BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [18] 



entirely without an operculum, and affords a conspicuous example of 

 tlie obsolescence of protective devices, originally acquired in shallow 

 water, resulting from long residence in the deeps. 



In the Unio and Melania of fresh-water streams and the pond snails 

 of our lakes and ponds, the waters of which from the decay of vege- 

 table matter are overcharged with carbonic acid, we find a dense thin 

 greenish epidermis developed as a protection against erosion. In the 

 deep sea where every xDortion of the shell must be permeated by the 

 surrounding element to equalize the external xDressure, and where car- 

 bonic acid exercises its usual malign influence on the limy parts of all 

 organisms, we find a strikingly similar protective epidermis developed 

 in most unexpected places. Thus it comes about that in the Trochi^ 

 Pleurotomidce and other characteristic abyssal animals we find those 

 puzzling and remarkable counterparts of land and fresh-water shells 

 which have astonished every student of the mollusca who has seen them. 



The influence of darkness upon the inhabitants of the Abyssal 

 Eegion has often been expatiated upon. The absence of visual organs 

 or their preter naturally excessive development beyond the normal of 

 the groups to which the individuals belong is evidence enough that the 

 deeps are markedly darker than the shallows. But this evidence proves 

 too much for the claim that the deeps are mathematically dark. What- 

 ever notions may be entertained or conclusions deduced by the physi- 

 cist from the premises, the jn^esence of large and remarkably developed 

 eyes in many abyssal animals shows that light of some sort exists even 

 on the oceanic floor. It is inconceivable that these organs should be 

 developed without any light and if the experiments and reasoning of the 

 physicist result in the apparent demonstration of absolute darkness in 

 the depths, the facts of nature show that in his premises or his experi- 

 ments there lurks some vitiating error. It is ridiculous to suppose that 

 the phosphorescence of certain animals in the deep-sea fauna is a factor 

 of sufficient importance to bring about the development of enormous 

 and exquisitely constructed eyes in a multitude of deep-sea species. 

 A greater or general phosphorescence, such as would amount to a gen- 

 eral illumination, has never been claimed by any scientific biologist and, 

 as a theory, requires a mass of proof which seems unlikely to be forth- 

 coming. 



In general, then, we find the physical conditions simpler than those 

 of the shallows, and yet much more energetic. The effect of tempera- 

 ture is marked in the distribution of life over cold and warmer areas of 

 sea bottom. The relative importance of the effects of pressure, partial 

 darkness, and of the quietness of abyssal waters, our knowledge is yet 

 too imperfect to allow us to i^recisely estimate. All doubtless have 

 their effect; some of the effects are more obvious than others, but it is 

 by no means certain that the most obvious are necessarily the most 

 important to the organisms concerned. 



