Respiratory system of Unio.

Respiratory system of the Unio.

The respiratory system of Unio consists of

(i) Respiratory organ and

(ii) Physiology or Respiration.

[I] Respiratory organs.

The respiratory organ is accomplished by two structures (i) the mantle and(ii) the pair of gills or ctenidia.The gills are in fact highly specialised derivatives of the mantle itself.

1. Gills.

These lie in the mantle cavity, one on either side of the body and are suspended from the visceral mass and the mantle lobe of their sides.

(a) Structure.

The gills are of typical eulamelli branch type and consists of two “half gills” or hemibranchs or laminae, lying side by side.The two lamellae of each gill are similar in structure that the inner one is little broader than the outer one. The lamellae are made of an up number of vertically parallel gill filaments or branchial filaments. The lamina is made up of two similar, elongated, somewhat rectangular plate, called the lamellae. Each gill plate or lamina flaps or lamellae joined to each other except dorsally, thus two lamellae of a gill plate form a narrow but long bag.The gill filaments are pass downwards and are then reflected upwards like a V –shaped rod called gill filaments forms a descending and the ascending limb.Adjacent gill filaments are joined by the fusion of tissue forming interfilament junctions. Thus, gill filaments and the interfilament junctions form the two lamellae of a gill plate. In the interfilament junctions are holes known as ostium which connects ventral inhalant chamber of the mantle with water tubes in the laminae.

The large mantle cavity has made it possible for the great length of ctenidia which lie laterally. Each ctenidium appears double but it is made of two-gill plates , an outer and an inner gill plate which have been derived by the folding of a single ctenidium.The ctenidium divides the mantle cavity on each side into a large ventral inhalant chamber or infra-branchial chamber and a smaller dorsal exhalant chamber or suprabranchial chamber.The gill filaments appear as verticle lines and their interfilament junctions appear as horizontal striations on a lamella.All the gill filaments are alike. This condition is known as homorhabdic, seen in most pelecypods.In some pseudo-lamellibranch, the condition is heterorhabdic, to types of gill occur.Each gill filaments are composed mainly of connective tissue and is supported by two chitinous rods. The entire structure is bounded by an epithelium which is richly ciliated.The cilia are of three types.(i) frontal cilia lining the external, ridge-like face of the filament,(ii) lateral cilia lining the lateral the lateral sides of the filaments which bound an ostium, and the (iii) lateral-frontal cilia which lie on the either side, between the frontal and lateral cilia. Between the two lamellae of the gill plate a space divided by verticle bars of vascular tissue forming interlamellar junction which contain blood vessel.

(i) frontal cilia lining the external, ridge-like face of the filament,

(ii) lateral cilia lining the lateral the lateral sides of the filaments which bound an ostium, and the

(iii) lateral-frontal cilia which lie on the either side, between the frontal and lateral cilia.

Between the two lamellae of the gill plate a space divided by verticle bars of vascular tissue forming interlamellar junction which contain blood vessel.

The interlamellar junctions between two lamellae divide the space into distinct compartments called water tubes which are closed all around except dorsally where they open into a suprabranchial chamber of the mantle cavity. A ventral food groove runs longitudinally at the lower edge of each inner gill plate.There are also two dorsal food grooves on each side at the base of a ctenidium,one between the mantle and the outer lamella of the outer gill plate.The inner lamellae of the outer lamina and the outer lamella of the inner lamina of each gill are attached with each other dorsally, along their entire length. The line of attachment is called ctenidial or gill axis. Thus, each gill presents a typical bipectinate structure consisting of a V-shaped gill filament. Each gill filaments consists of a descending limb arising from the ctenidia axis and ascending limbs. Each gill appears in W-shaped in transverse section.

(b) Blood supply to ctenidia.

The ctenidia are supplied by the afferent branchial vessel carrying deoxygenated blood from the kidneys and divides to gives rise branches into the interlamellar junctions. These branches unite to open into the efferent branchial vessel which carries away oxygenated blood to the heart. During the flow of blood from the branches of an afferent branchial vessel to the efferent branchial vessel, the blood is oxygenated. The oxygenated blood is finally conveyed to the heart by the efferent blood vessel.

(c) Attachment.

The ctenidial axis formed by attachment of gill plates shows that the outer lamella of the outer gill plate is attached to the mantle, the inner lamella of outer gill plate and the outer lamella of inner gill plate are joined to the visceral mass, the inner lamella of inner gill plate is attached to the visceral mass anteriorly, but further back it is free, and behind the foot it is joined to its fellow of the other side, so that the inner lamellae of inner plates are united with one another.Each gill divides the mantle cavity of its side into-(i) a spacious infra-branchial chamber that lies below the attached dorsal margin of the lamellae and communicates with the exterior through the inhalant siphon, (ii) two narrow longitudinal suprabranchial chambers partitioned from each other by the ctenidial axis.

2. Mantle.

The mantle lobe in addition to a shell-secreting function of the shell. It also helps in respiration.It is richly supplied with the blood vessel and it remains contact with water, hence gaseous exchange takes place through its thin wall.Being thin and highly muscular, they are well suited for the purpose.

[II] Physiology of respiration.

When water passes through the water tubes in the gill, the gaseous exchange takes place; in fact, carbon dioxide from the blood is diffused out in the water and dissolve oxygen from water is diffused in the blood. Thus, deoxygenated blood become oxygenated which is carried to the heart for distribution. The outgoing current of water, in addition to carbon dioxide, carries away the nitrogenous excretory products and faecal matter as well.

Reference.

Aggrawal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

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