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 fréquently 
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 Alge. 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 medusz, 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 24 tons per square inch; the cold 
water in sinking from the polar regions brings down much 
