392 ANNUAL. EEPOKT SMITHSONIAN INSTITUTION, 1923 



where. On the ear bones and the teeth manganese slowly collects, 

 in time inclosing them in characteristic nodules of various sizes. As 

 over the red mud oceanic life is so very scanty as to be practically 

 nonexistent, these nodules for the most part probably represent the 

 remains of decrepit sharks and whales which have strayed out here 

 and died. 



Below the twilight zone the variety of animal life rapidly lessens, 

 and, on account of the uniformity of conditions in all oceans at these 

 levels, becomes practically the same everywhere. 



The basic food here consists of detritus from the plants along the 

 shores, decreasing rapidly in amount with distance from the land, 

 and a correspondingly increasing amount of organic matter derived 

 from the bodies of the creatures in the layers above which, dying, 

 sink gradually to the bottom, where further decay is arrested by the 

 perpetual cold and the great pressure which prevent, or at least 

 inhibit, the action of bacteria. The foraminifera, pteropods, di- 

 atoms, etc., and the Sargassum and other floating seaweeds dying 

 and going to the bottom carry there at least a portion of their or- 

 ganic substance, which mixes with the mud. This bottom ooze or 

 mud when brought on deck seems absolutely clean, but in the warm 

 air it soon gives forth a most offensive smell, proclaiming the organic 

 matter it contains. 



This mud is swallowed by many of the bottom animals, particu- 

 larly by all of the numerous echinoderms, except the crinoids, and 

 by many of the crustaceans, which digest the organic matter out of 

 it, living in the same way that some of their relatives do along the 

 shores and the earthworms do on land. For the other animals, such 

 as the sponges, sea squirts, stalked crinoids, and coelenterates, inter- 

 mediates are necessary to make this food available, and these inter- 

 mediates seem to be the numerous forms of radiolarians and related 

 types which, judging from the long stalks of the attached animals, 

 must in some places form a thin mist for some distance above the 

 sea floor. 



Deep-sea animals are much more common near land off precipit- 

 ous coasts than at the same depths farther out, in correspondence 

 with the greater density of life in the layers above, and also under 

 cold surface water. They are, after all, only littoral types with 

 sufficiently adaptable natures to enable them to descend to the 

 greatest depths, and fundamentally they differ very much less from 

 the shore types than would be supposed. In the Tropics the dif- 

 ference between the littoral and the abyssal animals is great, and the 

 change from one sort to the other rather abrupt, but in the cold 

 regions many of the deep-sea types come up into shallow water. 



The reasons why all sea animals are most abundant near the 

 land and gradually decrease in abundance and in size with increas- 



