ABYSSAL AND PELAGIC FORMS. IxXXV 



whereas at the most profound depths both eyes and tactile organs are 

 absent. Deep-sea forms are either pale^ colourless, or of a single tint; 

 and as already remarked (page xiv), the tissues which connect their 

 hard structures together are weak; while such as are brought up from 

 great depths have their bodies expanded, and even burst, due to the 

 removal of the pressure of superincumbent water, as has been observed 

 (page xliii) . 



The temperature of the sea at 500 fathoms is as low as 40° P., even 

 under the equator in the Atlantic and Pacific Oceans ; below 2000 fathoms 

 it is not more than a few degrees above freezing-point, except in pecuhar 

 instances of land-locked seas. Almost everywhere, at 500 fathoms, and 

 everywhere at 1000, there is nearly an absence of currents, and move- 

 ments must be very slow and probably quite imperceptible to the resident 

 animals ; as a consequence of this similar condition of life the deep-sea fauna 

 show no zones of distribution in depths below 500 fathoms. 



M. Regnard has made experiments with water respecting the amount of 

 pressure a fish will sustain : one destitute of an air-bladder, or in which it 

 had been evacuated, he found might be submitted to a pressure of 100 

 atmospheres, equivalent to a depth of 650 fathoms, without injurious efiects ; 

 at 200 atmospheres it became torpid, but soon recovered on being removed; 

 at 30O atmospheres, equivalent to about 2000 fathoms, it died. But among 

 other reasons for failure in this last experiment, the pressure was more 

 rapidly induced than would occur in nature, were a fish to change its 

 habitat. 



Some abyssal forms are dwarfed, perhaps from cold or deficiency of food ; 

 others, perhaps due to absence of foes, are almost gigantic. 



Many deep-sea forms have an enormous development of the mouth and 

 stomach, thus enabling them to swallow fishes even larger than themselves, 

 probably in order to retain a stock of food sufficient for some time, all being ' 

 carnivorous, for at certain depths Pteropods dissolve, at greater distances the 

 Globegerina are similarly lost, possibly the sea-water itself assisting, as 

 when alkaline it can absorb an additional amount of carbonate of lime. 

 Doubtless oxygen decreases with depth of water, but some is present even 

 in the deepest. : 



The number of animals in the. sea decrease as the depth increases, and in 

 the deep abysses a varied repast sinks from the surface ; while as no plants, 

 unless parasitic, are foundj all food must descend from above, assisted by 

 shore debris and vegetable matter carried down by rivers and which reach 

 the sea-bed. 



Pelagic fishes (see page Ixxx) are more numerous in tropical regions than 

 in our own, and not a few are cosmopolitan, while they often follow their 

 food into the littoral zone. Some are rapid swimmers and pursue their 



