xiV DISTRIBUTION 633 



and Elasmobranchs — are also littoral, and Penguins, Seals, and 

 Sirenia may be included in the list. 



Next in abundance to the.littoral is the pelagic or ocean-surface 

 fauna, including animals which live habitually on the surface or 

 at slight depths of the ocean, often far from land. Amongst them 

 are many Foraminifera, such as Globigerina and Hastigerina, the 

 Eadiolaria, the Siphonophora, the majority of Medusae, both 

 hydrozoan and scyphozoan, the whole class of Ctenophora, many 

 Entomostraca and Schizopods, the hemipterous Insect Halobates, 

 the Pteropoila, Heteropoda, and some other Gastropods, such as 

 Glaucus, most Cephalopods, Pyrosoma and the Salps, numerous 

 Teleosts, such as Herrings, Flying-fish, Mackerel, &c., the greater 

 number of Sharks, and the majority of Cetacea. 



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, Echinoderms, 

 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 glohigerina-oose, consist- 

 ing 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 Hoxactinellida ; a few Medusas and Corals ; examples 

 of all classes of Echinoderms, Stalked Crinoids, and Holothurians 

 being especially abundant ; Crustacea, particularly Schizopods and 

 Prawns; and Teleostei. Crabs, Molluscs, and Annulata are rare. 



Many abyssal animals are blind, including several of tlie 

 Crastacea; 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. 883). Other forms, such as the Ribbon- 

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



