Respiration is with water brought into the mantle cavity via an inhalent passage (1) through the connective tissue by the left of the head. In the cavity it first passes through the reddish osphradium (2) which tests the water quality and probably removes large particles of detritus. It then passes through the dull yellow ctenidium (3) which acquires oxygen from the water for the haemolymph (‘blood’) passing through it and releases carbon dioxide which passes with the exhalent water out of the animal via an exhalent passage (4) through the connective tissue on the right side of the body. The rectum (5) and anus are near the exhalent passage where faeces exit the animal with the exhalent water.

marinvert.senckenberg.science/image-browse/