262 MANUAL OF ZOOLOGY. 



siphons. The animal withdraws into its shell by a retractor- 

 muscle, which passes into the foot, or is attached to the oper- 

 culum ; its scar or impression being placed, in the spiral uni- 

 valves, 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 

 Jfranchifera, or Branchiogasteropoda. 



SUB-CLASS A. BI:AN> mi I:RA, 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 ofintesti-m-- //<//,/"/. 



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

 Iranchiata, the Opisthobranckiato, and the Nucleobranchiata 

 or HetiM-i<in>i1<i. 



ORDER I. PROSOBRANCHIATA. The members of this order are 

 denned as follows : * Abdomen well developed, and protected 

 liv 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. Hrcm-hi'ii' pectinated or 

 plume-like, situated (proton) in advance of the heart. 

 distinct.' (M. -Edwards. See Woodward's Manual.) 



The order Prosolranchiata includes all the most charac- 

 teristic members of the Branchiate Gasteropods, and is divisihlr 

 into two sections, termed respectively tiiphonostomata 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 (/;//-,/. 

 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: ,S'//v.///iW</ (Wing- 

 shells), MuricidoB t Buccinidce (Whelks), Conidce (Cones), Fo- 

 , and Cyprwidce (Cowries). 



