INVERTEBRATE BOTTOM FAUNA 545 



there are altogether 45 species in the two areas, 10 oi which 

 are common to both. A comparison between the West 

 Atlantic (American) and the East Atlantic (European- African) 

 deep-sea crustaceans shows an equally small number of common 

 forms. 



These instances show that, in spite of temperatures and 

 salinities appearing identical in widely separated localities, it is 

 possible to distinguish between the faunal communities of the 

 deeper tracts of the ocean, and we perceive accordingly that 

 temperature and salinity are not the only factors which regulate 

 the distribution of species. Unquestionably there are other 

 physical conditions which are of considerable importance, and it 

 must further be remembered that biological factors, such as 

 competition between species, exert a decided influence.^ 



Murray showed in 1895 that the results of the " Challenger " 

 Expedition afforded no confirmation of the opinion that a 

 universal deep-sea fauna was spread all over the floor of the 

 ocean ; he compares the catches at six deep-water stations 

 scattered over the Atlantic, Pacific, and Southern Oceans, 

 the total number of species recorded being 290, but not a 

 single species was common to the six stations."^ At the same 

 time we must remember that whole groups of forms, showing 

 common characteristics in bodily structure, and belonging to 

 types quite distinct from the littoral ones, belong either entirely 

 or principally to the deep sea. These types are as a rule 

 very extensively distributed, even if their species and genera 

 may be limited to more circumscribed areas. Among fishes, 

 for instance, we have the Macrurus-type, which is to be found 

 in all the greater depths of the oceans of the world, although 

 particular species have a comparatively limited distribution. 

 The big group of holothurians known as Elasipoda is a 

 particular type, separated in all essentials from the littoral and 

 sub-littoral forms of holothurians. They belong almost entirely 

 to the archibenthal and abyssal tracts of the different oceans, 

 and are often abundant enough to give a distinct character 

 to the deep-sea fauna. The same is true also of the Echino- 

 thuridae, though in their case there are littoral and sub-littoral 

 species ; some species, however, have a comparatively limited 

 distribution. Among crinoids we find survivals from remote 

 ages of the earth, namely, the stalked genera [Rkizocrimis, 

 Bathycrinus, Pentacrinus, etc.), as typical inhabitants of widely 



^ I must, however, point out that in all probability some faunal groups show a greater 

 uniformity in widely separated localities than others. 

 - See Summary of Results Chall. Exp., p. 1438. 



2 N 



