18 M"B. J). J. SCOTJEFIBLD ON ENTOMOSTEACA 



be able to secure additional morsels of food floating on the 

 surface, but so far this has not been observed. 



There is yet another way in which the surface-film is utilised 

 bv some Copepods. In these instances the animals do not 

 break the surface at all, but make use of the property which 

 a small drop of water possesses of tenaciously adhering to 

 even vertical and overhanging surfaces of solids, by reason of 

 the tension of its enclosing surf ace- film. The process by which 

 the animals referred to make use of this property is as follows : — 

 When they attempt, as they often do, to force their way up the 

 side, for instance, of a glass vessel, above the general level of the 

 contained water, they become surrounded by a small quantity of 

 water, which most persistently clings to them and to the gla?s, 

 thereby binding them, so to speak, to the latter. By means of 

 the support thus afforded, some Copepods can raise themselves 

 up the sides of a glass vessel far above the water, and they 

 no doubt raise themselves in a similar way up the exposed 

 parts of the stems &c. of some water-plants. The forms that do 

 this most constantly are certain species of Canthocamptus, e. g. 

 0. minutus, O. E. M., and of Cyclops, e. g. C. qfinis, Gr. 0. Sars, 

 and O. phaleratus, Koch. The last-named species affords perhaps 

 the best example of all. I have repeatedly watched individuals 

 wriggle their way up the sides of a bottle partly filled with 

 water, until they have reached the underside of the cork, where 

 they would stay for very long periods. It may be said quite 

 confidently that, in captivity at least, this species spends more of 

 its time above than below the water ; yet its powers of locomotion 

 in this way are not unlimited, for it is practically unable to force 

 itself over dry surfaces. As to the advantages, disadvantages, 

 and other problems connected with such a semi-aquatic mode of 

 existence, nothing definite is known, and so here for the present 

 the subject must be left. 



Briefly summarised, the principal views advanced in this paper 

 are as follows : — (1) To many Entomostraca the surface-film of 

 water is a very dangerous element in their environment. To 

 this category belong large numbers of the Cladocera and Ostra- 

 coda. (2) To some others, on the other hand, the surface-film 

 affords peculiar advantages. This class includes, so far as is yet 

 known, only a few specially modified Cladocera and Ostracoda, 

 and some Copepoda, which do not, however, present any apparent 



