LIMNOLOGICAL PROBLEMS 
411 
forms. At a temperature of 12-14° or 14-16° C, which in the 
Baltic lake territory mainly occurs at the end of May and beghming 
of June^ all seasonal variations are fully formed. Diatoms change 
their shape of colony, in Ceratium Mrundinella the fourth horn is 
developed, or new, narrower, and longer seasonal forms are observed, 
the longitudinal axis in Asjjlanchna increases in certain localities, the 
series of variations in Synchasta and Anurcea arise, the growth in the 
tip of the crest in Daphnia and Hyaloda/phnia proceeds, the hump in 
Bosmina coregoni grows upwards and the first pair of antennae increase 
in length ; B. longirostris as well as many others of the above-named 
forms decrease in size. At the same time the summer forms appear 
with their highly developed floating apparatus, viz., Holopedium, 
BytJiotrephes^ Diaplianosoma, Leptodora. By an abrupt change the 
whole plankton community has by form variations decreased its rate 
of sinking and augmented its floating capacity ; the rate of sinking 
for the plankton of June is therefore much less than for the plankton 
of May. 
Towards winter the species has again the external appearance 
which we are accustomed to regard as normal. Simultaneously the 
summer forms with their often highly bizarre appearance disappear. 
What has happened in these three weeks, in which the whole 
plankton changes its form and augments its floating capacity ? The 
investigations have now established the following facts. 
The demands made by the variations in the outer conditions on 
the floating power of the species can generally not he satisfied hy the 
transformation of the single mature individuals. In Diatoms it is 
probable that some species, on the rate of sinking increasing, occur 
in colonies instead of singly, or, if they are generally colony forms, 
they change the shape of colony through the use of gelatinous masses, 
which are different qualitatively as well as quantitatively {Tabel- 
laria) ; the individual itself remains, so far as is known, as a rule 
unchanged. With regard to the Cladocera and Rotifers it may be 
pointed out, that if the demands become too great, the individual 
may respond to them by migrating to deeper waters, the temperature 
of which agrees more with that under which the organisms were 
hatched and grew up ; but doubtless the response most frequently is 
death. The demands made by the variations in outer conditions are 
mainly met by the species in such a manner that the individuals 
hefore death produce new broods in which, ivhen hatched, the demands are 
on essential points already satisfied. Thus we also find that those 
individuals which have survived the winter in water layers with high 
bearing-power, and which are distinguished by relatively plump form, 
die out in spring. In the brood-pouches of the roand-headed winter 
forms we find broods with pointed heads in the above-named three 
