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 light 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 light limit, 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 light. At other times, 
as in some protozoa, and such forms as the sunfish, 
