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



Proceeding with the examination of the specimen clinging to 

 the surface-film it will be seen that the basal disc, either wholly 

 or in part, is in almost absolute contact with the surface. Some- 

 times it appears to be very slightly below the actual surface, 

 but it is certainly never higher than the surface because the 

 extreme limit of the ectodermal cells of the base can always be 

 clearly seen. 



Further scrutiny will often reveal the presence of minute 

 strands and irregular masses of a gelatinous substance running 

 from the basi-/ disc to the surface film and spreading out con- 

 siderably as they do so. The appearance described, which is 

 shown in Fig. 1, produces at first the impression that the Hydra 

 is stuck on to the underside of the surface-film by a transparent 

 adhesive cement or mucilage. That there is such a substance 

 normally present on the basal disc is of course highly probable 

 from the fact that the animal can cling so tenaciously to weeds 

 and to the sides of any vessel in which it is placed. Its presence 

 can moreover be directly observed if the animal be examined in 

 a live -box, so that a side view can be obtained of the basal disc. 

 The gelatinous material can then sometimes, more especially in 

 young specimens, be seen to be comparatively large in amount 

 and almost symmetrical in outline, forming a kind of cap (see 

 Fig. 2) ; but more often it is very irregular and not present in 

 great quantity. The extreme tenacity of this substance can 

 be proved by allowing the animal under observation to attach 

 its basal disc either to the cover-glass or to the glass table, and 

 then forcing it to alter its position by movements of the lid of 

 the live-box. When the basal disc is at last shifted it will be 

 seen that the gelatinous substance is pulled out into irregular 

 streamers which eventually break, leaving a portion of the original 

 mass adhering to the glass. Such a detached portion is shown 

 in Fig. 3. The jelly-like material, although transparent, is not 

 by any means homogeneous, but contains comparatively large 

 globular and ovoid bodies, usually granular but sometimes 

 structureless, and numerous smaller particles. This want of 

 homogeneity seems to point to the conclusion that the adhesive 

 substance, w^hatever its actual nature may be, is produced by 

 the disintegration of the outer ends of the large ectodermal 

 cells of the basal disc, and that it is not merely an exudation 

 therefrom. 



