Respiratory Organs of Phylum Mollusca and Its Modification

1. Skin and mantle.

2. Molluscan gills or ctenidia or branchiae and

3. Lungs or pulmonary sac.

The outer covering of the body (skin) and mantle usually act as accessory respiratory organs.

1. Skin and Mantle — as Respiratory Organs:

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.

Ctenidium — as the Respiratory Organ:

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.

Mechanism of Aquatic Respiration:

The movement of water is effected through the mantle cavity by the beating of cilia. Each ctenidium contains afferent and efferent blood vessels which run through the ctenidial axis.

The body of the ctenidium receives deoxygenated blood from the body of the animal through the afferent blood vessel and after oxygenation in the gill fila­ments the oxygenated blood is sent back to the heart through the efferent blood vessel.

Number of Gills:

In Polyplacophora the gills vary 6-80 pairs. In Monoplacophora the gills vary 3-6 pairs. In primitive gastro­pods, either two bipectinate (e.g., Pleurotomaria, Haliotis) gills or single bipectinate gill (e.g., Patella, Trochus, Nerita) are present.

In Mesogastropods (e.g., Viviparus, Pila, Lambis, Cypraea, etc.) or Neogastropods (e.g., Murex, Xancus, Conus, etc.), the single monopectinate gill is present. In Bivalves, a pair of plate­like gills and in Cephalopods, 4 gills in Nautilus or 2 gills in Coleoidea (e.g., Sepia, Loligo, Octopus, etc.) are present.

Based on the topography, the ctenidia present in molluscs can be divided into fol­lowing categories:

1. Holobranchiate type

2. Merobranchiate type

3. Plicate type

4. Monopectinate type

5. Bipectinate type:

(a) Unequal,

(b) Equal.

6. Feathered type.

1. Holobranchiate type:

This type of arrangement of ctenidia is found in Polyplacophora. In this form the ctenidia extend all over the body. The number of ctenidia varies from fourteen to seventy pairs and in some cases the number may be about eighty pairs as in Acanthopleura.

In Patella (Gastropoda) a circlet of gill lamellae ex­tends completely around the margin of the mantle. It resembles that of Chiton superfi­cially which made Cuvier to include Patella and Chitons under one class Cyclobranchia.

But the true nature of ctenidia differs greatly. In Chiton the ctenidia are present along the margin of the body excepting the head and anus, but in Patella the ctenidia are extended throughout the body.

2. Merobranchiate type:

When the ctenidia remain restricted to a particular area of the body, it is called the merobranchiate type. The merobranchiate type of ctenidia can be subdivided into the following types depending on the arrangement of leaflets.

3. Plicate type:

This type of gill com­prises in simple flat transversely folded pro­jecting integumentary laminae. In Neomenia a tuft of filaments arises from the cloacal wall.

4. Monopectinate type:

This type of ctenidia consists of flattened gill filaments arranged in only single side of the ctenidial axis as observed in Pila, Triton.

5. Bipectinate type:

This type of ctenidium has flattened gill filaments ar­ranged in two rows.

They may be of two types:

(a) Unequal:

When both of them are present, but right one is smaller as observed in Fissurella, Haliotis.

(b) Equal:

When both of them are of same sizes. This is the characteristic of the Bivalves. Amongst Bivalves they become variously modified (Fig. 16.66). Nucula possesses short flat leaflets (Fig. 16.66A). In some forms long filamentous leaflets are present.

These filaments may be free as in Area (Fig. 16.66B, C) or may be joined by ciliary connectives as in Mytilus (Fig. 16.66D). In Unio (Fig. 16.66E) the ciliary junctions are replaced by membrane. In Poromya (Fig. 16.66F) the ctenidium becomes degenerated and is repre­sented as a transverse partition.

6. Feathered type:

This type of ctenidium is characteristic of the Cephalopods. Detailed structure is described in the biology of Sepia.

Modifications:

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.

Relationship between heart and gills:

The heart and the gills are intimately related because the main function of the gills is to aerate blood on its way to the heart. The number of gills are directly proportional to the number of the auricles (Fig. 16.67). For example, when the gills are two in number, two auricles are present as encountered in Octopus and Loligo.

In Chiton two auricles correspond to the two sets of the multiple gills, In Nautilus there are four gills and four auricles. When the gill is unpaired, the heart has one auricle as seen in Opisthobranchs, Mesogastropods, Neogastropods, etc.

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.

Circulatory System:

The circulatory system in Molluscs is quite well-developed. Almost all the members of the phylum (excepting Scaphopoda which lacks a distinct heart) have distinct heart which receives oxygenated blood from the respiratory organs and conveys it to the different parts of the body.

Blood:

The blood in Molluscs is colour­less. The respiratory pigment is haemocyanin, which contains both iron and copper. Haemoglobin is also present in some rare cases, e.g., Planorbis. Special blood corpus­cles with haemoglobin are present in Solen legumen, Area noae.

Heart:

The heart is composed of muscu­lar ventricle and thin-walled auricle. The auricle gives origin to aorta which in turn divides into arteries to supply the various parts of the body.

Blood from the different parts of the body is returned to the respira­tory organs. After oxygenation blood is re­turned to the heart from the respiratory organs to complete the circuit. The ventricle is usually single, but the number of auricle varies from one to four.

The number of auricle corresponds to the number of ctenidia present. Majority of the Molluscs possess one auricle. Double auricles are present in Chitons, Dibranchiate Cephalopods and Bivalves. Tetrabranchiate Cephalopods pos­sess four auricles. The number of aortae originating from the ventricle varies from one to two.

Single aorta is present in Chitons, Solenogastres, Prosobranchia, etc. In some forms there are two aortae leading out of the two ends of the ventricle. In most Gastro­pods, a single aorta bifurcates into an ante­rior cephalic aorta and a posterior visceral aorta. The topography of heart also shows variation. The heart is placed within the pericardium.

In bilaterally symmetrical forms the heart lies on the median line of the body, while in asymmetrical forms the heart is shifted to one side of the body, usually on the right side.

Course of circulation:

In Molluscs the blood from the ventricle is carried through aorta/aortae into the arteries. The arteries supply blood to the different parts of the body. After making an excursion through the different organ systems, the blood is col­lected into irregular spaces—the lacunae which in turn open into sinuses.

Existence of capillaries is said to occur in Dibranchiate Cephalopods and in some Bivalves. The re­lationship between the foot-pore and the circulatory system in some Molluscs has not yet been fully established. In the opinion of many workers these foot-pores help the incoming of water into the circulatory system.