2 6 HIBERNATION AND AESTIVATION chap. 



temperature is but low, as to share the comforts of a cosy 

 retreat in common. Shigs generally hibernate alone, excavating 

 a sort of nest in the earth, in which they encyst themselves, 

 contracting their bodies until they are almost round, and secret- 

 ing a covering of their own slime. The Helices usually close 

 up the mouth of their shell by the formation of a memljranous 

 or chalky einphragm, which will be further described below. 

 Both snails and slugs take care to be in good condition at the 

 time their winter sleep begins, and for this reason the former 

 are said to be most esteemed by foreign epicures if captured just 

 at this period.^ 



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 ^ some interestino- 

 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. 



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 ^ rvfescens gave live 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. radiaiula, 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 io 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 Drcisscnsia casts off its byssus and retires under the 



^ Moquin-Tandon, jl/oU. de France, i. p. 116. 

 2 Journ. of Conch, iii. p. 321 f . ; iv. p. 13; Science Goss. 1866, p. 158. 



