250 MARINE ANIMALS 



The abyssal benthic community. — The lightless benthal exhibits 

 different habitats corresponding to the different biotopes of the littoral, 

 principally based on differences in the substratum. Rock bottom is 

 present in deep water only at very isolated places where special con- 

 ditions cause a current of depths of more than 400 m. (see p. 154). 

 With these exceptions the bottom of the sea is covered with deposited 

 materials, of varied origin and composition. Murray distinguishes 

 terrigenous and pelagic deposits, the former derived from land areas 

 and made up of decomposed rock, principally quartz sand. Such mate- 

 rials are in general deposited near land, but they may occasionally 

 be carried to great distances by icebergs, though not in sufficient 

 quantity to produce a uniform deposit. The coarser deposits are laid 

 down closer to the land, and abyssal terrigenous deposits consist only 

 of the finest materials. Large rivers may carry such material to the 

 border of the continental shelf at a depth of 200 m. 



The sea-bottom deposits. — Deposits beneath the open ocean con- 

 trast with the terrigenous in being composed of the remains of pelagic 

 organisms. They are regularly found in waters beyond the 200-m. line. 

 They are composed of the calcareous or siliceous skeletons of animals, 

 and their nature varies with locality and with depth. Murray dis- 

 tinguishes pteropod ooze and Globigerina ooze as calcareous, and 

 diatomaceous and radiolarian ooze as siliceous. The calcareous de- 

 posits are developed especially in subtropical and tropical regions, 

 where lime deposition by animals is facilitated. The siliceous deposits 

 appear in polar regions and in regions where an abundant influx of 

 argillaceous material supplies the plankton with silica, as in the East 

 Indies. At very great depths the pressure prevents the deposition of 

 either lime or silica, which are redissolved, and the only deposit is a 

 red clay, the product of the decomposition of volcanic materials. 

 Pteropod ooze is found in relatively shallow water and relatively small 

 areas. The Globigerina ooze is widespread, covering 29.2% of the ocean 

 bottom (105,000,000 sq. km.), second only to the red clay in extent. 

 It prevails especially in the North Atlantic, broken only by isolated 

 areas of red clay. The red clay is the typical deposit of the Pacific. 



Globerina ooze, on account of its lime content, is especially favor- 

 able to the development of animal life. 7 At certain of the Challenger's 

 stations in the Indian Ocean, where diatom skeletons composed the 

 principal deposit, the higher animals contained only small amounts of 

 lime in their shells. Thus among the echinoderms there are some very 

 thin-shelled and irregular sea urchins, and an excess of holothurians. 7 

 Life on the red clay is everywhere poorly developed, consisting of 

 shell-less holothurians and worms. This fact may be in part due to the 

 great depths in which the clay is found. 



