the deli- 

 cate and skillful work of the tailor 

 bird, we find a long series of nests 

 of many degrees of complexity in 

 structure. But with the exception 



FIG. 190. Nettling cell of 

 jellyfish 



This specialized skin cell, A, con- 

 tains a fine coiled thread suspended 

 in a capsule of acid fluid. When the 

 surface is disturbed at the trigger, 

 /, the coil suddenly straightens out, 

 shooting the sharp needle into the 

 surrounding space, and at the same 

 time the acid fluid from the cell 

 passes through the hair. The sting- 

 ing sensation is probably produced 

 by this fluid. B, the discharged cell 



of homes made in hollows, like that 

 of the woodpecker, whatever shelter nests may furnish serves 

 almost exclusively for the protection of the young (Fig. 188). 

 Indeed, we may say that the making of shelter among the 

 higher animals is closely related to the protection of the young, 



