AND THE SURFACE-FILM OF WATEE. 19 



structural modifications. (3) In all cases (except where some 

 €opepods possibly make use of the properties of the surface-film 

 to attach themselves to aquatic plants above the general water- 

 level) the relation to the surface-film, whether beneficial or 

 the reverse, depends fundamentally upon the same physical 

 principles, namely, the upward pull of the surface-film when 

 forming a capillary depression, and the possession by the animals 

 of water-repellent shells, ridges, scales, or setae, capable of pene- 

 trating the surface-film and producing capillary depressions. 



In conclusion I wish to express my best thanks to Prof. L. C. 

 Miall, r.R.S., for his kind sympathy shown during the progress 

 of this inquiry, and for many helpful suggestions. 



EXPLANATION OF THE PLATES. 

 Plate I. 



Fig. 1 . Scapholeberis mucronata, 5 . Side view. X 55. 



2. The same. Ventral view as seen when in contact with the surface-film. 



X55. 



3. The same. Ventral margin. XllO. 



4. The same. Last of the anterior series of ventral setse. X 1000. 



5. The same. Diagrammatic section across the flattened ventral margins of 



the valves, showing setae and hyaline scales. 



6. Notodromas monacka, $ . Side view. X 35. 



7. The same. Ventral view as seen when in contact with the surface-film. 



X35. 



8. The same. Diagrammatic section across the flattened ventral margins 



of the valves showing ridges. 



Plate II. 



Fig. L Scapholeberis mucronata, 2- Front view of ventral margin. x200. 



2. The same. Anterior series of setse of ventral margin. X 700. 



3. The same. Three setae with hyaline scales from inner row of middle 



series. x700. 



4. Notodromas monacha, $. View of ventral flattened area. x85. 



2* 



