INTEODl'CTOEY. ii> 



Apart from darkness, which we have already considered, 

 deep-sea animals are snbjected to enormous pressure : at a 

 thousand fathoms this pressure is something like a ton to the 

 square inch. 



The temperature, again, is low ; the heat of the sixn does not 

 apparently extend to a greater depth than a hundred and 

 fifty fathoms. The investigations of the Challenger have 

 shown that there is everywhere a progressive decrease in heat 

 from the surface water downwards in the great sea basins. In 

 the deep abysses the average temperature is about 34° ; it is 

 occasionally below freezing point. 



These temperatures, it must be observed, do not fluctuate. 

 In the shallow waters there is of course fluctuation. 



As to the colours of the deep-sea animals, the tints are 

 sometimes deeper. Crustaceans are often brilliant scarlet, the 

 pigment being much more abundant than in the pelagic 

 forms. Professor Agassiz states that " the echinoids dredged 

 beyond the hundred-fathom line are many of them dark- 

 coloured " ; black is a common colour in deep-sea fishes. In 

 examining the Challenger collection of the curious Isopod 

 genus Serolis, which bears so striking a superficial resemblance 

 to the extinct Trilobites, I found that three out of the four 

 deep-sea species were much more darkly and uniformly 

 coloured than any of the shallow-water species ; I imagine 

 that the pigment is the same, only more abundant. 



Change of Colour after Death. 

 It is, of course, well known that the colours of insects, 

 lizards and other animals frequently change after death ; the 

 change generally takes the form of a diminution in depth of 

 colour, — the colours fade: the delicate greens of many moths— 

 the Emerald moths, for example— become a pale straw coloiir. 



