Order IV. PULMONIFEEA. 



PULMOXATA. PuLMOGASTEROPODA. 



Eespiration either aerial or aquatic, carried on by the lining 

 membrane of a pulmonary cavity. Head distinct. Hermaphro- 

 dite. Larva with the velum absent or inconspicuous. 



The pulmonary sac or cavity is placed near the neck on the 

 right side, and communicates with the external air by a single 

 aperture. Many of the terrestrial species have cutaneous foli- 

 cular glands which secrete a granular mucus. A curious organ 

 is a pyriform muscular sac, containing one or two slender conical 

 styles, which can be thrust out through the aperture of the sac ; 

 they are found in certain snails, and with them they pierce each 

 other's skin. They are known as " love-darts." 



In some species the ova are of very large size. Gadinia, Sipho- 

 naria, &c. have simple patelliform shells. Some have no shells, 

 or the shell is very small or is hidden within the mantle. The 

 common garden-snail (Helix aspersa) and the slugs, of which we 

 have five or six species belonging to Arion and Limax, are mem- 

 bers of this order. Amphibola is the only genus with an oper- 

 culum ; it is also without tentacles. 



Many species hibernate ; during hibernation the shell is closed 

 by an epiphragm, a calcareous secretion of the foot, which is 

 dropped when hibernation is over. 



Helicidse [ = G-eophila ; Stylommatophora] is the only exclu- 

 sively land family. They have their eyes, except in Onchidium, 

 placed on retractile stalks or tentacles (two or four). The re- 

 maining families [included under the name Basommatophora] are 

 found in fresh or brackish waters. About 6500 species are con- 

 tained in this order. 



