COPEPODA 749 



net dragged behind a boat, but that collections made in the night 

 were much more successful than those in the daytime. It was at 

 first inferred from these collections that the animals shunned the 

 light, and sank beneath the surface during the day, to rise again 

 at night. Careful studies of the subject, however, show that the 

 migration of these animals is by no means so simple a matter as 

 had been thought, and that very complex forces are at work con- 

 trolling their movements. While some of them are sensitive to 

 the influence of light, it appears that temperature is much the 

 stronger factor, and that, generally speaking, they move up or down 

 as the result of changes of temperature rather than because they 

 seek or avoid the light. This, without doubt, explains the fact 

 that Limnocalanus remains in the deeper waters in the summer 

 and gradually rises higher as the waters cool off in the fall. On 

 the other hand, Cyclops prasinus has a marked preference for 

 warmer waters. During the summer it is found in the upper 

 layers of water, but in the winter it is inclined to avoid the imme- 

 diate surface and seek the deeper and warmer waters. 



Epischura lacustris is a very interesting species in its vertical 

 distribution; for it is large and a strong swimmer, and changes 

 its location from hour to hour during the day. It likes warm 

 water, but dislikes the light, and its vertical migrations both daily 

 and seasonal are the resultant of these two forces, so that its move- 

 ments sometimes seem quite erratic. 



It is a curious fact that the Copepoda differ in the character of 

 the habitat they like best at different times of their lives. Most 

 of the larval forms are found close to the surface in the daytime, 

 while the maturer animals are found at a greater or less depth. 



It seems probable that the movements of the nauplii and larval 

 Copepoda are caused by comparatively slight changes of tempera- 

 ture, and that a somewhat elaborate determination of the changes 

 of temperature in the upper layers of water may explain their 

 movements, which now seem rather strange. 



Through the study of the geographical distribution one may 

 hope to get some knowledge of the evolution of the species and 

 genera of the Copepoda, and it is on this account that this phase 

 of the study of any group of animals is especially interesting to 



