484 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Most of them are transparent and colourless, and thus invisible in 

 water ; where colour occurs, it is usually violet or blue, resembling 

 that of the water ; the fishes are chiefly steel-blue above, silvery-white 

 below. Most forms are naked ; the shell, if present, is comparatively 

 delicate. A large number are viviparous, even when their nearest 

 allies are oviparous. 



A great number of pelagic animals are phosjmorescent. Nearly 

 all are admirable swimmers. Some have their surface-area largely 

 developed, e. g. the tests of Eadiolaria, of Globigerina, Hastigerina, &c, 

 probably in order to hinder sinking. As to the manner of life, they 

 are almost without exception social, they mostly have a very wide 

 distribution, and are found alike in the Atlantic. Indian, and Pacific 

 Oceans ; the genera are almost all identical in these seas, although 

 polar seas are distinguished from warmer waters by possessing few 

 forms besides Crustacea, Pteropoda, Cephalopoda, and Cetacea. In 

 connection with their usually delicate structure stands the fact that it 

 is only in the calmest weather that they live on the surface ; storms 

 may drive them to a depth of more than 50 fathoms. Further, far 

 the majority only come to the surface in the night, a point to be con- 

 sidered in connection with the prevalence among them of phosphor- 

 escence ; the time of appearance of the phosphorescent fish is more 

 often connected with the night than with any other time. Pelagic 

 animals seldom occur except over deep water, and at great distances 

 from coasts, hence their scarcity in the German Ocean, their poverty 

 in littoral and their abundance in deep-sea deposits. 



The deep-sea fauna is distinguished by the appearance or pre- 

 dominance of certain individual species, genera, and families, and 

 exhibits little variation in the different parts of the world. It com- 

 mences at a depth of about 50 fathoms in all seas, but it is only in 

 the tropics that anything like a sharp line of demarcation is found 

 between it and the littoral fauna. Examining these points to ascertain 

 the reason for a bathymetric limit of this particular nature, Fuchs 

 finds that it cannot be due to temperature, although this diminishes as 

 the depth increases, for in the Bed Sea the warm zone extends much 

 below 50 fathoms, while in polar waters even the surface has a low 

 temperature, and currents operate besides so as to introduce great 

 irregularity into the bathymetric relations of temperature ; however, 

 the fact that 43 to 50 fathoms has been ascertained to be the limit to 

 the penetration of light into the sea appears to him good evidence 

 that the presence or absence of light is the determining agency sought 

 for, and that the littoral fauna is simply the fauna of the light, the 

 deep-sea fauna that of darkness. This view is supported by the more 

 superficial distribution of deep-sea forms in some places in which 

 the limit of light lies at an inferior depth, and the deeper range of 

 littoral forms in fresh waters, where the light has greater penetration. 

 The large eyes or blindness of so many, the pale or monochromatic 

 colour of most, and the phosphorescence of a large number of the 

 animals which compose this fauna is evidently connected with the 

 absence of light. The resemblance of the pelagic to this fauna is 

 intelligible if it is remembered that it too is most in its element in 



