THE HAUNTS OF LIFE 77 



several horny plates separated by deep grooves, the petrels 

 appear to have been very successful, for they are to be 

 found in all the oceans including the Arctic and Antarctic 

 and they are represented by a great variety of types from 

 the tiny storm petrel to the gigantic albatross which may 

 have a spread of wing twice the height of a man. As we 

 have already said, there are many larval forms of shore 

 animals which pass through a pelagic phase. They cannot 

 be counted in except for the time being, and the same 

 must be said of the Leptocephali or transparent young 

 stages of various eels. Nor can we include such fishes 

 as salmon and sea-trout, which really belong to the fresh 

 waters, though so much of their energy is acquired during 

 their visits to the sea. There are some pelagic animals, 

 such as the arrow- worms (Sagitta and the like), which we 

 can think of as having always lived in the open sea, but 

 most seem to bear the impress of lessons which the open 

 sea could never have taught them. In such a case as 

 the Halobatidae (pelagic insects) it is obvious that the 

 open sea is a secondary home. 



Adaptations. Among the adaptations to pelagic life, 

 the following seem most important. In several ways the 

 floating capacity is increased : by the formation of gas 

 reservoirs, such as the great float like a glorified cock's- 

 comb of the Portuguese Man of War ; by the development 

 of light and buoyant tissue, as in the jelly of Medusae ; 

 and by the enormous development of delicate outgrowths 

 which give the creature a wide surface of contact with 

 the water, as we see, for instance, in many of the pelagic 

 Crustaceans. We cannot glance at them without feeling 

 that architecture of this sort could not survive the sea- 

 shore conditions for a day. 



