REPORT ON THE ECHINOIDEA. 249 



at the average depth of the Atlantic and Pacific basins, which does not seem to have 

 practically any influence whatever on the distriljution of species ; whUe the slight range 

 of temperature which affects the upper layers of the waters so as to form a littoral faima 

 within the limits where we have the greatest possible differences between the extremes 

 of heat and cold, due to the daily changes of the temperature, and a continental fauna 

 extending to those depths which we may assume are affected by the slower action of the 

 heat of the sun at different seasons ; while at last we find the abyssal regions in which 

 the changes of temperature can be considered as null and in which there is a most 

 remarkable uniformity of temperature though the conditions of pressure at the extremes 

 of depths to which the species belonging to this abyssal fauna range are immensely 

 greater than corresponding extremes due to the difi"erence of atmospheric pressure at 

 different levels on the surface of the earth. 



If we examine the physical conditions which prevail within the 100 fathom line, 

 within the continental limits, and within the oceanic limits, we, of course, at first would 

 be inclined to look upon the great differences of pressure as the important element in the 

 limitation of faunas. The fact, however, that one and the same species so readUy adapts 

 itself to the enormous diflferences of pressure occurring in the oceanic, continental, and 

 even littoral districts would seem to show that this element is not an all-important one. 

 It is among the littoral districts that we find the greatest diversity of faunae, and these 

 littoral districts mainly differ in their temperature, but it is not the greatest amount of 

 heat which apparently forms the limits of these districts. 



They are separated by Hnes of lowest temperatures. Thus we find an arctic and 

 boreal district, a tempei'ate and a tropical district in which the extremes of temperature 

 range within comparatively narrow limits. A similar condition of things exists within 

 the littoral, continental, and abyssal districts ; they represent the depths at which as a 

 general rule certain well marked conditions of temperature exist, regulating as they do 

 for the littoral districts the principal faunistic features of the bathymetrical districts. 

 The littoral where the changes of temperature are greatly affected by the action of the 

 sun ; the continental extending from this limit into regions where we find the tempera- 

 ture gradually diminishing till we come to the abyssal or oceanic depths at which we 

 have practically a uniform temperature. 



With the exception of the abyssal species found in the Southern Ocean near the 

 Antarctic circle, none of the species of Echinids seem to extend to very great distances from 

 the continental ranges. This agrees well with my own observations in the "Blake," where 

 I found that at even a comparatively short distance from the land there was a marked dimi- 

 nution in the number of species, and also in the number of specimens collected. The 

 greatest distances from any land or banks of moderate depths at which Echinids have 

 been collected are off Tristan da Cunha, at Stations 133, 334, 335 ; at Stations 153, 156, 

 157, 158 in the Southern Ocean, south of the Heard Islands, which, however, may not 



(ZOOL. CHALL. EXP. — PART IX. — 1881.) I 32 



