628 Sir John Murray [Feb. 7, 



sentativL'S of the warm- blooded air-breathing animals, whose blood- 

 temperature remains more or less constant whatever the external 

 temperature may be. 



Temperature conditions are thus of paramount importance in the 

 life-history of the cold-blooded marine fauna, animal metabolism 

 being retarded in cold water and accelerated in warm water. This 

 explains the relatively large number of individuals found living in 

 cold polar waters as compared with the warm tropical waters, for in 

 the colder waters eggs and young and adults (some of them ten or 

 twelve years old) are found at all seasons, while in the warmer waters 

 the various stages are passed through rapidly, so that the age of the 

 oldest individuals may l)e reckoned in days, weeks or months. While 

 pelagic larval stages are characteristic of most animals living in 

 warm tropical waters, polar marine animals have usually a direct de- 

 velopment. The secretion of calcium carbonate by animals to form 

 shell and skeleton is much greater in warm than in cold water : thus 

 the numerous pteropods and foraminifera of the tropics have only 

 one or two small representatives in polar regions, while coral reefs 

 and calcareous bottom-deposits are limited to the warmer regions 

 of the ocean. The viscosity of sea-water is greater at low than 

 at high temperatures, and this has an effect upon the floating 

 powers of organisms and upon the development of suspension- 

 organs. 



The pelagic animals (Zooplankton) include nearly all the marine 

 groups, the principal ones being : foraminifera, radiolaria, Crustacea, 

 pteropods, cephalopods, tunicates, and fishes ; there are no sponges, 

 only one pelagic echinoderm (though many pelagic larvae), and only 

 one group of insects {Hal abates). The colouring and amount of 

 pigmentation in plankton animals, as well as the development of eyes 

 and light-organs, appear to be correlated with the intensity of sun- 

 light and the depth of water ; thus at the surface the animals are 

 usually colourless, silvery, or bluish, except in the case of those living 

 in the vicinity of floating objects, which exhibit a remarkable pro- 

 tective colouring ; in depths of 100 to 200 fathoms the fishes 

 are grey, or have silvery sides and brown backs, while in depths 

 beyond 300 fathoms red or dark colours predominate. Many 

 plankton animals emit phosphorescent Hght by the aid of a luminous 

 secretion or special light-organs more or less complicated in structure ; 

 both light-organs and eyes appear to be larger in depths down to 

 800 fathoms than in deeper water. 



There is a bathypelagic (or twilight) fauna living near the lower 

 limit of the photic zone, where a great change in the viscosity, 

 temperature, and other physical conditions takes place, probably 

 retarding the fall of dead organisms and thus forming a sort of arti- 

 ficial bottom, or seaward extension of the continental mud-line. The 

 animals inhabiting the intermediate zone of water between the lower 

 limit of the photic zone and the bottom seem to be less numerous 



