334 MANUAL OF ZOOLOGY. 



the whorls, termed the " body-whorl" (fig. 136). 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 former is often expanded or fringed with spines. 

 When these expansions 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, 



be e b 



Fig. 136. Anterior and posterior views of Cassis cancellata, a Spiral Gasteropod. a 

 " Spire," placed at the posterior end of the shell ; b " Mouth, " placed at the 

 anterior end of the shell ; c Inner or collumellar lip ; d Outer lip ; e Notch for the 

 passage of a respiratory siphon. 



and are called " varices." In most of the phytophagous 

 Gasteropods (Holostomatd) the aperture of the shell (fig. 137) is 

 unbrokenly round or " entire," but in the carnivorous forms 

 (Siphonostomatd] it is notched, or produced into a canal (fig. 

 138). 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. 



In 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. 



