436 Proceedings of Royal Society of Edinburgh. [sess. 
the cells in the plankton is eaten away by Phycomycetes before 
reaching the bottom : my observations prove that an organism in 
the latter part of the period of maximum development may very 
often be infected by Phycomycetes, which feed upon the proto- 
plasm and kill it, leaving the skeleton intact. 
All the decayed matter derived from the plankton or from the 
littoral organisms, on settling upon the bottom, will be mixed with 
the inorganic material washed out by the waves from the shores 
or carried by the rivers out into the lakes. In our country this 
material consists mainly of lime and clay, but as yet the inorganic 
constituents of our lake-bottoms have not been thoroughly studied. 
The percentage of lime in our deeper lake-deposits is very variable, 
but in most cases it is extremely high, often 15 to 25 per cent., 
and in the Fureso 35 ’30 per cent., while in other lakes it may 
rise to 46*98, and even 59*44 per cent, (see my bottom explora- 
tions, 1901, p. 93). We have no chemical analyses of the water 
of the greater lakes, and therefore cannot speak of any deposits 
due to chemical precipitations from the water of the lakes. 
The rich bottom-fauna, consisting mainly of Chironomus, 
Oligochseta, Ostracoda, and Pisidium, obtains its nutriment from 
the rain of organic and inorganic matter which drops down 
through the water and reaches the bottom. I have studied the 
life of this fauna in aquaria at the fresh- water biological laboratory 
at Fureso. If we take the mud from the greatest depths of our 
lakes and place it in aquaria, we shall observe, after the lapse of 
some days, upon the surface of the mud, elevations consisting of 
granules, as well as some jelly tubes covered with mud, and sur- 
rounded by similar granules. Beneath the elevations and in the 
tubes we find respectively Oligochseta and Chironomus larvae ; we 
can detect the granules being pushed out, and we know them to 
be excrementa. If we take some mud from the deep lake-bottoms 
and sift it through a very fine sieve we shall find enormous 
quantities of these granules, and if we allow the mud to remain 
sufficiently long in the aquaria the whole surface becomes con- 
verted into granules, that is, into excrements. From these 
observations we conclude that the upper layers of the deeper lake- 
bottoms become, consequent upon the digestive action of the 
fauna, converted into layers of excrements. 
