THE COMMON HYDRA OP BENGAL. 357 



their bodies to the utmost, and stretch out the former organs like a halo round the 

 mouth. In this position they have almost the same specific gravity as water and are 

 carried along most readily by slight extraneous movements, generally with the tentacles 

 uppermost. Practically this mode of progression is the same as that adopted by some 

 young Actinians, although it is brought about in the case of the latter by the body 

 assuming a globular form and the tentacles contracting. Older individuals sometimes 

 float tentacles downwards with their bases attached to small objects, such as cast 

 skins of Entomostraca, on the surface or in mid-water ; while, occasionally, the aboral 

 pole is thrust through the surface film, the tentacles being spread out in a circle below 

 and giving the body some support. In this position the base is not attached to any 

 other object. On one occasion I saw a polyp bend down its " head " and emit from 

 its mouth a mass of slime, to which the basal disk attached itself ; but I have not 

 been able to repeat the observation and can offer no explanation of the action. 



The polyp crawls for short distances with considerable rapidity. The body is 

 stretched out and bent down in such a way that it is in contact for the greater part of its 

 length with some foreign surface, the basal disk still maintaining its attachment. The 

 tips of the tentacles are then applied to the surface as far away as possible, the disk 

 is set free and the body contracts. The disk is then applied again to the surface and 

 the same process is repeated. The body, however, is not bent into an arch, and indeed 

 is only bent at all for a very small part of its length just above the aboral pole, and 

 this only at the commencement of each forward movement. I have not observed 

 any spiral twisting. On several occasions I have noticed a polyp which was attached 

 to the root of a floating plant stretch out one of its tentacles and apply the tip to 

 an adjacent root. Then, releasing its basal disk, it contracted its body and, as 

 it maintained a firm hold with the tip of the tentacle, passed from one root to the 

 other. Finally it bent down the proximal part of its body until it lay nearly parallel to the 

 second root and fixed its basal disk thereon. Zykoff * has stated that pseudopodia are 

 extruded from the tentacles of Hydra during progression and has even figured the struc- 

 tures. Other observers, including myself, have failed to see these pseudopodia ; but 

 it is difficult to be certain that they do not exist. Possibly the cnidocils may assist 

 the tentacles in maintaining a hold, and it is noteworthy that in Hydra orientalis, 

 in which the body adheres to the surface along which the animal is crawling, 

 the cnidocils are long and abundant on the body, especially on its anterior part. 



The periodic movements which Wagner (op. cit.) has observed in European species 

 of Hydra, take place to some extent also in the Bengal form ; but I have been 

 unable to satisfy myself that these movements are strictly periodic or rythmical. Ap- 

 proximately once in half an hour the direction of the tentacles is changed ; but the 

 period varies within very wide limits. ! I have obtained no evidence, moreover, that there 

 is any correlation in this respect between the movements of a parent polyp and those 

 of buds still attached to it. Indeed the buds are often completely contracted while 

 the parent is expanded, and vice versa. 



1 Biol. Centralblatt, XVIII, p. 272, fig. 1, 1898. 



