194 STRUCTURE OF THE MOLLUSCA. 



This order is extremely numerous, and comprehends the 

 families of the Turbonma, Trochina, Cyclostomina, Neritina, 

 JBuccinina, Fusina, Muricina, Volutina, Cyprceina, and 

 others. 



The Trochina, which are destitute of respiratory siphon, 

 have a spiral shell of a conical form, with the aperture entire, 

 closed by an operculum, and somewhat quadrangular. They 

 are represented by the genus Trochus. 



The Turbomna, to which belong the genera Turbo, Lit- 

 torina, and others, have the shell globose, oval, or oblong, 

 with the aperture completed internally by the penultimate 

 turn. They have two long tapering tentacula, bearing the 

 eyes at their base externally. On the sides of the foot 

 are membranous expansions, sometimes simple, sometimes 

 fringed. Their operculum is sometimes horny, sometimes 

 calcareous, or extremely thick. 



The Cyclostomina differ from the rest in respiring air, and 

 in not having branchial organs in their respiratory cavities, 

 but are otherwise so nearly allied to the families of this 

 order, that they cannot well be separated from them. The 

 aperture of the shell is round, margined, and closed by a thin 

 circular operculum. They are terrestrial, and live in woods, 

 among moss, or under stones. 



The Vahatina have a shell somewhat resembling that of 

 the Planorbes, but with a circular aperture, furnished with 

 an operculum. 



The Paludinina, which resemble the Cyclostomina, have 

 the aperture of the shell thin-edged, and are furnished with 

 branchiae for respiring water. 



The Phasianellce, AmpuUarice, and MelanicBy which live 

 chiefly in warm climates, are nearly allied to the Turbonina. 



The lanthincE, of which the shell somewhat resembles 

 that of a snail, differ from the animals spoken of above, in 

 having no operculum, and in being furnished with a vesicular 

 organ under the foot, resembling a water-bubble, and which 

 prevents the animal from crawling, but enables it to float. 

 The common species, lanthina fragilis, on being touched, 



