MOLLUSCA : GASTEROPODA. 335 



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 

 respectively 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 hczmal. 



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 (prosori) in advance of 

 the heart. Sexes distinct" (M.-Edwards). (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 (Bucci- 

 num undatum} may be taken as an example, are all marine, 

 and are mostly carnivorous in their habits. The following 

 families are comprised in this section : Strombidcz (Wing- 

 shells), Muricidce, Bucrinida. (Whelks), Conidce (Cones), Volu- 

 tidce, and Cyprceidce (Cowries). 



The Holostomata, of which the common Periwinkle (Litto- 

 rina littored) is a good example, are either spiral or limpet- 

 shaped, in some few instances tubular, or multivalve ; the aper- 

 ture of the shell being in most cases entire. They are mostly 

 plant-eaters, and they may be either marine or inhabitants of 

 fresh water. The following families are included in this sec- 

 tion : Natirida, Pyramidellida, Cerithiada, Melaniada, Turn- 



