Anatomy of finfish and shellfish

Unit 13- Mollusca

13.4.4. Respiratory System
Respiratory organs
These include a pair of large, plume- shaped cienidia, lying in the mantle cavity, one on either lateral side. Each ctenidium or gill is bipinnate, with nunierous delicate lamellae on either side of a central axis. The surface of the lamellae is much folded to increase the respiratory sur¬face. Cilia are absent as removal of sediment is not a problem in pelagic animals and water current is created by mantle contractions Each gill receives venous blood through an afferent branchial vessel from the branchial heart of its side. Inside the gill it passes through a system of minute branches through the lamellae and is collected finally into an efferent branchial vessel leading to the auricle.
Respiratory mechanism
The muscular mantle rhythmically expands and contracts, so that the mantle cavity alternately increases and diminishes in size. Consquently, the oxygen bearing inhalent current of water enters the mantle cavity through the wide aperture around the neck, and the exhalent current escapes through the funnel. Exchange of gases occurs when the water passes over the ctenidia which are richly vascular.