AlfD THE SUEFACE-PILM OF WATER. 9 



the ventral margin of each valve, it becomes clear that ScapJio- 

 leleris exhibits very considerable specialisation, and the question 

 naturally arises as to the meaning of the latter. To answer 

 this, attention must be turned to the living animals and their 

 peculiar mode of existence in relation to the surface-film. By 

 watching them in their native ponds, it may be seen, with a little 

 patience, that they have the habit of disporting themselves quite 

 close to the surface, especially in sunny weather ; and this fact 

 has been noticed and recorded by many observers. But some- 

 thing more than this seems necessary to justify the elaborate 

 modifications described ; audit is therefore particularly fortunate 

 that by very simple means a most intimate connection with the 

 surface-film can be demonstrated. If a single individual be 

 isolated, say, in a watch-glass, and carefully observed, it will be 

 found sooner or later to come up and apply its straight ventral 

 margin close to the underside of the surface-film. In this position 

 it will usually continue to move about more or less rapidly for 

 considerable periods, very often, in fact, until purposely dis- 

 turbed, when it will at once dive below the surface. Occasionally, 

 when conditions are favourable, the animal may even be seen to 

 remain motionless at the surface, with its swimming-antennse 

 held rigidly almost at right angles to the body (PL I. fig. 2). 

 But what does this mean ? Since the animal is heavier than 

 water, it can only mean that the difi'erence in weight between the 

 animal's body and water is borne by the surface-film, and this 

 again further implies the existence of a capillary depression. 

 Such depression can be actually seen if looked for in the following 

 way : — Place the watch-glass containing the specimen in such a 

 position that the light from a lamp or windoAV foils upon the 

 surface of the water at an angle anywhere between 20° and 30° 

 with the horizontal. If the eye, aided by a lens or low power of 

 the microscope, be now placed in the path of the reflected rays, 

 the surface will appear like a sheet of polished silver, upon which 

 the smallest speck of dust or break in continuity can be instantly 

 detected. Now whenever the animal comes into contact with 

 the surface-film, it will be found that there is a very evident 

 break, or rather several breaks, produced in the continuity of 

 the surface-film. Purther, these breaks will be found to persist 

 as long as the contact is maintained, no matter whether the 

 animal be actively moving about or stationary. It is not pre- 

 tended that these minute irregularities in the surface-film can be 



