4 ME. D. J. SCOUBFIELD ON ENTOMOSTEACA 



depression is at once formed of such size that it is at least suf- 

 ficient to sustain the difference in weight between the animal's 

 body and water. In the case of large specimens, the capillary 

 depression can be readily seen with a pocket-lens. In the case 

 of very small forms, e. g. Bosmina, it is proved to exist by the 

 way in which they are repelled from a clean glass rod or tube ; 

 for it is a familiar fact that while similar capillary curves attract, 

 dissimilar ones repel one another, and the capillary curve formed 

 against glass is of course an ascending one. A very simple 

 experiment may be made which illustrates the whole action on a 

 scale large enough to be watched by ordinary vision. If a lenti- 

 cular piece of some such water-repellent substance as paraffin- 

 wax be taken, say, about 2 inches in diameter, and weighted until 

 slightly heavier than water, it will be found that as soon as one 

 of its convex faces is pressed from below against the surface-film, 

 the latter rapidly retreats down the sloping sides, producing a 

 large capillary depression, and that the wax then remains sus- 

 pended from the surface. When it is remembered that, owing 

 to the small size of the Water-fleas, their area, compared with 

 that of the piece of wax, must be enormously greater in pro- 

 portion to their bulk, it will be seen that the suspending power 

 of the surface-film must also be comparatively greater in 

 their case ; for, other things being equal, the force exerted 

 by the surface-film is proportional to the length of the line of 

 contact. 



There are two other factors which, although they do not help- 

 in the actual floating, are of considerable importance, because 

 together they account very largely for the inability of the animals 

 to get below the surface when once caught by the film. One of 

 these is the tendency which they have, in common with all floating 

 bodies of approximately lenticular shape, to take up a horizontal 

 position, that being their only position of stable equilibrium ; and 

 the other, the situation and range of movement of their swimming- 

 organs. The tendency to turn upon their sides is obscured occa- 

 sionally in consequence of a more or less considerable departure 

 from the lenticular form, or by the possession of wide-spreadino- 

 antennae ; but it remains true, nevertheless, that in the great 

 majority of eases the animals are actually found floating in this 

 position. It will readily be seen that, when thus floating on their 

 sides, the Cladocera lose the use of one of their swimming- 

 antennge altogether, it is out of the water, and that both Cladocera 



