754 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 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 translu- 

 cent ; 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 character- 

 istic. 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 Abyssal Fauna. 



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 intermediate zone 

 between the light-limit and the greatest depths. We know, 

 too, that there are representatives of most types from 

 Protozoa to Fishes, though Sponges and Echinoderms 

 preponderate, and that the distribution tends to be 

 cosmopolitan, in correspondence with the uniformity of the 

 physical conditions. 



The abyssal fauna includes many flinty sponges, some 

 corals and sea-anemones, possibly 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, many more than half-blind, others 

 catching with darkness-eyes the fitful gleams of phos- 

 phorescence. 



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 34 F., for 

 the sun's heat is virtually lost at about 150 fathoms; the 



