380 TROCHOIIXE. IANTHINA ; NERITA ; AMPULLARIA. 



fragile violet-coloured shell, which presents the general form 

 of that of the Helix, although differing from it in some 

 important particulars. This she] 1 has no oper- 

 culum; but this appendage is replaced by a 

 curious apparatus, which serves as a float, 

 enabling it to swim at the surface of the sea 

 without any effort. To the foot are attached 

 a number of vesicles, full of air, and resem- 

 bling foam-bubbles; and the animal seems to 

 have the power of compressing or emptying 

 these at will, so as cause itself to sink. 

 When irritated or alarmed, it pours out a violet-coloured 

 secretion, resembling its shell in colour, which darkens the 

 water around it, and thus serves for its concealment, in the 

 manner of the ink of the Cuttle-fish. The genus Nerita, which 

 is nearly allied to this, contains several species which inhabit 

 fresh water ; and some which even live out of the water alto- 

 gether, by the aid, probably, of some provision analogous to 

 that which enables the Climbing Perch, or the Land Crab, to 

 live at a distance from the water. The Ampullaria (Fig. 577)? 



has a shell still more like that of the 

 Helices ; and although provided with 

 a branchial comb, like that of the 

 Trochoidas in general, it has also the 

 rudiment of a pulmonary sac, like 

 that of the Snails, which is said to 

 be filled with water, and thus to 

 keep the gills moist for a long time. 

 The animal inhabits the fresh or 



F I0 .577.-AMP ULLARIA UGO s,. brackish waters of warm climates, 

 and forms a calcareous opercu- 



lum. It is remarkably tenacious of life, partly in consequence 

 of the curious provision just alluded to. Thus specimens have 

 been brought from Egypt to Paris alive, although packed up in 

 sawdust ; and in one instance, a box containing a large number 

 of river-shells from the Nile having been delayed four months 

 on the road, the Ampullarise were found to have remained alive 



