AND THE STJEFACE-FILM OF WATEE. 13 



curious problems that suggest themselves in connection with, the 

 modifications of the two forms described. No certain answer 

 can be given, for instance, as to the reason for the division of 

 the ventral setae in ScapJioleheris mucronata into three sections, 

 or about the function of its shell-spines, although it is almost 

 certain that they are both related in some way or another to the 

 animal's peculiar habits. Again, there is the strange rectangular 

 plate projecting from the posterior end of the ventral margin of 

 the left valve in Notodromas monacha ; while it seems probable 

 enough that this plate also is related to the use made by the 

 animal of the surface-film, nothing is definitely known about 

 it at present. 



Leaving these and similar queries, it will be useful to turn to 

 a consideration of the benefits derived from a close connection 

 with the surface. The most important of these are certainly the 

 support afi"orded, the probable abundant food-material obtained, 

 and the easiness of respiration. The first is very evident, for, 

 owing to the greater specific gravity of these animals than water, 

 a large amount of muscular effort is required to enable them to 

 maintain themselves at any particular level, apart altogether 

 from making onward or upward progress, and this will naturally 

 be entirely saved by suspension from the surface. In regard to 

 the food-supply, two questions ai'ise which need answering before 

 any special indebtedness of the animals to the surface-film on 

 this account can be demonstrated: — (1) Can particles of food 

 floating on the surface be appropriated ? and (2) What is the 

 extent to which such particles occur in that position ? The first 

 question can be answered in the afiirmative without hesitation. 

 If a little finely-divided material, such as flour, be lightly dusted 

 upon the water, it can be seen, if the animals are watched under 

 the microscope when they come to the surface, that the floating 

 granules are taken between the shell-valves in a continuous 

 stream owing to the current produced by the branchial append- 

 ages ; and from this stream any particles suitable for food would 

 evidently be picked out in the usual way. The second question 

 is not quite so easily answered. Direct observation does indeed 

 show that a number of small fragments of all descriptions can 

 actually be seen upon the surface of all open waters, especially 

 near their margins, but these would only be available for food to 

 a very small extent by the animals under review, which no doubt 

 depend much more lai'gely upon particles so minute as to be 



