LIMNOLOGICAL PROBLEMS 
405 
organisms. The theories and suppositions set forth in the following 
are based upon abundant material : here, of course, only the main 
points can be advanced ; with regard to all details I may refer to my 
main work. 
1. Seasonal Variation. — The plankton organisms of the fresh 
water may at different seasons have quite a different appearance. This 
fact is, in my opinion, connected with the above-mentioned regular 
oscillations in viscosity and specific gravity of the fresh water. As the 
rate of sinking at ^5° C, i.e. in the summer, is probably twice as great 
as in the winter, the organisms, unless they are able to augment their 
buoyancy in the summer half-year, will sink down into deeper water- 
layers ; thereby thev will be shut out from the light and high tem- 
peratures which for other reasons form a life-condition for them. 
We are now able to show that very many plankton organisms, 
and especially the perennial, are subject in the course of the year to 
regular morphological variations which are exactly in accordance 
with the variations in the bearing power of the fresh water. As far as 
we are able to understand these variations, it seems as if the organisms, 
by means of either an increase in the cross-section resistance or an 
increase in the superficial area by diminution of volume, try to 
diminish the rate of sinking, and that just in the season (summer) 
when the rate of sinking is greatest. These variations of the fresh- 
water plankton organisms we call " seasonal variations.^'' 
We may now mention some examples. 
It has been noticed that the Hyalodaphni^e (fig. 52 ^) in the summer 
half-year increase their longitudinal axis to a very high degree. 
While the distance from the eye to the point of the crest in the 
Avinter half-year is only about 100 yot, it is in summer up to 700 ju. The 
most correct interpretation of this very peculiar fact is probably 
that the prolongation of the crest causes a shifting of the centre of 
gravity of the body ; the effect of this again is that the original 
vertical axis, with each beat of the swinnnerets, becomes the horizontal, 
the result of which is an increase in the cross-section resistance. In 
other Cladocera (Bosmina corego?ii, fig. 53) the body in the sunmier 
half-year is higher than long, in the winter half-year longer than 
high ; in summer the antennae are more than twice as long as in the 
winter half-year. In some Rotifers (Asplanchiia, fig. 54) it is known 
that the body, which in winter is almost isodiametric, is in summer 
about five times longer than broad. Here the aim is probably to 
remove the shape of the body in summer as far as possible from the 
1 With regard to the figs. 52-62 it mnst be remeiiil;)ered that the size of the 
figures is quite conventional ; the same species is in the difi'erent figures drawn 
in difi'erent sizes, and young brood and growth stages are often figured larger than 
mature animals. 
