GENERAL INTRODUCTION TO THE ZOOLOGICAL REPORTS. 



41 



Tlie conditions of i^ressure at great depths are very extraordinary. Pressure increases 

 at the rate of about 1 ton on the square inch for each thousand fathoms of increasing 

 de-pih ; so that the inhabitants of the floor of the ocean at its average depth of about 

 2500 fathoms, sustain a pressure of 2^- tons on each square inch of surface, compared 

 with the 14 lbs. of atmospheric pressure sustained by the inhabitants of the upper earth. 

 Sea-water is, however, almost incompressible, so that its density at 2500 fathoms is 

 scarcely perceptibly increased. At a depth of a mile, under a pressure of about 159 

 atmospheres, sea-water, according to the formula given by Jamin, is compressed by the 

 i-^th of its volume. Any free air suspended in the water, or contained in any compres- 

 sible tissue of an animal, would be reduced, at a depth of 2500 fathoms, to a mere 

 fraction of its bulk ; Ijut an organism permeable throughout, and supported through 

 and through and on all sides by incompressible fluids at the same pressure, need not 

 necessarily be incommoded by that amount of pressure. We have been long familiar, 

 chiefly through the researches of the late Professor Michael Sars, with a long hst of 

 animals of all the marine invertebrate orders living at depths of from 300 to 400 

 fathoms, and consequently subject to a pressure of 1120 lbs. on the square inch ; and ofi" 

 the coast of Portugal there is a great fishery of sharks {Centroscymnus ccelolepis, Boc. 

 and Cap.) carried on beyond that depth. 



Other physical conditions, such as the specific gravity (salinity) of the water, the 

 relative proportions of the dissolved salts, the total amount of gases dissolved in the 

 water, and the relative proportions of free oxygen and carbondioxide, vary slightly in 

 difierent parts of the ocean and at difi'erent depths ; but although such differences are 

 often valuable in tracing the source from 

 which the water occupying a certain area 

 or stratum is derived, they apparently 

 never occur to such an extent as to aftect 

 animal life. 



The Nature of the Bottom. — The ele- 

 ment next in importance to temperature 

 in regulating the distribution of the 

 abyssal fauna is the nature of the bottom. 

 Two kinds of sediment, very different in 

 their character, may he, said broadly to 

 cover the area inhabited by the abyssal 

 fauna. The first of these, the now well- 

 known Glohigerina Ooze, is a fine calcareous dej^osit, somewhat resembling the chalk 

 of the Cretaceous period in itsjnicroscopic character, and composed to a great extent of 

 the shells, more or less l^roken or decomposed, of pelagic Foraminifera, chiefly of tlie 



Fig. IS.—Gloiigei-inabuUoidcs, D'Orl). 



