226 MARINE ANIMALS 



The most effective means of all for reducing the excess of weight 

 is the inclusion of air or other gases in the animal body. Among 

 siphonophores, air sacs (pneumatophores) are present, which are filled 

 with gas from a gas-producing gland. In Velella, which floats on the 

 surface, branched air sacs are connected with the atmosphere, and 

 filled with air by rhythmic breathing motions. The cephalopods 

 Nautilus and Spirula have a chambered shell with air in the chambers. 

 The pelagic snail Glaucus (Fig. 53a) has intestinal gases which are 

 supposed to play a similar part. An air sac is best developed among 

 the bony fishes, which usually have an air bladder whose gas content 

 is under control, so that their weight may be exactly adjusted to the 

 water displaced; thus their independence from the bottom is complete. 

 The bony fishes are by far the most numerous of the vertebrate 

 animals of the pelagial. One may say that the change from a littoral 

 to a pelagic existence, where no place of rest is available, was made 

 possible by the acquisition of an air bladder, for only the extremely 

 powerful sharks and a few bony fishes (without an air bladder) are 

 capable of the unceasing muscular exertion necessary to keep from 

 sinking. The air bladder has also made the bony fishes more inde- 

 pendent in the matter of body form, freeing them from the necessity 

 of having flattened lower surfaces and large pectoral fins, so that they 

 may be deep bodied and even wedge-shaped ventrally, as the selachians 

 never are. 10 Among the air-breathing vertebrates which have taken 

 up a marine existence, such as the sea turtles and water snakes, the 

 whales, sirenians, and seals, the lungs serve the same purpose as does 

 the air bladder of fishes. 



These different means of reducing the density may occur in varied 

 combinations, such as air chamber and jelly tissue in the siphonophores, 

 and fat accumulation and air bladder in the moonfish. When an 

 excess of density still is present, it must be overcome by increased 

 friction with the water and resistance offered by the surface. The 

 amount of resistance in the water, aside from friction, depends on the 

 sum of the distances to which the water particles are forced when 

 the body is moved. The amount of such displacement is increased with 

 increase in size in the horizontal plane of a sinking body. Obviously 

 a sheet of lead will sink more slowly if laid in the water on its side 

 than if placed edgewise. One may speak in this sense of the form 

 resistance of a body, which increases with the increase of the lower 

 surface. Such water resistance may be actively increased by an animal 

 by swimming. 



The retardation of sinking by changes in form is a widespread 

 phenomenon among pelagic invertebrates. It is possible to equalize 



