26 HIBERNATION AND AESTIVATION chap. 



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

 retreat in common. Slugs generally hibernate alone, excavating 

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

 tracting their bodies until they are almost round, and secreting 

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

 the mouth of their shell by the formation of a membranous 

 or chalky epiphragjii^ 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 Pulmo- 

 nata ceases almost entirely. This appears to be directly due to 

 the effect of cold. Mr. C. Ashford has related ^ some interest- 

 ing experiments made upon H. liortensis 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 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 sud- 

 denly 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 b3^ssus and retires under the 



1 Moquin-Tandon, Moll, de France, i. p. 116. 



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



