208 CRUSTACEA 



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 Holopedium 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 teniperature 

 and the agitation of the water, is 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 Dio/itum //*. AVith the excep- 

 tion of Holopedium, by far the commonest Crustacean in Grasuiere 

 during July was found by Air. 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 us in such numbers as Diaptomua. 



The life-cycle of the pelagic Entomostraca has been studied in 

 both the Cladocera and the Copepoda. In some of the Cladocera 

 AA r eismann 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 



