RESPIRATION. 85 



isolated, as it escapes by a hole in the shell, at or near its apex ; 

 consequently far removed from the point of ingress. Near this 

 aperture are found the anal, renal and generative orifices. 

 Yibratile cilia on the surface of the branchise determine the 

 current of water towards the anus after it has completely bathed 

 the gills. 



Falmonifera. The land snails and a large portion of the fresh- 

 water species possess a lung formed by a folding of the mantle, 

 the interior of the chamber thus constructed being lined with 

 arborescent vessels forming small fossets or alveolae. The 

 opening of this chamber is relatively large, on the right side of 

 the animal, and dilates and contracts at irregular intervals. 

 The air within the cavity appears to be renewed with sufficient 

 rapidity by the law of diffusion. In Auricula the respiratory 

 apparatus is a true lung, but in nearl}'' all the pulmonifera it is 

 as described, and in some mollusks the breathing is performed 

 by an organ of an intermediate nature. In Ancylus so equivocal 

 is it that it has been called a branchia, a branchial operculum, 

 and a lung. Ampullaria, a fresh-water mollusk, the amphibious 

 seashore genus Siphonaria, etc., possess both a pulmonary sack 

 and a branchia, Littorina, a branchiferous mollusk, usually 

 inhabits stations where it is out of water a considerable portion 

 of each day. I have known specimens to live for months, but 

 without exhibiting much activity, in the air of Philadelphia — 

 which is rather &\:y. On the other hand, Ilelampus bidentatus, 

 a pulmoniferous mollusk, inhabiting exclusively the seashores 

 of the Atlantic coast of the United States, seeks situations 

 where at high water it is covered by the sea. Manj' examples 

 might be cited of this adaptability^ in mollusks, and of modifica- 

 tion in the breathing organ, which modify the sharp distinction 

 formerl}^ supposed to exist between the prosobranchiata and 

 pulmonata. The pulmoniferous Limnsea usually visits the 

 surface of the water, from time to time, for the purpose of 

 renewing its supply of air, yet it can pass several days immersed 

 without asphyxiation : indeed a species inhabiting great depths 

 in the Lake of Geneva (L. abyssicola) normally supplies its sack 

 with water, instead of air, thus justifying the name pulmo- 

 branchiata, which has been given to the androgynous pulmonata. 

 Terrestrial mollusca will survive an immersion of a day's 

 duration in water, during which their teguments have absorbed 

 more than their own weight of the element.* 



The production of heat is verj^ feeble in a number of land 

 snails which have been examined by Spallanzani, Graspai'd, 

 Hunter, Becquerel, etc., varying from "IS to 3-90 degrees above 

 that of the surrounding atmosphere. The Helicidse of temperate 



* Fischer, Jour, de Conch., 101, 1861. 



