During hibernation, the action of the heart in land Pulmonata ceases almost entirely. This appears to be directly due to the effect of cold. Mr. C. Ashford has related[39] some interesting experiments made upon _H. hortensis_ and _Hyal. cellaria_, with the view of ascertaining the effect of cold upon their pulsations. His observations may be tabulated as follows:-- Number of pulsations per minute _Helix hortensis_ _Hyal. cellaria_ At degrees Fahr. 22 21 52° 14 12 44° 10 11 38° 4 9 30° At low temperatures the character, as well as the number of the pulsations changed; they became imperfect and intermittent, although exceptionally at 31° F. a _H. rufescens_ gave five or six pulsations a minute, very full and deliberate. The result of taking the _Hyalinia_ suddenly into the heat of a greenhouse was to bring on palpitations. Further experiments resulted in evidence of a similar kind. _Hyal. radiatula_, placed upon a deal table in a room, showed 52 pulsations per minute at 62° F. Placed upon the palm of the hand, the action soon rose to 108. _Hyal. alliaria_, similarly treated, rose from 72 pulsations to 110. Floated upon water, the action of the heart of the latter suddenly fell to 29. Fresh-water Pulmonata do not appear to hibernate. _Unio_ and _Anodonta_, however, bury themselves more deeply in the mud, and _Dreissensia_ casts off its byssus and retires under the mud in deeper water.[40] _Limnaea_ and _Planorbis_ have often been noticed to crawl about under the lower surface of a thick coating of ice. In periods of prolonged drought, when the water in the ponds dries up, the majority of genera bury themselves in the mud. I have known _Limnaea peregra_ bury itself three inches deep, when surprised by a sudden fall of the water in the ditch on Coe Fen, behind Peterhouse, Cambridge. _Physa hypnorum_ frequents by preference ditches which dry up in summer, as does also _Planorbis spirorbis_, the latter often forming a sort of epiphragm against evaporation. _Ancylus_ has been observed to spend the whole winter out of water, and _P. spirorbis_ has been noticed alive after four months’ desiccation.[41]