The Gasteropoda are divided into two primary sections or subclasses, according as the respiratory organs are adapted for breathing air directly or dissolved in water: termed respectively 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 haemal.

This sub-class comprises three orders - viz., the Prosobranchiata, the Opisthobranchiata, and the Nucleobranchiata or Heteropoda.