AND THE SUEFACE-riLM OF "WATEE. XT 



I have observed several species of Cyclops do this, but there are 

 only two in which the habit appears to be in any degree a 

 constant one, namely G. signatus, Koch (not including G. tenui- 

 cornis, Claus, although this form does sometimes suspend itself 

 from the surface), and G. prasinus, Jurine (= G. magnoctavus, 

 Cragin). "When one of the former is closely watched, it will be 

 seen that the animal suspends itself from the surface by means 

 of the long setge on the second pair of antennae, exactly in the 

 same way as it would do from a piece of weed. It should be 

 especially noticed that the action is simply one of suspension, the 

 animal always remaining quite motionless after taking up this 

 position. Under the microscope there will be no difficulty in 

 seeing that the two longest of the terminal setae of each of the 

 second antennae have penetrated the surface-film, and are lying 

 upon it for a small part of their length. If the reflected 

 beam of light be used, as previously recommended, it will be 

 found that four minute irregularities in the surface-film exist 

 where the setae break through. 



Since animals of this type are denser than water, though but 

 slightly, these irregularities must be capillary depressions. It 

 follows, therefore, that in all probability the explanation of the 

 surface-using power of these species of Gyclops is exactly the 

 same in principle as in the previous cases, notwithstanding that 

 it has not been proved directly that the antennal setae alluded 

 to are water-repellent. One very curious point must not be 

 omitted in regard to G. signatus and G. prasinus, as it is one 

 which these forms have in common with Scapholeheris and Noto- 

 droonas, but which at the same time distinguishes them from 

 all other species of Gyclops known to me. It is that they are 

 normally very dark-coloured, sometimes appearing almost black 

 to the naked eye. The pigment producing this dark colour, 

 while not by any means uniformly distributed over the bodies of 

 these two species, is not so markedly ventral as is the case in 

 either Scapholeheris or Notodromas. As, however, they do not 

 bring any definite area of their bodies into contact with the 

 surface-film, but simply hang from it obliquely, the view that 

 here also the dark colour is protective may be provisionally 

 accepted. Beyond the support afforded in water free from 

 weeds, and the probable easiness of respiration, it is difficult to 

 see what benefits these species of Gyclops derive from their 

 power of clinging to the surface-film. They may, of course, also. 



LINN. JOUEN. — ZOOLOGY TOL. XXY. 2 



