322 ' Prof. F. A. Forel on the 
pletely from the waters they inhabit, and can only be found 
in the state of resting-eggs; that this period during which 
the animals disappear varies according to the different 
species, occurring for one in the summer, for others in the 
winter, spring, or autumn. According to this a revision of 
the pelagic population of a lake, if it is to be complete, must 
be drawn up on the results of numerous observations made 
at different seasons of the year. 
The characters common to the animals of the pelagic 
region are due to their mode of life. They must swim inces- 
santly without ever being able to rest upon a solid body; and 
instead of any organ of adhesion they possess a highly deve- 
loped natatory apparatus; their specific gravity, which is 
nearly the same as that of the water *, enables them to swim 
about in the water without any great muscular exertion. 
They are rather sluggish animals, and escape the enemies 
that pursue them rather by their transparency than by their 
activity ; they are, indeed (and this is their characteristic - 
peculiarity), perfectly transparent, like crystals ; and only their 
strongly pigmented black, brown, or red eye appears distinctly. 
This nearly perfect transparency of the pelagic animals may 
be regarded as a mimicry acquired by natural selection ; only 
the animals which are as transparent as the medium in which 
they live have held their own. 
They feed upon vegetable or animal structures ; some feed 
upon pelagic Alge (Anabena circinalis, Pleurococcus angu- 
losus, P. palustris, Tetraspora virescens, Palmella Ralfsit) ; 
the rest upon animal prey, eating the smaller and weaker 
species which live in the same water. 
The pelagic animals perform daily migrations, as was found 
independently by Weismann and myself in 1874: during the 
night they swim at the surface; during the day they descend 
into the depths. Fric thought that in the Bohemian lakes 
he found that the different species selected a particular depth 
at which they preferred to dwell; but neither Pavesi nor my- 
self have been able to ascertain that there was any such con- 
stancy of habitat. The different species form groups or flocks, 
in which the net obtains an abundant booty; but, at least in 
the great Swiss lakes, these associations of animals of the 
same species maintain no definite and permanent position. 
As regards the greatest depths at which they are met with, 
I have captured them in the Lake of Geneva as far down as 
* They are a little heavier than the water; and when they die the dead 
bodies sink to the bottom of the lake and then form an important part of 
the food of the fauna of the depths. 
