460 



ANIMAL RESPIRATION 



is covered by water. In the case of those kinds of Periwinkle 

 which Hve high up on the shore we should naturally expect to 

 find the gill more reduced than in allied species living further 

 down, since in the former case its work would be inconsiderable, 

 as its owner would only be covered with water for a brief period 

 of time. Such an expectation is actually realized in one British 

 species {Littoriim ritdis) which lives near high-water mark, and 

 which has a smaller gill than the common kind {L. littorea) that 



spends a larger part of its life 

 under water. In the former spe- 

 cies, too, folds richly supplied with 

 blood-vessels are beginning to 

 appear on the inner surface of 

 the gill-cavity (fig. 566), so as to 

 increase its efficiency as a lung. 

 After what has been said it will 

 l^HEART not surprise the reader to hear that 

 there are certain tropical forms of 

 Periwinkle (native to Central 

 America and the West Indies) 

 which live to a considerable ex- 

 Fig, see-Roof of GiiiMvit,' in a spedes of Peri- ,-gjj(- above hio-h-water mark, and 



winkle [Litteruia mats), which lives near high-water *-* 



mark (enlarged). Front end Is at top of figure. ^j-C CVCn foUnd UDOIl trCCS. SucH 



X, Commencing net-work of lung-folds; Int., intestine: ^ 



A and V, auricle and ventricle of heart; K, kidney; CaSCS lead OH tO Certain faiTlilieS of 



i?.j., water-testing organ (osphradium). ^ ., ,. . 



Snails hvmg on land, in which the 

 gill has entirely disappeared, but which otherwise are probably 

 closely related to Periwinkles. These families are among those 

 known as Land Operculates, on account of the possession of an 

 opercnlum or plate by which the aperture of the shell is closed 

 when the animal is completely withdrawn into it. It must not be 

 supposed, however, that all the families of land operculates are 

 related to Periwinkles, for some of them can be shown with 

 considerable probability to be allies of other sorts of marine 

 Snails. 



It is quite possible that some kinds of snail which are now 

 adapted for a life on land, and breathe by gill -cavities which 

 simply serve as lungs, may have been derived from amphibious 

 freshwater ancestors. We know at any rate that double-breathers 

 of the kind actually exist, the most striking case being that of 

 the Apple- Snail [Ampullaria), native to both Africa and South 



