114 HEAT OF THE MoLLUSCA. 



of life. Others live in confined and dirty places, where 

 they are obliged to breathe foul and noxious gases, 

 which would suffocate animals of more active respiration. 

 It is curious to see how long they can live when utterly 

 deprived of air. Turtles have been known to live a 

 month and more, with their jaws tied closely together 

 and stopped up with sealing-wax. Dr. Edwards com- 

 pletely cut out the lungs from two frogs, and found that 

 one lived 33 and the other 40 days, in wet sand. 



Emily. I have read of toads being found alive, com- 

 pletely immured in solid rocks, or in the heart of trees. 

 My faith has been always staggered by such stories, but, 

 I suppose now there is some foundation for them in truth. 



Dr. B. It is probable that in all these cases, the an- 

 imals were either in a torpid condition, or that they had 

 communication with the air by means of some' hole or 

 fissure in the rock. 



In the Mollusca or shell-fishes, we find considerable 

 variety in the construction of the respiratory organs. In 

 those which live on the land, and those which are obliged 

 to come to the surface of the water to breathe the air, 

 we observe a lung which differs from other lungs, by 

 consisting of a single membranous bag, communicating 

 externally by a narrow aperture which can be closed or 

 opened at the will of the animal, while the bag contract- 

 ing and dilating, expels or admits the air. On the sides 

 of this cavity are ramified an almost infinite net-work of 

 blood-vessels, containing the blood which is to be reno- 

 vated by the influence of the air. In other species, we 

 observe gills projecting externally, and sometimes re- 

 sembling tufts of branches, forming a kind of hedge all 

 around the body. In others, the gills are contained in 

 the mantle which surrounds their body, and the water is 

 admitted and expelled by the successive contraction and 

 dilatation of this mantle. 



The heat of the Mollusca is very nearly the same as 

 that of the surrounding medium; in the snails it is a few 

 degrees above. Mr. Hunter found the lungs of a sndl 



