CHARACTERISTIC DEEP-SEA TYPES. -- GASTEROPODS. 65 



vegetation, except as a sediment from near the surface, does not 

 exist, so that the flesh-eating mollusks of the deep, when within 

 reach of pelagic food, or of the carcasses of dead fishes and 

 other decaying organic matter, are not obliged to prey upon 

 each other to the same extent as do the shallow- water forms. 

 The latter take part in a fierce struggle for existence amidst the 

 vicissitudes of tidal and storm waves, variation in elevation of 

 land, and a vastly denser population of all sorts. Compara- 

 tively few of the shells dredged from deep water show the frac- 

 tures and injuries so common in shells from littoral dredginffs, 



O O ' 



or the drill-holes made by the so-called lingual ribbons, a terri- 

 ble boring weapon of enemies of their own kind. Most of the 

 enemies of deep-water mollusks are blind, or at any rate can 

 have little power of vision for objects not luminous. The ab- 

 sence of violent motion in deep water removes any mechanical 

 effects of that medium from the category of modifying, in- 

 fluences upon the animal. Thus it is evident that the factors 

 affecting the restriction of tendencies to variation in the form, 

 color, and sculpture of littoral species are nearly eliminated in 

 the abyssal regions ; so that we may expect in the deep sea a 

 very wide range of variation in form and sculpture within the 

 specific limits of the " flexible " species, and an almost complete 

 uniformity over very wide areas of the forms which we may con- 

 sider as " inflexible " species. 



Many of the gasteropods must lead a more or less roving life 

 in search of their prey ; others, like Dentalium, live buried in 

 ooze. A great number of the mollusks are blind. The lamelli- 

 branchs live either buried in the ooze, or on the surface of harder 

 bottoms anchored by the byssus. Most of them are stationary, 

 though, judging from analogy with some of the shallow-water 

 genera, they may be capable of considerable change of locality. 



Those mollusks which subsist upon other animals, with a hard 

 covering, so that they have to bore or break their way to their 

 food, are much less numerous in the deep sea than those which 

 feed upon soft tissues, or kill their living prey by bites with poi- 

 sonous fangs. The latter, the PleurotomidaB, outnumber any 

 other group of mollusks in the abyssal fauna ; they are charac- 

 terized by a notch near the junction of the outer margin of 



