D. J. SCOURFIELD ON HYDRA AND SURFACE-FILM OF WATER. 141 



probability be found to hold good in the latter also. Many 

 molluscs in particular (including some •which are not air- 

 breathers and cannot therefore possibly be lighter than 

 water) attach themselves directly to the surface-film, and 

 some even suspend themselves from it by means of gelatinous 

 threads. 



As the subject of the relation of aquatic animals and plants 

 to the surface-film, in spite of its attractiveness, is but little 

 known, I wdll conclude with the following short list of the papers 

 bearing on the question : — 



Brockmeier, H. " Ueber Slisswassermollusken der Gegend von 

 Plon." Forschungsberichte cms der Bioloyischen Station zii 

 Plon, Theil 3, 1895, pp. 191—193. 



HoRNELL, J. " On Surface Tension as an aid to Locomotion 

 among Marine Animals." Journal of Marine Zoology and 

 Microscopy, No. 7, 1896, pp. 59 — 60. 



MiALL, L. C. " Some Difficulties in the Life of Aquatic Insects." 

 Nature, vol. 44, 1891, p. 457. ''The Surface-film of Water 

 and its relation to the Life of Plants and Animals." Xa,ture, 

 vol. 46, 1892, p. 7. 



Plateau, F. " Observations sur lArgyronete aquatique." Bull. 

 Acad. Sci. Bruxelles, 1867, pp. 96 — 125. 



ScouRFiELD, D. J, " Entomostraca and the Surface-film of 

 Water." Journal of the Linnean Society — Zoology, vol. 25, 

 1894, pp. 1 — 19. " Note on Scapholeheris mucronata and 

 the Surface-film of Water." Journ. Quekett Micro. Club, 

 vol. 7, 1900, pp. 309-312. 



Explanation of Plate 8 (Upper Portion). 



Fig. 1. Basal portion of Hydra vulgaris clinging to the surface- 

 film of water. x 60. 



Fig. 2. Basal disc of a young Hydra viridis, showing an almost 

 symmetrical gelatinous cap. 



Fig. 3. A detached portion of gelatinous material from the 

 basal disc of H. vulgaris. X 200. 

 JouRN., Q. M. C, Series II. — No. 49 10 



