Chap, xxii.] DEPTHS AT WHICH ANIVlALS LIVE. 



490 



times, especially at night when safe from enemies, and again 

 descending. It is quite uncertain to what depth they ext1:nd 

 their range, and whether there is a zone of water intermediate 

 between that near the bottom and that near the surface, which 

 is devoid, or nearly devoid, of life, as is believed by Sir Wyville 

 Thomson to be probably the case. 



The trawl-net used on board the "Challenger" swept, in 

 going down to the deep-sea bottom and in coming up, the 

 entire depth of the sea, and animals were constantly being 

 found in the net, about which it was quite uncertain as to what 

 depth they came from. Amongst these were, for example, 

 some MeduscE, which have been found by Prof. Haeckel to be 

 of peculiar structure, and which may possibly be deep-sea 

 forms ; they may, however, have come from a few fathoms' 

 depth only. 



To settle this question a net of some kind is required 

 which shall be capable of being sent down completely closed 

 to any required depth, then opened and towed for some time, 

 and then again closed before it is raised. It is by no means 

 an easy matter to devise such a net which will be practically 

 available. There are numbers of animals, fish, Meduscc, and 

 Actinia for example, which are found in the deep-sea trawl, 

 and about which it is a matter of speculation only as to the 

 depth from which they came. 



Mr. Murray hit upon the expedient of using the ordinary 

 towing-net at considerable depths,* and with great success, 

 since he constantly obtained large catches of pelagic animals, 

 when very few were obtainable at the surface. 



Pelagic animals are most widely spread, closely similar 

 forms occurring in widely distant oceans. In this particular, 

 the pelagic fauna resembles that of the deep sea. In the case 

 of the sea surface winds and currents are present either to aid 

 or Hmit the range of species, and the variety of climate acts as 

 a barrier. In the deep sea all these forms of restriction are, 

 however, absent. 



The Deep Sea and its Fauna. — I have above briefly de- 

 scribed the vegetation and fauna of the ocean surface, because, 

 did these not exist, life would be impossible, or only extremely 

 scanty, at the deep-sea bottom. Before referring to the fauna 

 of the deep sea, it will be well to consider briefly the conditions 

 under which it exists. 



If a globe, 40 feet in diameter, be taken to represent the 



* A. Baur was, I believe, the first to use the towing-net at considerable 

 depths. " Beitrage zur Naturgeschichte der Synapta digitata." Verhandl. 

 der K.L.C.D. Akad. 1864. Mr. Murray, however, invented the method 

 independently. 



