65 



worked by hand from the shore, and a net attached to a line for 

 throwing out to greater distances than can be reached by hand, are 

 usually sufficient for this region. 



The abyssal region of a great lake is a complete contrast to that 

 of the littoral, for it offers the most wearisome uniformity of con- 

 ditions that it is possible to imagine. The temperature is nearly 

 uniform, or at any rate never subject to sudden fluctuations ; there 

 are no violent movements of the water ; there is practically perpetual 

 darkness, and there is a blanket of fine mud everywhere and over 

 everything. There are no living plants except bacteria, but animal 

 life is fairly well represented, and may sometimes be abundant. The 

 most remarkable forms are perhaps blind species of Asel/us, 

 GamiiiaiKS, etc. ; Limnaeas, which fill their air-sacs with water ; and 

 larvae of winged insects, which seem to only reach the pupa stage 

 and then become sexually mature. To collect from this region it is 

 necessary, of course, to use various kinds of dredges and sampling 

 tubes worked by means of long lines from a boat. 



It is in the pelagic region, however, that the most characteristic 

 features of lake-life are to be found. Here the conditions, though 

 not nearly so uniform as at the bottom, are much less liable to 

 sudden variations than in the littoral region, while there is a com- 

 plete absence of all solid objects to which organisms can attach 

 themselves or in which they can find hiding-places from their 

 enemies. It follows from this that all genuine pelagic organisms 

 have to be provided with physical or mechanical means of maintain- 

 ing themselves permanently suspended in the water, and that the 

 only possible chance they have of rendering themselves incon- 

 spicuous, either for the purpose of avoiding their enemies or for 

 approaching their prey unobserved, is to become as transparent as 

 possible. As a matter of fact, we do find that the majority of 

 pelagic organisms either possess powerful swimming organs, or sur- 

 round themselves with jelly, or develop various outgrowths in order 

 to increase their area as compared with their bulk, and also that 

 they are usually extremely transparent. With the exception of the 

 fishes, all the organisms found in the pelagic region — and they belong 

 to many groups : AlgK, Rhizopods, Infusoria, Rotifers, Entomos- 

 traca, etc.— are small and practically at the mercy of any currents in 

 the water. They have on this account been grouped under the 

 collective name of " plankton," and it is with the study of these 

 creatures that the most recent advances in the domain of limnology 

 have been made. By the use of specially constructed nets, and even 

 pumps, it has been found possible to collect the life contained in a 

 vertical column of water of known depth and area, and from the 

 captures thus made to estimate, by means of counting small samples 

 under the microscope, the probable number of each constituent of 

 the plankton. Researches of this kind carried on for a year or more 

 have already given us a very large amount of information on the 

 remarkable changes in the abundance, vertical distribution, diurnal 



5 



