120 THE LIFE OF CRUSTACEA 



At a depth of only 33 feet of water this pressure is 

 doubled, and at greater depths the pressure increases 

 in proportion, till at 2,000 fathoms it is more than 

 2J tons on the square inch. As a matter of fact, 

 however, the animals at the bottom of the sea are 

 probably but little affected by this enormous pressure. 

 Only, when they are brought up by the dredge the 

 sudden release of pressure causes the fluids of the 

 body to expand and destroys the tissues, so that the 

 animals are generally dead or dying when they reach 

 the surface. 



More important than the pressure in its influence 

 on life is the darkness of the depths. The light of 

 the sun only penetrates the water of the sea to a 

 comparatively small depth. At 200 fathoms there 

 is not enough light to produce any effect on a photo- 

 graphic plate. Even at a considerably less depth 

 the absence of light puts an end to all plant-life, 

 except for the ubiquitous bacteria, and it follows 

 that all the animals of the deep sea ultimately depend 

 for their food-supply on the rain of dead bodies of 

 surface animals which, as already mentioned, is con- 

 stantly falling on the sea-bottom. 



The temperature at the bottom of the deep sea is 

 always very low. Dr. Alcock states that " in the 

 open part of the Bay of Bengal, where the mean 

 surface temperature is about 80° F., the temperature 

 at a depth of 100 fathoms is only about 60° P., at 

 a depth of 300 fathoms not quite 50° F. ; while at a 



