CRUSTACEA OF THE DEEP SEA 121 



depth of 2,000 fathoms the temperature all the year 

 round is only 3° above freezing-point." 



Finally, it is important to notice the uniformity of 

 the conditions at the bottom of the sea ; not only 

 are the alternation of night and day and the progress 

 of the seasons unfelt in the abysses, but the con- 

 ditions are practically the same over vast areas in all 

 the oceans. 



In the case of deep-sea Crustacea, we are fre- 

 quently confronted with a difficulty which does not 

 occur in the case of some other groups of animals — 

 Corals or Echinoderms, for example — the difficulty, 

 namely, of deciding whether the animals really lived 

 on or near the bottom, or were captured by the open 

 mouth of the trawl on its way to the surface. When 

 the animals are plainly not well adapted for swim- 

 ming — as, for instance, most of the Crabs — it may 

 be assumed that they did actually live on the 

 bottom ; but, with the prawn-like forms, the possi- 

 bility that they may really be inhabitants of the 

 intermediate depths must always be taken into 

 consideration. 



In animals that live in perpetual darkness we 

 should expect to find, in accordance with the prin- 

 ciple of adaptation which runs through the whole 

 of organic nature, that the eyes are wanting or 

 imperfectly developed. In a great many deep-sea 

 animals this is indeed the case. The deep-sea 

 Lobsters of the genus Nephropsis (Fig. 42), which 



