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The pelagic Invertebrates are mostly distinguished by great 

 transparency, and by being either colourless or of a blue or violet 

 hue. Pelagic Fishes are usually grey or steel-blue above, white 

 beneath, presenting none of the brilliant colours, varied mark- 

 ings, and extraordinary forms so often found among Shore-fishes. 



It must be remembered that many littoral animals are pelagic 

 in the larval condition, or during some phase of their life-history, 

 e.g., many Sponges, fixed Hydrozoa and Actinozoa, Echinodermata, 

 Annulata, Mollusca, Crustacea, and Fishes. 



The abyssal or deep-sea fauna is far more abundant than 

 might be supposed from the physical conditions immense pres- 

 sure and absence of light and of vegetation. In most parts of the 

 world the bed of the ocean, at depths from 400 to 2,000-2,500 

 fathoms, is formed of a greyish mud called globigcrina-ooze, con- 

 sisting largely of the shells of Foraminifera, such as Globigerina, 

 Orbulina, &c., which have for the most part sunk to the bottom 

 after death. Below 2,500 fathoms the sea-bottom is formed of a 

 red clay, in which shells are absent, having apparently been 

 dissolved during their descent to the greater depth. 



Living on the sea-bottom, and most abundant on the globigerina- 

 ooze, are representatives of many groups of animals : Sponges, 

 especially Hexactinellida ; a few Medusa3 and Corals ; examples 

 of all classes of Echinoderms, Stalked Crinoids and Holothurians 

 being especially abundant ; Crustaceans, particularly Schizopods 

 and Prawns ; and Teleosts. Crabs, Molluscs, and Annulates 

 are rare. 



Many abyssal animals are blind, including several of the 

 Crustacea ; many others are phosphorescent, and thus supply their 

 own light in an otherwise dark environment. The deep-sea 

 Teleosts are often of very grotesque appearance, with immense 

 heads, wide mouths furnished with long, pointed teeth, extremely 

 distensible stomachs, and phosphorescent organs arranged in rows 

 along the body (see Fig. 894). Other forms, such as the Ribbon- 

 fish (Regalecus), attain a great size, and are toothless. When 

 brought to the surface, the expansion of the gases in the interior 

 of the deep-sea Teleosts often bursts the air-bladder, and produces 

 a general disintegration of the tissues. 



Plankton, Nekton, and Benthos. Besides being arranged 

 with regard to their relations to the shore, the surface of the 

 ocean, and its bed, marine animals are also conveniently classified 

 on the basis of their capacity for movement. Many forms, such 

 as Medusae, Siphonophora, Ctenophora, Salps, and numerous 

 pelagic larvae are carried along passively by oceanic currents, their 

 own powers of progression being of the feeblest. Such animals 

 together constitute the Plankton, or " drif ting-fauna." Others 

 swim actively by means of fins or other appendages, such as the 

 pelagic Teleosts and Elasmobranchs, Schizopods, Prawns, and 



