2QO MANUAL OF ZOOLOGY. 



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 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- 

 stomata) the aperture of the shell (fig. 103, a) is unbrokenly 

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

 mata) it is notched, or produced into a canal (fig. 103, 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. 



In the multivalve Gasteropods, the shell is composed of 

 eight transverse imbricated plates, which succeed one another 

 from before backwards. 



CHAPTER XLVIII. 

 DIVISIONS OF THE GASTEROPODA. 



THE Gasteropoda are divided into two primary sections or 

 sub-classes, according as the respiratory organs are adapted 

 for breathing air directly, or dissolved in water: termed re- 

 spectively the Pulmonifera or Pulmogasteropoda, and the 

 Branchifera or Branchiogasteropoda. 



SUB-CLASS A. BRANCHIFERA or BRANCHIOGASTEROPODA. 

 In this sub-class respiration is aquatic, effected by the thin 

 walls of the mantle-cavity, by external branchial tufts, or by 

 pectinated or plume -like gills, contained in a more or less 

 complete branchial chamber. Flexure of intestine hamal. 



This sub-class comprises three orders viz., the Proso- 



