RESPIRATION. 257 



small portable receiver," carefully exhausted ; " but, though 

 they did not lose their motion near so soon as other ani- 

 mals were in our vacuum wont to do, yet, coming- to look 

 on them after some hours they appeared moveless and very 

 tumid ; and, at the end of twelve hours, the inward parts 

 of their bodies seemed to be almost vanished, and they 

 seemed to be but a couple of small full-blown bladders ; 

 and on the letting in of the air they immediately so shrunk, 

 as if the bladders had been pricked : the receding air had 

 left behind it nothing but skins ; nor did either of the 

 snails afterwards, though kept many hours, give any signs 

 of life."* In this experiment, it is obvious that the snails 

 were killed from the mechanical eifects of the expansion 

 of the air within them, and not from its ingress to the pul- 

 monary cavity being prevented. 



But there are on record some extraordinary facts, which 

 seem to prove that, under certain conditions, all of which 

 are not yet known, the resj)iration of many mollusca, more 

 especially the terrestrial, may be suspended for an indefinite 

 period, and again renewed by the application of heat and 

 moisture ; life, as it were, keeping watch, and holding at 

 bay every destructive agent, but without giving any outward 

 sign of her presence and constant wakefulness, until the 

 return of those influences in which she joys. " All the 

 land testacea," to use the words of Dr. Fleming, " appear 

 to have the power of becoming torpid at pleasure, and in- 

 dependent of any alterations of temperature. Thus, even 

 in midsummer, if we place in a box specimens of the Helix 

 hortensis, nemoralis, or arbustorum, without food, in a day 

 or two they form for themselves a thin operculum, attach 

 themselves to the side of the box, and remain in this dor- 

 mant state. They may be kept in this condition for several 

 years. No ordinary change of temperature produces any 

 effect upon them, but they speedily revive if plunged in 

 water. Even in their natural haunts, they are often found 

 in this state during the summer season, when there is a 

 continued drought. With the first shower, however, they 

 recover, and move about ; and at this time the concholo- 

 gist ought to be on the alert." f I may illustrate these 

 remarks, which are perfectly correct, by some additional 

 examples ; one or two of which you may find to require 

 an exercise of faith for which you may not be altogether 

 prepared. Some AmpuUariae from the Nile, being packed 

 in saw-dust, revived after their arrival in Paris on being 



* Phil. Trans. ICuO, p. 20r)0. f Pliil. Zuol. ii. 77. 



S 



