208 CRUSTACEA CHAP, 
the commonest of all in Grasmere lake, but not so frequently 
met with in .the other English lakes, is peculiar in that its 
body is enveloped in a spherical mass of transparent jelly, some- 
times a quarter of an inch in diameter, so that the contents 
of a tow-net jar full of Holopediwm have something of the con- 
sistency of boiled sago. The enormous quantities in which 
these animals often occur during summer is very astonishing ; 
but to be truly appreciated tow-nettings should be taken at the 
surface of the lake either during night-time when there is not 
much moonlight, or else on a dark still day when there is 
a quiet drizzle falling on the surface of the water. In bright 
sunshine the plankton passes below the surface into the lower 
strata, and can be usually taken by sinking the tow-net some 
10-20 feet, or to even greater depths in the water. The exact 
reason of these periodic migrations out of the light, and their 
dependence on other physical conditions, such as temperature 
and the agitation of the water, 1s not clearly understood. It 
appears, however, that when the water is rough, plankton always 
passes into the deeper regions. Besides the species mentioned, 
the minute Bosminidae, whose trunked heads are suggestive 
under the microscope of elephants, and Polyphemus pediculus 
are among the commonest pelagic Cladocera, though neither 
Polyphemus nor Bythotrephes ever form shoals. The above- 
mentioned genera are characteristic of the larger lakes in the 
Northern Hemisphere. Our knowledge of the Crustacean 
plankton of tropical lakes and of those of the Southern Hemi- 
sphere is limited (but see p: 216). 
A very important constituent of lake-plankton is furnished by 
the Copepoda, especially of the genus Diapiomus. With the excep- 
tion of Holopedium, by far the commonest Crustacean in Grasmere 
during July was found by Mr. Gibson and myself to be D. caeruleus. 
At the same time a number of Cyclopidae, e.g. Cyclops strenuus, 
may occur in the pelagic region in considerable quantities, 
though they were never found by usin such numbers as Yiaptomus. 
The life-cycle of the pelagic Entomostraca has been studied in 
both the Cladocera and the Copepoda. In some of the Cladocera 
Weismann at first supposed that males had altogether dis- 
appeared, and that reproduction was entirely parthenogenetic. 
It appears, however, that all the pelagic species have at least 
one sexual period, namely, in the autumn, when resting eggs are 
