EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 405 



for instance in about 500 fathoms, hundreds of specimens of holothu- 

 rians,pycnogouids, and crustaceans have been procured in a single haul, 

 and just beneath the mud line at a depth of about 100 fathoms around 

 continental shores enormous numbers of individuals belonging to one 

 species have been procured. This is the great feeding ground in the 

 ocean. To this depth the herring, salmon, whales, narwhals, descend 

 to feed upon the immense numbers of individuals belonging to species 

 of Calanus, Euchceta, Passiphcea, Crangon, Hippolyte, as well as species 

 of schizopods, amphipods, isopods, fishes, and cephalopods. 



Probably the majority of deep-sea species live by eating the surface 

 layers of the mud, clay, or ooze at the bottom, and by catching or pick- 

 ing up the small organisms, or minute particles of organic matter which 

 fall from the surface or are washed away from the shallower reaches of 

 the ocean, and ultimately settle on the bottom beyond the mud line. 

 These mud-eating species are in turn the prey of numerous rapacious 

 animals armed with peculiar tactile, prehensile, and alluring organs, for 

 phosphorescent light plays an important role in the economy of deep- 

 sea life, aud is correlated with the red and brown tints of the majority 

 of deep-sea organisms. Some species are blind, and others, in addition 

 to large eyes, are provided with a sort of bull's-eye lantern, from which 

 streams of light are thrown out at the will of the animal. Phospho- 

 rescent organs act sometimes as a lure, sometimes they indicate the 

 presence of prty or the passage of an enemy. 



Some species of deep-sea organisms are of gigantic size when com- 

 pared with their shallow water allies. Some of the hexactinellids are 

 3 or 4 feet in diameter; the hydroid Monocaulus is 3 feet in height; the 

 legs of some pycnogonids extend for over a foot on either side of the 

 body, and the largest echini and isopods are found in deep water. 



Before the systematic investigation of the deep sea it was believed 

 by some naturalists that the remnants of faunas which flourished in 

 remote geological periods would be found in the great depths of the 

 ocean. This expectation has not been realized. Discina and some 

 other brachiopods undoubtedly represent a very ancient group ; still the 

 king-crabs, lingulas, trigonias, Port Jackson sharks, Heliopora, AmpM- 

 oxus, Ceratodus, Lepidosiren, and other shore and shallow-water forms 

 undoubtedly represent older forms than anything to be found in the 

 deep sea at the present time. 



Sir Wyville Thomson was of opinion that from the Silurian period to 

 the present day there had been, as now, a continuous deep ocean, with 

 a bottom temperature oscillating about the freezing point, and that 

 there had always been an abyssal fauna. It is much more probable 

 that in palaeozoic times the oceau basins were not so deep as at the 

 present time; that the ocean had then a nearly uniform high tempera- 

 ture throughout its whole mass, and that life was either absent through- 

 out all the greater depths or represented only by bacteria, as in the 

 Black Sea at the present day. 



