V. 



Distribution of marine life. — Nature of tlie deep-sea fauna. — Oceanic pelade 

 fauna. — Littoral fauna. — Pelagic faunas of lakes.— Deep-lake faunas. 



DISTRIBUTION OF MAEINB LIFE. 



One of the most important results of deep-sea explorations is 

 the establishment of the fact that the distribution of oceanic life 

 has no depth-limit. Contrary to the opinion so long entertained 

 by naturalists that this life was confined to a shallow zone, extend- 

 ing but a few hundred feet beneath the water's surface, it is now 

 known that representatives of all the marine invertebrate classes, 

 and probably also fishes, exist at the greatest depths that have been 

 reached by the dredge, and that in all likelihood a fair proportion of 

 these penetrate even to the profounder abysses of four or five miles. 

 The most extensive organic deposits accumulating in the trough of 

 the sea are made by the Radiolaria and Poraminifera, whose world- 

 wide distribution and prodigious development give the determining 

 character to the oceanic floor. The well-known " Globigerina," or 

 "Atlantic" ooze, a composition in principal part of the calcareous 

 tests of four or five genera of Foraminifera (Globigerina, Orbulina, 

 Spheroidina), constitutes the bed of the sea at nearly all points be- 

 tween the depths of four hundred and two thousand, or two thou- 

 sand five hundred fathoms, except where, as in the immediate 

 neighbourhood of coast-lines, the bottom is formed by the conti- 

 nental debris. Over this vast calcareous area animal life is far more 

 abundant than over the comparatively sterile region where, for any 

 reason, the ooze is wanting, and where, consequently, there is a 

 marked deficiency in the material necessary for the proper develop- 

 ment of many of the lower forms of life. Thus, it has been noticed 

 that in such localities chiefly the shell-less orders of animals, as the 

 Holothuroidea and the Annelida, are represented. Beyond a depth 



