312 MANUAL OF ZOOLOGY. 



are coiled round a central axis in the same plane, when the 

 shell is said to be "discoidal" (as in the common fresh- 

 water shell Planorbis). In most cases, however, the whorls 

 are wound round an axis in an oblique manner, a true spiral 

 being formed, and the shell becoming " turreted," " trochoid," 

 " turbinated," &c. This last form is the one which may be 

 looked upon as most characteristic of the Gasteropods, the shell 

 being composed of a number of whorls passing obliquely round 

 a central axis or " columella," having the embryonic shell or 

 "nucleus" at its apex, and having the mouth or "aperture" 

 of the shell placed at the extremity of the last and largest of 

 the whorls, termed the "body-whorl." The lines or grooves 

 formed by the junction of the whorls are termed the " sutures," 

 and the whorls above the body-whorl constitute the " spire " of 

 the shell. The axis of the shell (columella) round which the 

 whorls are coiled is usually solid, when the shell is said to be 

 " imperforate ; " but it is sometimes hollow, when the shell is 

 said to be " perforated," and the aperture of the axis near the 

 mouth of the shell is called the "umbilicus." The margin of 

 the " aperture " of the shell is termed the " peristome," and is 

 composed of an outer and inner lip, of which the fonner is 

 often expanded or fringed with spines. When these expan- 

 sions or fringes are periodically formed, the place of the mouth 

 of the shell at different stages of its growth is marked by 

 ridges or rows of spines, which cross the whorls, and are called 

 " varices." In most of the phytophagous Gasteropods {Holo- 

 stomafa) the aperture of the shell (fig. no, a) is unbrokenly 

 round or "entire," but in the carnivorous forms {Siphonosto- 

 mata) it is notched, or produced into a canal (fig. no, b). 

 Often there are two of these canals, an anterior and a posterior, 

 but they do not necessarily indicate the nature of the food, as 

 their function is to protect the respiratory siphons. The 

 animal withdraws into its shell by a retractor-muscle, which 

 passes into the foot, or is attached to the operculum ; its scar 

 or impression being placed, in the spiral univalves, upon the 

 columella. 



[n the multivalve Gasteropods, the shell is composed of 

 eight transverse imbricated plates, which succeed one another 

 from before backwards, and are imbedded in the leathery or 

 fibrous border of the mantle, which may be plain, or may be 

 beset with bristles, spines, or scales. 



