MOLLUSCA: GASTEROPODA. 41 1 



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 embedded in the leathery or fibrous 

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

 with bristles, spines or scales. 



CHAPTER XLVI. 



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 respec- 

 tively the Pulmonata, Pulmonifera, or Pulmogasteropoda, and 

 the Branchiata, 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 hcemal. 



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

 branchiata, the Opisthobranchiata, and the Nudeobranchiata or 

 Heteropoda. 



ORDER I. PROSOBRANCHIATA. The members of this order 

 are defined as follows: "Abdomen well developed, and pro- 

 tected by a shell, into which the whole animal can usually 

 retire. Mantle forming a vaulted chamber over the back of 

 the head, in which are placed the excretory orifices, and in 

 which the branchiae are almost always lodged. Branchice. 

 pectinated or plume -like, situated (proson) in advance of 

 the heart. Sexes distinct " (Milne- Ed wards). (See Woodward's 

 * Manual.') 



The order Prosobranchiata includes all the most character- 

 istic members of the Branchiate Gasteropods, and is divisible 

 into two sections, termed respectively Siphonostomata and 

 Holostomata, according as the aperture of the shell is notched 

 or produced into a canal, or is simply rounded and " entire." 



The Siphonostomata, of which the common Whelk (Buccinum 

 undatum, fig. 212) may be taken as an example, are all marine, 



