896 GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



The fauna of the open sea is representative, but there are 

 few of the types which we can suppose to have Uved there 

 always. It may be that forms hke the minute water-fleas 

 have been there almost from the first, but most bear the 

 impress of lessons which the open sea could never have 

 taught them. 



Pelagic animals tend to be delicate and translucent ; 

 many are phosphorescent. The number of species, differ- 

 ing from one another within a relatively narrow range, is 

 often enormous ; thus about 5000 species of Radiolarians 

 are known. The huge number of individuals, which 

 frequently occur in great swarms, is equally characteristic. 

 Perhaps both facts indicate that the conditions of life are 

 relatively easy, as is also implied in the limitless food-supply 

 afforded by the unicellular Algae. The pelagic fauna is 

 richest in the colder seas. 



Abyssal. — Through the researches of the Challenger and 

 similar expeditions, we know that there is practically no 

 depth-limit to the distribution of animal life, though the 

 population is denser at moderate depths than in the deepest 

 abysses, and though there is probably a thinly-peopled 

 zone between the light-limit and the greatest depths. 

 We know, too, that there are abyssal representatives of 

 most types from Protozoa to Fishes, and that the distribu- 

 tion tends to be cosmopolitan, in correspondence with the 

 uniformity of the physical conditions. 



The abyssal fauna includes some Foraminifera and 

 Radiolarians, many flinty sponges, some corals, sea- 

 anemones, and Alcyonarians, a few medusae, annelids and 

 other " worms " on the so-called red clay, representatives 

 of the five extant orders of Echinoderms, abundant Crusta- 

 ceans, representatives of most of the Mollusc types, and 

 peculiarly modified Fishes, some with small eyes, others 

 with large eyes, which probably catch the fitful gleams of 

 phosphorescence. 



As to the physical conditions, the deep-sea world is in 

 darkness, for a photographic plate is not influenced below 

 250-500 fathoms ; it is extremely cold, about the freezing- 

 point of fresh water, for the sun's heat is virtually lost at 

 about 150 fathoms ; the pressure is enormous — thus at 

 2500 fathoms it is about 2J tons per square inch ; the cold 



