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



when a Hydra clinging to the surface-film is examined from 

 the side, as in Fig. 1, is, without doubt, due to their very minute 

 size. Even in the experiment already described, where the 

 capillary depression is readily noticeable from above, it will 

 be found to be very faintly marked w^hen viewed from the side. 

 As regards the water-repellent nature of the mucilaginous 

 material on the basal disc, I do not think there can be any 

 reasonable doubt. I have repeatedly found, wdien manipulating 

 a Hydra in a live-box, that when once the basal disc is brought 

 into contact with the air it is with the greatest difficulty that 

 the surface of the water can be made to leave the gelatinous 

 material. In some cases the surface of the water only succeeds 

 in getting away from the basal disc by pulling out long threads 

 of the jelly, or by leaving an air-bubble entangled in the mass. 

 Both these phenomena occurred in the case shown in Fig. 5. 

 This last observation also explains a very remarkable appearance 

 once noticed of a Hydra suspended in mid-water as shown in 

 Fig. 6. I have no doubt that this was due to some of the 

 water-repellent gelatinous substance remaining at the surface 

 and producing the necessary capillary depressions, while it was 

 at the same time connected with the basal disc by a very fine 

 thread. In fact, by very careful examination with a pocket lens 

 (the animal was not in a favourable position for observation 

 under a microscope) I thought I could just discern such a thread, 

 but it was exceedingly faint, as indicated. 



The foregoing explanation of the means by which the attach- 

 ment of Hydra to the surface-film is accomplished is, it will 

 be noticed, essentially the same as that w^iich I advanced some 

 years ago in connection with the Entomostraca (see Linnean 

 Society's Journal — Zoology, vol. xxv., 1894, pp. 1-19). The only 

 difl[erence, in fact, between the two cases is that, whereas in the 

 Entomostraca the capillary depressions are produced, when formed 

 at all, by the water-repellent nature of theii^ chitinous coats,, 

 scales, or setae, in the present instance the capillary depressions 

 are caused by the action of a specially foi'med water-repellent 

 gelatinous substance on the basal disc. 



In addition to Hydra there are many other examples of 

 animals utilising the surface-film for support by means of 

 gelatinous or mucilaginous substances, and if the explanation 

 now brought forward is true in the former case it will in all 



