Respiratory Organs and Its Modification in Mollusca

Lecture Notes: Respiratory organs and its modification in Mollusca

Respiratory organs and its modification in Mollusca Correspondence: [email protected]

The members of the phylum Mollusca show different modes of living. Some of them are aquatic, some are terrestrial and others are amphibious in their habitats. As a consequence of living in diverse environmental conditions, the respiratory organs are modified accordingly. The respiratory organs encountered in Molluscs are mainly the ctenidia and the lungs or pulmonary sac. The outer covering of the body and mantle usually act as accessory respiratory organs.

Respiratory organs for aquatic mode of living

1. Skin and Mantle Skin appears to be the simplest type of respiratory structure encountered in Molluscs. It acts as a respiratory organ in certain forms where there is no special respiratory device. Such type of respiratory organ is found in Cenia, Limapontid, parasitic Entoconcha, etc.

In most of the members of Aeolididae the dorsal surface of the body is provided with papillae. The papillae are variable in size and communicate with the heart by veins. Most of the Nudibranchia respire through skin. In some forms (e.g., Neomenia, Chaetoderma, Aplysia, Dentalium, etc.), the mantle is used for respiration.

2. Ctenidium Aquatic molluscs respire through ctenidia. These are the comb-like outgrowths of the mantle and are located within the mantle cavity.

Structure Each ctenidium (pl. ctenidia) or molluscan gill consists of a long flattened axis hangs from the anterior wall of the mantle cavity. The ctenidium contains afferent and efferent blood vessels, muscles and nerves. A row of flattened, triangular gill filaments or lamellae are attached to each side of ctenidial axis. The surface of ctenidium is generally covered with cilia.

Types of ctenidia Based on topography, the ctenidia in molluscs are placed in following categories: 1. Holobranchiate The ctenidia extend all over the body. The number varies from 14-80 pairs. Example: Polyplacophora.

2. Merobranchiate The ctenidia are restricted to a particular area of the body. Depending on the arrangement of leaflets the merobranchiate type of ctenidia can be subdivided into following types:

A. Plicate Simple, flat, transversely folded, projecting integumentary laminae constitute the gill. In Neomania a tuft of filaments arise from the cloacal wall. Correspondence: [email protected] Page 1 of 3

Lecture Notes: Respiratory organs and its modification in Mollusca

B. Monopectinate Flattened gill filaments are arranged in a single row. Examples: Pila, Triton, etc.

C. Bipectinate Flattened gill filaments are arranged in two rows. They may be of two types-

a. Unequal: Both rows of filaments are present, the right one being smaller in size. Example: Fissurela, Haliotis. b. Equal: Both the rows of filaments are of same sizes. This is the characteristic of the bivalves. Amongst bivalves they become variously modified. Example: Nucula, Mytilus, Unio etc.

D. Feathered Assumes the shape of a feather. Examples: Cephalopods.

Modifications of Ctenidia The ctenidia become modified in some forms.

Anal gills In Doris (see Fig. 16.49B) delicate leaflets form a rosette round the anus and is designated as the anal gills. In Pterotrachea the mantle fold is absent and the filamentous branchial leaflets project freely and remain uncovered. In most cases the mantle may serve as respiratory organ.

In Solenogastres (= Subclass Neomeniomorpha) the cloacal gills are present. Gradual degradation of the cloacal gills is encountered. In Chaetoderma the gills are two in number and are symmetrically placed one on each side of the cloaca. Single distinct gill is present in Neomenia and in Proneomenia, the gills are nothing but a few folds on the cloacal wall.

Cerata In Aeolis, Aedida many highly vascular secondary gills (Cerata) are present on the dorsal surface of the body (see Fig. 16.49C) serving for gas exchange.

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Lecture Notes: Respiratory organs and its modification in Mollusca

Respiratory organs for terrestrial mode of living

Terrestrial habit leads to complete loss of gills and a variety of respiratory organs develop to suit the particular environment. They are as follows-

Pulmonary sac: In most Pulmonata the mantle cavity forms a pulmonary chamber, the inner surface of which is highly vascularised.

Trachea: In some Pulmonata, the pulmonary chamber gives off breathing air- tubes, called trachea.

Nuchal lobe: In Monotocardia (Mesogastropoda) the left nuchal lobe is better developed and forms a long respiratory siphon.

Amphibious forms

These forms are exemplified by Pila. It possesses both ctenidium as well as pulmonary sac. The genus Siphonaria is furnished with a lung-cavity and a ctenidium. Both the forms represent a transitional stage between aquatic and terrestrial life.

References Brusca R.C. and Brusca G.J. (2003). Invertebrates. 2nd Edition. Sinauer Associates Inc. Publishers, USA. Ganguly B.B., Sinha A. and Adhikari S. (1977). Biology of Animals, Vol. I. 3rd Edition. New Central Book Agency (P) Ltd., Culcutta. Kotpal R.L. (2013). Modern Text Book of Zoology: Invertebrates. Tenth revised edition. Rastogi Publication, Meerut. Parker T.J. and Haswell W.A. (1972). Textbook of Zoology, Vol. I: Invertebrates. Seventh Edition. The Macmillan Publishers Limited, London.

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