30 RESEARCHES UPON THE HYDROBIIN^ 



Society of Natural History, above referred to,' observes that the 

 lapidaria in habits is evidently air-breathing, but that in water 

 they " seemed not to be embarrassed in their movements, though 

 they soon made their way out, apparently preferring to be out of 

 it." Aecording to my own observations they exhibit considerable 

 uneasiness when placed in the water, which caused me some sur- 

 prise when on dissection I found them to possess a true gill and 

 no trace of a vascular "lung." We have, however, analogous 

 cases among the Crustacea, in such genera as Cai^disoma, Uca, 

 Gecarcinus, and Ccenohita, which breathe air, although their 

 breathing organs consist of gills, of similar structure with those 

 of the aquatic Crustacea. There is no difficulty in understanding 

 that a gill may perform the function of respiration in air, so long 

 as its surfaces are kept damp. Even bivalve Mollusca may be 

 kept out of water for great lengths of time, provided the sur- 

 rounding atmosphere be sufficiently humid, and the temperature 

 cool. In this case they can of course breathe only air unmixed 

 with water. 



That the Pomntiopsis is truly terrestrial in its habits, notwith- 

 standing its preference to the vicinity of water, I can have no 

 doubt. Its peculiar mode of progression is, indeed, adapted for 

 land travel only. I have found it living in company with Suc- 

 cinea oralis, S. avara, and Helix electrina, in places not liable to 

 desiccation, that is, near the margins of streams or marshes which 

 do not dry up in summer. Its occurrence in such places only, is 

 in consequence of the necessity of having some moisture for its 

 breathing organs, it being unable, like the Pulmonates, to prevent 

 evaporation, and the consequent desiccation of those organs, by 

 the formation of an epiphragra closing up the aperture of the 

 shell.* The animal may be said to be amphibious, but only in 

 the sense that Succinea and some other terrestrial Mollusca are 

 so ; that is, like them it is capable of living for a long time under 



' Proc. Bost. Soc. Nat. Hist., VIII, 255. 



2 Since writing the above, I have received a letter from Mr. Tryon in 

 which he informs me that " Mr. Conrad lately captured a number of speci- 

 mens of Pom. lapidaria, and laid them away in a dry place. Upon ex- 

 amining them some time afterward he found the animal so much retracted 

 that the operculum was out of sight, showing that, unlike Amnicola, it is 

 provided against dry weather, and can exist out of water or even moisture 

 for some time." 



