HABITS AND FACULTIES. 9 



epiphragm. It is formed in this manner : The animal being withdrawn 

 into the shell, the collar is brought to a level with the aperture, and a 

 quantity of mucus is poured out from it and covers it. A small quan- 

 tity of air is then emitted from the respiratory foramen, which detaches 

 the mucus from the surface of the collar, and projects it in a convex 

 form, like a bubble. At the same moment the animal retreats farther 

 into the shell, leaving a vacuum between itself and the membrane, which 

 is consequently pressed back by the external air to a level with the 

 aperture, or even farther, so as to form a concave surface, where, having 

 become desiccated and hard, it remains fixed. These operations are 

 nearly simultaneous, and occupy but an instant. As the weather be- 

 comes colder, the animal retires farther into the shell, and makes another 

 septum, and so on, until there are sometimes as many as six of these 

 partitions ; the circulation becomes slow ; the pulsations of the heart, 

 which in the season of activity vary from forty to sixty in a minute, 

 according to the temperature of the air, decrease in frequency and 

 strength, until they at length become imperceptible ; the other func- 

 tions of the body cease, and a state of torpidity succeeds, which is inter- 

 rupted only by the reviving heat of the next spring's sun. During the 

 months of April or May, 1 on the accession of the first warm weather of 

 the season, the animal breaks down and devours the membranous parti- 

 tions, and comes forth to participate in the warmth and freshness of the 

 season. At first it is weak and inactive, but, recovering in a short time 

 its appetite, resumes its former activity. The peculiar epiphragm of 

 Binneya is described when treating that genus. 



The season of hibernation continues from four to six months. The 

 final cause of this extraordinary condition is undoubtedly to enable the 

 animal to resist successfully the extreme reduction of temperature, and 

 to survive through the long period when it must, in northern climates 

 at least, be entirely destitute of its usual food. With a view to the first 

 purpose, a place of shelter is provided, and the aperture of the shell is 

 hermetically sealed by the epiphragm or the hibernaculum ; for the 

 second, the state of torpor is adopted, during which the functions of 

 digestion, respiration, and circulation being suspended, and all the secre- 

 tions and excretions having ceased, there is no drain upon the strength 

 and vitality of the animal, and no exhaustion of its forces. Hence it 

 comes forth, at the end of the period, in much the same condition in 

 which it commenced it, and resumes almost immediately its usual func- 



1 In New England, earlier in more southern latitudes. 



