Respiration in Mollusca

Molluscs have a soft body and an exoskeleton made of a hard, calcareous shell. They primarily live in water. Molluscs have gills in their mantle cavity, which are used for respiration to exchange oxygen and carbon dioxide. Gills are employed by some bivalves for both filter feeding and breathing. Land molluscs have pulmonary sacs or cavities, which are aerial respiration organs. Here, let’s discuss the types of respiration in molluscs.

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Respiration in Mollusca

Most molluscs breathe using gills known as ctenidia or comb-like gills. In terrestrial molluscs, this respiratory apparatus is smaller, but pallial cavity respiration still occurs. Thus, the three types of respiration seen in molluscs are:

  1. Cutaneous respiration
  2. Branchial respiration
  3. Pulmonary respiration

Cutaneous Respiration

The simplest sort of respiratory system found in molluscs is cutaneous respiration. When there is no specific respiratory device present, it functions as a respiratory organ in some forms. Such respiratory organs are present in parasitic Entoconcha, Cenia and Limapontids.

The dorsal surface of the body is covered in papillae in the majority of the Aeolididae species.

The papillae differ in size and are connected to the heart by veins.

The majority of Nudibranchia breathe through their skin. The mantle is employed for respiration in some forms of Nudibranchs such as Aplysia, Neomenia and Chaetoderma.

Branchial or Ctenidial Respiration

Molluscs living in water breathe through ctenidia. These are the mantle’s comb-like protrusions that are found inside the mantle space. The cilia’s beating causes the water to travel through the mantle cavity.

Afferent and efferent blood arteries can be seen in each ctenidium, and they pass through the ctenidial axis. Through the afferent blood artery, the ctenidium’s body receives deoxygenated blood from the animal, and through the efferent blood vessel, oxygenated blood is returned to the heart after being oxygenated in the gill filaments.

Modifications of Ctenidia

  • Anal gills are the little, fragile leaflet-like structures that develop in a rosette around the anus in the Doris species.
  • On the dorsal surface of the body, Aeolis (sea slug) has a large number of highly vascular secondary gills called the cerata, which are used for gas exchange.
  • Pallial gills are seen in Patella. Their pallial grove houses a row of modified gills on each side.
  • Pleural gills are seen in Pleurophyllida which has many rows of branchial leaflets beneath the mantle.

See more:Snail – Skeletal System

Pulmonary Respiration

A real ctenidium is absent in terrestrial pulmonates, and the mantle cavity develops into a pulmonary sac or lung for aerial respiration. The pulmonary sac’s ceiling is well-supplied with blood vessels. Air rushes in and out of the mantle cavity as a result of the alternate muscular contraction and relaxation of the mantle floor.

A small, circular pulmonary opening or pneumostome is present on the right side of the sac with a valve. It allows air to enter or exit. The mantle cavity’s compression increases the partial pressure of oxygen and makes it easier for it to be absorbed.

The mantle cavity of lower pulmonates like Lymnaea serves both for aquatic and aerial respiration.

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Frequently Asked Questions


How do molluscs respire?

In the case of aquatic respiration, the ciliary or muscle pumping is used to move water across the ctenidial surface. Whereas in terrestrial respiration, the air enters and exits the pallial cavity as a result of pulmonate muscle contraction and relaxation. Both mechanisms aid in the exchange of gases.

How do gills help in respiration?

The majority of molluscs only have one pair of gills, or perhaps only one gill. The gills are often shaped like feathers and some molluscs have gills with filaments present on only one side. They partition the mantle cavity such that water enters near the bottom and escapes towards the top. Their filaments feature 3 different types of cilia, out of which 2 keep the gills clean and one propels the water stream through the mantle cavity.

What are pulmonary sacs in molluscs?

In some amphibious molluscs like Pila, the pallial cavity is only partially divided into a left pulmonary chamber and a right branchial chamber by a fleshy fold. The pulmonary chambers’ highly vascularized roof produces a pulmonary sac which is the aerial respiratory structure in molluscs.

What is a pneumostome?

Air-breathing molluscs have a respiratory opening on their exterior body called a pneumostome or breathing pore. It is a component of the gastropods’ respiratory system. The pneumostome allows air to enter the animal’s lung.