174 THE DISTRIBUTION OF 



a depth of 50 fathoms, if it can be proved that neither 

 has any dependence on the proximity of a substratum. 

 But here, obviously, a difficulty occurs. A sponge with 

 an attaching stalk, taken in deep water, is obviously an 

 abyssal form, for it was attached to the bottom. But 

 if a net which has been dragged along the bottom at 

 a depth of say 2,000 fathoms proves, when examined, 

 to contain fish, how can we tell whether these fish were 

 truly abyssal forms, or were merely caught as the net 

 was hauled up ? The difficulties in coming to a decision 

 are great, and until recently there has been a tendency 

 to assume that all fish obtained in deep water, and 

 having a bizarre form and colouring, large eyes or no 

 eyes, and so on, were abyssal fish, feeding on the 

 bottom. Dr. Hjort, however, believes that the recent 

 (1910) expedition of the Michael Sars, taken in con- 

 junction with some previous observations, has yielded 

 evidence which proves that many of these curious fish 

 are truly pelagic, living at or below the fight limit. If 

 he is right, then pelagic animals may be divided into 

 at least three groups : (1) surface forms, most abundant 

 in the upper 100 fathoms or so ; (2) mesopelagic or 

 intermediate forms, found about the fight fimit, which 

 varies with the latitude ; (3) bathypelagic forms, which 

 occur below the limit to which the rays of light pene- 

 trate, but yet not upon the sea-floor, of which they 

 are independent. 



We must now consider some of the characters of 

 pelagic organisms, beginning with the plankton. Plank- 

 ton animals are those able to float in water without 

 exertion, and have usually flattened and expanded 

 bodies. Sometimes, as in the very abundant copepods, 

 a globule of oil makes the body fight. At other times, 

 as in some protozoa, and such forms as the sunfish. 



