THE GASTEROPODA. 



191 



Nearly all Gasteropoda are unsymmetrical, the body being coiled up spirally, having the breathing 

 organs on the right side developed and those on the left absent. A few are like the Cephalopoda in 

 being bilaterally symmetrical, as Chiton and Dentalinm, in which the gills and ovaries are found on 

 each side. With a few exceptions, in which the young Snails are bom like their parent, the greater 

 part deposit eggs, either in the water or in damp situations on the land. All are provided with a 

 shell when first hatched, but this (in some genera) is found to become concealed by growth in the 

 adult, or disappears altogether in later life. 



The creeping Snails are, like the Cephalopoda, divided into two natural groups by their breathing 

 organs in the one they breathe air, and are hence called Pulmonifera,* in the other the gills are 

 bathed in water, and they are named Branchifera. 



Before proceeding further, it may be well if we clearly understand what are " the points " about 

 a univalve shell, so that in enumerating the salient features of A 



each genus our readers may comprehend and follow us in these 

 descriptions. 



The subjoined lettered figure of a shell of the genus Triton 

 may serve to indicate, better than whole chapters of description, 

 by what terms each portion of the shell is named. 



This shell may be described as fusiform : f the apex (A) 

 manmiillated ; 1 the whorls (w) ventricose, strongly ribbed or 

 corrugated, with non-continuous varices || (v), and distinct sutures 

 H (su) ; the columella ** (i) is denticulated ; ff the outer lip (o) 

 is internally plicato-dentate ; %+ the anterior canal (c) is elon- 

 gated ; the body- whorls (bw) are large ; the aperture (a) ovately- 

 elliptical. 



bw 



In 



ORDER I. PROSOBRANCHIATA. 



the first order of Gasteropoda, called Prosobranchiata 



SHELI- OF TKITON. 



in habit, though some, as the 



by Milne-Edwards, the gills are pectinated, or comb-like, and are 

 placed in advance of the heart. The soft parts are protected by 

 a shell into which the body of the animal can usually be with- 

 drawn. The eye pedicels and the tentacles, or feelers, are on the 

 same stalk. 



Division a. SIPHONOSTOMATA.|| || The proboscis is long and 

 retractile, and the breathing chamber is provided with a tube, 

 or siphon, to convey a fresh current of water to the comb-like 

 gills. The members of this section are carnivorous and marine 

 " Strombs," are carrion-feeders. 



FAMILY I. THE STROMBID.E. 



The Strombidoi, or "Wing-shells," are very active; they have large eyes, placed on thick pedicels, 

 which are more perfect than those of other Gasteropods, or even of many fishes. They have powerful 

 lingual teeth of a type peculiar to the carnivorous Sea Snails. 



The Strombs (Strombus H11) generally have a widely-expanded outer lip with an elongated 

 aperture, lobed above and sinuated near the notch of the anterior canal. The wtorls of the spire 

 are often covered with tubercles or spines ; and the spire of the shell is usually short 



* Puhno, a lung ; and fero, I bear. 



t Spindle-shaped, pointed at both ends : from fusus, Lat., a spindle. 

 J From Lat., mamilla, a teat, a rounded summit. 

 From Lat., ventricosus, inflated. 



|| Lat., varix, a swollen vein ; in reference to the periodic mouths or ridges on the whorls of some shells, marking rests, 

 in growth. 



H Lat. svtura, a seam ; the point where two whorls of a spiral shell are united. 



; * Lat., columeUa, a small column or pillar ; the axis of a spiral shell, around which the whorls of the shell grow. 

 ft From Lat., dcm, a tooth ; hence denticulated, bordered with small teeth. 



J Lat., plica, a fold, and dens. Proson, in front ; branckiata, gilled. 



|||| From siphon, a tube, and stoma, a mouth. ^fH" From stromlos, a top. 



