THE COMMON HYDRA OF BENGAL. 355 



there pierced the comparatively weak integument, this was not the case as regards the 

 mite's body, which was covered with a greatly thickened chitinous layer pierced by 

 pores through which sensory hairs emerged. The threads were arranged in groups 

 at these pores, their barbed heads pointing towards them, and a few had actually 

 penetrated the integument at the base of the hairs. I have repeatedly seen small 

 Hemiptera 1 (which have a thick integument) and Ostracods temporarily paralysed 

 by coming in contact with the tentacles of the polyp ; and, occasionally, the same 

 thing has occurred in the case of large Cladocera, which have touched the tentacles with 

 their shell only. In these cases the polyp has made no attempt, so far as could be seen, 

 to retract the tentacles, but has remained still, with the tentacles fully extended, until 

 the insect or crustacean fell off, apparently owing to its own weight. The Hemiptera, 

 although thev did not struggle, were little affected and swam away immediately or 

 almost immediately after being released ; but the Crustacea sank through the water for 

 some little distance before swimming away, although the limbs of the Ostracods were 

 sometimes observed to move slowly even while the animal was held prisoner. Even the 

 Crustacea recovered their normal powers of movements before reaching the bottom. 

 I have seen soft-bodied insect larva larger than these Ostracods and Hemiptera 

 swallowed without difficulty, and also Cladocera as large as those which escaped. 



One important factor in the swallowing of food which appears to have escaped 

 notice, is the mucus secreted by the alimentary canal. This substance is so trans- 

 parent that it can hardly be detected when uncontaminated ; but if the upper part of the 

 canal contains faeces when food is swallowed, they are ejected with the mucus and the 

 latter is stained. When suitable prey has been seized and killed the tentacles 

 contract, bringing the food towards the mouth, which then opens and is thrust out 

 like a proboscis. As the tentacles approach it a cloud of mucus is ejected from it and 

 envelopes the food. This greatly assists in the process of ingestion, 2 as the food is now, 

 as it were, anchored to the interior of the gut, the mucus being extremely tenacious. 



After swallowing food, the polyp, as I have noted above, assumes temporarily the 

 form of a wine-glass, owing to the food being retained in the upper part of the alimen- 

 tary canal and so causing the walls of the body to bulge out in this region. The soluble 

 parts of the food pass down into the lower half of the body, where they remain until 

 the whole or a part of their digestible constituents has been removed. The residue is 

 then ejected from the mouth together with the harder tissues, which are thus got rid of 

 unaltered, never having reached the inferior section of the gut. Complete digestion, 

 after which the faeces consists of a white floculent mass, takes from three to four days ; 

 but digestion is by no means always complete. This is a point to which I must return 

 later when dealing with the colours of the polyp. 



Hydra orientalis feeds chiefly in the early morning, before the sun has risen high 

 above the horizon. In an aquarium the polyps remain with their tentacles partly retract- 

 ed during the night. If they are hungry, however, they may extend these organs to 



1 Identified by Mr. W. L. Distant as Plea pallescens. This is perhaps the commonest insect in the Museum "tank'' during- 

 winter. — N. A. 4-7-1906. 



2 Cf. Duerden on the " Role of Mucus -in Corals,'' Quart. Joum. Micr. Sci., XLIX, 1906, p. 591. 



