6 TJniversitij of California Publications in Zoology. [Vol.8 



as is Eucalanus, leads to speculation as to the meaning of diurnal 

 migrations. (Compare Morgan, 1903, pp. 391-394, 404-405). 

 Here we find that a species which is entirely without need of the 

 protection that is probably afforded by the dim light at 200 

 fathoms, remains, for the most part, at that depth during the 

 day. In a species not very transparent, this behavior would at 

 least suggest that protection from predaceous organisms is 

 secured. It is well known that many fish, for example, feed 

 largely upon copepods, but it is indeed difficult to conceive how 

 a fish could detect a Eucalanus by sight. This is suggestive of the 

 possibility that the degree to which an animal is adapted to a 

 life in the plankton through transparency and lack of color has 

 something to do with the vertical migrations. In other words, 

 it might be possible to show that there is a relation between the 

 coloration or opacity of an animal and its vertical movements in 

 a body of water. If it should be found that vertical migrations 

 are more characteristic of forms that w^ould be easily visible, and 

 less so of colorless and transparent forms, it would be an indica- 

 tion that the migrations are themselves in the nature of an 

 adaptation. This would, of course, be more true of the movement 

 to deeper levels for the day. It is suggestive, at any rate, that 

 Eucalanus does not appear to have a behavior that would be in 

 part protective while, at the same time, protection is secured in 

 another way. 



It would be worth while to know whether species that live 

 below the depth to which light penetrates execute the vertical 

 migrations. If "so, it could not be urged that the movement is 

 protective or adaptive. It is not to be expected that the behavior 

 of one species will be characteristic of all others, yet it does 

 not seem too much to hope that there are some far-reaching funda- 

 mental principles underlying the movements of plankton organ- 

 isms. We could not expect an adaptive behavior in all cases, for 

 diurnal migrations may be of no more use to some organisms 

 than the migration of pigment in the eye of a crayfish, under the 

 influence of heat. The latter is probably a manifestation of one 

 of the general properties of protoplasm, and the same may be 

 true in a general way of the movements of plankton organisms. 



In conclusion, while the distribution of Eucalanus as pre- 



