448 THE GENERAL PRINCIPLES OF ZOOLOGY 



higher vertebrates, such as Chelonia and Sirenia, while some 

 birds and the Pinnipedia may be added. The littoral fauna 

 may be said to extend outward along the continental shelf to 

 a depth of about three hundred meters. Beyond this point the 

 continental shelf descends more or less precipitously into the 

 deep sea, and along this slope we find animals more or less inter- 

 mediate between these of the littoral fauna and the representa- 

 tives of the deep-sea fauna ; they have been called the continental 

 fauna, extending downward to a depth of 1500 or 1600 meters. 

 Below this depth, at which we have reached a temperature at 

 least as low as 4.5 C. (40 F.) we come to the deep-sea or abyssal 

 fauna. These limitations of depth for the various faunae must, 

 however, be taken only in the most general way ; the animals of 

 one fauna are not sharply marked off from those of another. 



Our knowledge of deep-sea life is of very recent date, and 

 there are still broad and alluring fields for investigation in this 

 direction. As late as the middle of the nineteenth century it 

 was believed by scientists that there was no life in the ocean be- 

 low a depth of about 600 meters, despite the fact that fisher- 

 men had taken sharks from much greater depths, and had 

 occasionally brought up siliceous sponges on their lines. In 

 1 86 1 a telegraph cable in the Mediterranean was raised from a 

 depth of 4000 meters, and living animals were found attached 

 to it. At about this time Scandinavian investigators dredged 

 several types of invertebrates from depths of from 2000 to 

 nearly 3000 meters. Thus it became clear that even the depths 

 of the ocean were inhabited by animals, and the question arose 

 whether this deep-sea fauna was continuous vertically with the 

 pelagic fauna. This was answered conclusively in the negative 

 for the first time in 1880 by American scientists, who used nets 

 which could be sent down closed to any depth, then opened and 

 dragged for any desired distance, closed again and then brought 

 to the surface ; these nets, when dragged at a depth below what 

 we have already noted for the extent of the pelagic fauna, in- 

 variably came up empty. Thus we see that along the edge of the 

 continental shelf the littoral fauna divides, as it were, one por- 

 tion extending outward over the surface of the ocean as the 

 pelagic fauna, the other descending into the depths along the 

 ocean bottom to form the abyssal fauna. 





