[17] INSTRUCTIONS FOR COLLECTING MOLLUSKS DA.LL. 



bers have beeu shown beyond all question to exist on the oceanic floor 

 wherever this has beeu explored. 



In general it seems as if we might safely assume that the (composition 

 of abyssal sea water shows no very important differences from that of 

 other sea water, and that the animals existing in it are not exposed to 

 any peculiar influences arising from this source alone. 



This can not be said of the physical conditions. Everyone knows 

 how oppressive to the bather is the weight of the sea water at only a 

 few feet below the surface, and how difficult it is to dive, still more to 

 remain on the bottom, if only for a few seconds. 



But it is difficult to convey any adequate idea of the pressure at such 

 a depth as 2,000 fathoms, or about 2 miles below the surface. 



Eope made impervious by tarring is said to have become reduced 

 one-third in its diameter by a descent into these depths. Any hollow 

 object not pervious or elastic is at once crushed. There is no doubt 

 that at some points on the ocean floor the pressure may amount to sev- 

 eral tons to the square inch. 



If we recall that the average pressure in steam boilers is probably 

 much less than 100 pounds to the square inch it may help toward an 

 appreciation of the abyssal conditions. 



The inevitable conclusion is, therefore, that all the animals living 

 under these conditions must have their tissues so constituted as to per- 

 mit the free j)ermeation of the water through every part in order that 

 the pressure may be equalized. How this is j)ossible without putting 

 an end to all organic functions is perhaps the greatest mystery of abys- 

 sal life. How can a large egg, like those of various deep-sea animals, 

 pass through the stages of segmentation and development, with every 

 molecule of its structure in actual contact with ordinary sea water and 

 every solid particle subjected to a pressure of say a thousand pounds 

 to the square inch"? 



Such questions are much easier to ask than to answer, in fact no 

 attempt at an answer has, so far as I am aware, ever been offered to 

 biologists. 



The looseness of tissue necessary to such a permeation is conspicuous 

 in abyssal animals, whose flabby and gelatinous appearance when they 

 reach the surface is notorious. It is perhaps most noticeable in the 

 fishes, which nevertheless are often armed with formidable teeth. But 

 under the great pressures of the deeps it is quite conceivable that each 

 of these loose and half dissolving muscles may be compressed and 

 reduced to a condition resembling steel wire; and that the organiza- 

 tion thus sustained may be as lithe and sinewy in its native haunts as 

 its shallow water relatives are in theirs. 



The operculum is generally horny in abyssal mollusks, frequently 

 disproportionately small, compared with that of congeneric litoral 

 species, and in a remarkably large number of cases is absent altogether. 



The genus most abundantly represented of all is Mmigilia, which is 

 21387— No. 39 2 



