796 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



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

 always. It may be that forms like 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, differing 

 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 distribution 

 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 z\ tons per square inch ; the cold 

 water in sinking from the polar regions brings down much 



