178 THE DISTRIBUTION OF 
with the latitude for the reasons already discussed, 
there seems to be a fauna of silvery or iridescent 
animals, of which the fish have been chiefly investi- 
gated, though medusae, and doubtless other forms also 
occur. The fish living at these depths inhabit an 
environment where a considerable amount of sunlight 
penetrates, especially blue and violet rays. They have 
often large eyes, which sometimes protrude at the end 
of short stalks, and are then called telescopic. Large 
light organs are present, which perhaps catch the last 
rays of sunlight. An example is the silvery fish Argyro- 
pelecus affinis, which lives in the Atlantic at depths of 
somewhere about 200 fathoms; at night, when the 
light conditions are the same near the surface as in the 
depths where it habitually dwells, this fish has been 
found within some 80 fathoms of the surface. Its 
young have the same colouring as their parents, and 
live in the same depths. Another form with the same 
colouring and characters is Cyclothone signata, which 
lives at a depth of about 300 fathoms. 
Still deeper, at depths of from 300 to 500 fathoms, 
where all trace of red and green rays have completely 
disappeared, occur forms in which the colouring is 
black or red. In the absence of any red rays both 
these colours must look the same, and either will pro- 
tect the animal. At these depths occur deep-red or 
chocolate-coloured medusae, the only pelagic echino- 
derm (Pelagothuria), bright-red prawns, such as 
Acanthephyra multispina, and red or black fish, Se- 
bastes norvegicus being an example of the former and 
Goniostoma elongatum of the latter. The last-named 
fish is purplish-black, with small light organs and 
small eyes. It has huge jaws, a common feature in 
these deep-water fish. A very curious point about it 
