There is large space between the mantle and the body in the anterior part called mantle or pallial cavity.
On the floor of the cavity, near the anterior edge of right nuchal lobe, a prominent ridge extends upto the extreme posterior of the cavity, called the epitaenia.
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It divides the mantle cavity into two chambers the right branchial chamber and the left pulmonary chamber.
Both the chambers play an important role in respiration. Following organs are found in different chambers.
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1. Organs of the branchial chamber
Ctenidium or gill:
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It lies at the extreme right side of the mantle cavity, attached to its dorsal wall. Being monopectinate (Comb like), its gill filaments or lamellae hang freely in the branchial chamber.
Rectum:
On the left of ctenidium rectum lies on the floor of the branchial chamber. Posteriorly it extends upto the intestine blindly and anteriorly opens outside through anus. Anus is surrounded by a rosette of minute papillae
Genital opening:
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Male or female genital ducts lie close to the rectum and open outside through genital openings. In male, the penis arises from the mantle wall in front of the male genital openings.
Hypobranchial gland:
It is found at the base of the penis and its secretion helps in copulation.
Renal opening:
Renal organ extends in the branchial chamber and opens near the posterior termination of the epitaenia.
2. Organs of the Pulmonary Chamber
Pulmonary sac:
It hangs downwards from the roof of the mantle cavity and communicates with the pulmonary chamber through an elongated opening. It helps in aerial respiration.
Osphradium:
It is a gill like organ situated on the left side of the pulmonary chamber. It is bipectinate (feather like) and consists of 22 to 28 fleshy and somewhat triangular leaflets.
It serves in testing the physical and chemical qualities of the entering water and aids in the selection of food.