| Cutaneous | Simplest method. Respiration by moist integument. Usually in Mollusca that have no differentiated respiratory organs — Scaphopoda, Aplacophora. Respiration occurs through the whole body surface. Restricted when body is covered with a shell or where skin is thick and bulky. Can be used to supplement respiration by other mechanisms. |
| Branchial | True Gills (Ctenidia) Most of the Molluscs are aquatic and respire by means of gills. Throughout the division of Mollusca, gill is a homologous organ, derived from the gill of a common ancestral type. Structure of a Ctenidium Projection of body surface or mantle. Gill consists of a horizontal main stem or axis attached to the body, and a two of alternately arranged, flexible respiratory lamellae on either side. Surface epithelium is covered by cilia, whose movement is responsible for constant renewal of water over the gills Gills receive blood from the body through afferent branchial vein and after becoming oxygenated, it is sent to the body through efferent branchial vein. Counter current: water flows over the gills in a direction opposite to that taken by flow of blood through the gills. Gaseous exchange here is a matter of simple diffusion without any active transport. Secondary or Adaptive Gills True ctenidium is absent and other morphologically different structure develop e.g. Docoglossa, Nudibranchia. Types of Adaptive Gills: -Anal Gills: Rosette of feathered, retractile secondary gills around the anus e.g. Doris -Cerata: Simple or pinnately branched on the dorsal surface of the body. Richly vascular. Break off and regenerate. Diverticula of the digestive glands open to the exterior at their tips. E.g. Aeolis Pallial Gills: Series of gills in a row on each lateral side in the pallial groove e.g. Patella |
| Pulmonary | In terrestrial pulmonates, a true Ctenidium disappears and the mantle cavity is transformed into a pulmonary sac or lung for aerial respiration. Roof of pulmonary sac is richly supplied with vessels. Pulmonate lungs are an adaptation for terrestrial existence. Alternate contraction and relaxation of mantle floor, causes air to rush in and out of the mantle cavity. Most lower pulmonates are Amphibious (Planorbis). Mantle cavity serves for both aerial and aquatic respiration. |