AMPHIBIOUS INVERTEBRATES 



461 



America (fig. 567). The gill-cavity is here divided into upper 

 and lower parts by an imperfect partition, and a long breathing- 

 tube {siphon) can be protruded from the left side of the neck. 

 The upper part acts as a lung, 

 and its lining is raised into 

 numerous folds so as to increase 

 the air-breathing surface, while the 

 lower part contains a gill on the 

 right side. Semper (in Animal 

 Life) thus describes the way in 

 which the two divisions of the 

 breathing apparatus are used: — 

 "The Ampullaria uses both 

 organs in rapid alternation; lying 

 not far from the surface of the 

 water, it protrudes above it a 

 breathing siphon, and inhales air 

 through it; then it closes its 

 lungs, reopens the siphon, and 

 admits a stream of water through 

 it into the branchial cavity ". We 

 are here strongly reminded of 

 certain freshwater fishes already 

 described (see p. 450), which 



frequently come to the surface to take air for breathing into their 

 swim-bladders. 



We have so far dealt with the origin of only operculate land- 

 snails, many of which are closely related to Periwinkles and other 

 sea-snails belonging to the Fore-gilled (Prosobranch) marine forms 

 (see vol. i, p. 318). Such terrestrial molluscs, though common 

 enough in tropical regions, are quite different from the ordinary 

 land-snails and slugs with which we are familiar in this country, 

 and which constitute the group of Lung-Snails and Slugs (Pul- 

 monates), which do not possess an operculum to close the mouth 

 of the shell, and may even have no shell at all. Some at least of 

 these have probably been derived from the marine forms known 

 as Hind-gilled Snails and Slugs (Opisthobranchs), exemplified by 

 Bubble -Shells, Sea- Hares, Sea- Slugs, and many others (see 

 vol. i, p. 324). The chief evidence in support of this view con- 

 sists in the existence of a family of shore-snails {SiphonariadiB) 



Fig. 567. — Apple-Snail [Ainpidlaria) 



A, Upper surface; b, vertical section across breathing- 

 organs {diagrammatic); w.t., water-testing organ 



