470 THE DISTRIBUTION OF ANIMALS 



ing surf. Animals, such as the barnacles, grow on the surfaces of rocks 

 and close their shells tightly when necessary. Clams and many marine 

 worms bury themselves in the sand and mud which remain moist until 

 the tide comes back. Tides vary in height from one and one-half inches 

 in the Mediterranean to over fifty feet in the Bay of Fundy. 



The Pelagic Zone. This zone includes the open sea to the depth 

 that light penetrates. Animals here must either swim or float, since 

 there is no bottom on which to attach themselves or to rest. Among 

 the larger animals we find the whales and porpoises. At the other ex- 

 treme we find many very tiny animals, including those which are micro- 

 scopic. These are often found floating together in great masses known 

 as plankton. Many animals in this zone have special organs which keep 

 them afloat. The Portugese man-o-war has a float filled with air which 

 is so large that it serves as a sail. Many of the smaller pelagic animals 

 achieve protection through transparency. Some are so transparent that 

 one can see through them so perfectly that it is hard to realize that they 

 are really present. Both the swimming and floating forms are similar 

 in the Atlantic and the Pacific oceans. 



The Abyssal Zone. This region occupies the ocean depths beyond 

 the point of light penetration, including the floor of the ocean in these 

 deep areas. According to William Beebe's observations in his bathy- 

 sphere, no light penetrates beyond 1500 feet. The water is always very 

 cold, and the pressure is tremendous ; but, because this pressure is dis- 

 tributed throughout animals' bodies, it has little effect except when ani- 

 mals are brought up. When raised to the surface they usually swell up 

 and the air bladders of bony fish may explode. No food is produced in 

 this area, since without light there can be no photosynthesis. The ulti- 

 mate source of all food is from the bodies of plants and animals which 

 sink down from the light areas up near the surface. This includes every- 

 thing from whales to the microscopic organisms, which sink gradually 

 down through the miles of water like gentle rain. 



Abyssal animals fall sharply into two groups, those that are scav- 

 engers, living on this shower of dead organic matter, and those that are 

 predators on the scavengers or on each other. The most abundant deep- 

 sea animals are the sea cucumbers ; others being snails, crustaceans, 

 tunicates, cephalopods, and fish. The predaceous fish have large mouths 

 filled with long, sharp teeth, and stomachs capable of great stretching, 

 for they actually swallow other fish larger than themselves. Many of 

 these have lures with a light on the end which attracts their prey, and 

 many also have a row of lights on their bodies. Light-producing organs 

 are also found in the coelenterates, echinoderms, annelids, crustaceans, 

 and cephalopods. Many of the fish have large telescopic eyes. It is 



