40 



EVOLUTION AND ANIMAL LIFE 



conditions which may determine the existence of hfe. The 

 pressure or weight of the atmosphere on the smface of the 

 earth is nearly fifteen pounds on each square inch. This 

 pressure is exerted equally in all directions so that an object on 

 the earth^s surface sustains a pressure on each square inch of 



Fig. 30. — Cephalopods. Lower figure, the devil-fish or octopus, Octopus punctatus. 

 The upper figure represents the squid, Loligo pealii, swimming backward by 

 driving a stream of water through the small tube slightly beneath the eyes. (From 

 life, one-third natural size.) 



its surface of fifteen pounds. That is, all animals liv'ng on the 

 earth's surface or near it live under this pressure and under 

 no other condition. The animals that live in water, how^ever, 

 sustain a much greater pressure, this pressure increasing with 

 distance. Certain ocean fishes live habitually in great depths, at 

 from two to nearly five miles, Avhere the pressure is equivalent to 

 that of many hundred atmospheres. If these fishes are brought 

 to the surface their eyes bulge out, their scales fall off because 

 of the great expanse of the skin, and the stomach is thrust 

 wrong side out. Indeed the body itself sometimes bursts- Oa 



