Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.10. Respiratory System
Respiration is aquatic and carried on simultaneously w the feeding process. The respiratory organs are the gills the mantle.
Gills or ctenidia
The freshwater muscle respires the oxygen dissolved in water by a pair of gills or cienidia or branchiae. On each side of the foot is a single gill, hanging the mantle cavity between the mantle and the visceral ma like a flattened, plate-like structure. The great length of i gills has become possible due to the large siz of the man cavity into which they extend antero-posteriorly.
Structure of a ctenidium
Each ctenidium is compos of two more or less rectangular plates or laminae, one innerand other outer. Each gill lamina is a hollow double-fold, formed of two thin parallel plates or lamellae, an inner and an outer one, united together at their anterior, ventral and posterior edges, but free dorsally. Each gill lamina thus forms an elongated narrow bag, opening dorsally into a supre branchial chamber. The lamellae are joined together by vertical cross partitions or inter-lamellar junctions, so that the thin space between the two lamellae is divided, at regular intervals, into a series of vertical narrow compartments or water tubes. The water tubes or each gill lamina are closed ventrally, but join a common supra-branchial chamber, dorsally. Each gill lamella consists of a large number of close-set, thin, vertical gill bars or gill filaments, which impart vertical striations to the outer surface of the lamella. The adjacent gill filaments are connected by small bridge-like, horizontal bars, the inter- filamentar functions, which impart horizontal striations to the laminae. The filaments of both the lamellae are continuous at the free ventral edge so that each lamina appears V-shaped in a transverse section, and the ctenidiurn of each side resembles a W. The gill, lamellae have a porous or sieve-like structure, being perforated by minute but frequent openings, the inhalent ostia, bounded by filaments and their junctions and leading into the water tubes. Thus, the structure, of each gill-plate is very complex like a piece of basket-work.

The ctenidia of the mussel are of the eulamellibranch type. Each ctenidium is bipectinate, with a central ctenidial axis, from which filaments arise in two rows, one on either side. The filaments of each row are folded in the middle to appear V-shaped in section. Of the two arms of V, one is descending, the other ascending. At the angle of the fold each filament is notched. The notches of all filaments form a continuous food groove, that extends the whole length of the underside of each lamina of the ctenidium.
The gill filaments are composed of connective tissue. They are strengthend by chitinous rods and covered by a ciliated epithelium. The cilia are of three types, those present on the outer ridge-like faces of the filaments are called frontal cilia, those on lateral parts are lateral cilia while those lying in between are the latero-frontal cilia.
Attachment of ctenidia
The mode of attachment of Sills to the body determines the course of water current in the body. The gill axis or ctenidial axis remains fused to the dorsal wall of the mantle cavity throughout, but becomes free near the posterior end of the body. The outer lamella of the outer lamina is attached to the mantle throughout. The inner lamella of the inner lamina is attached to the viscero-pedal mass anteriorly, becomes free in the middle region, and fuses with that of the other gill posteriorly. In this way, a supra¬branchial chamber is formed above each lamina in the anteriol and middle regions, while in the posteriar region, the dorsal edges of all the four laminae form a sort of continuous hon. zontal partition separating the infra-branchial chamber of thc mantle cavity below from a supra-branchial or cloacal cavity above. The inhalent siphon leads into the infra-branchia chamber of the mantle cavity, while the cloacal cavity leads t the outside through the exahient siphon.

Blood supply of gills
The gills receive venous blood in the kidneys, through the afferent branchial vessels. From s the oxygenated blood is returned to the auricles through efferent banchial vessels.
Corse of water current
The constant bearing of lateral a covering the gill filaments draws a continuous current of water through the inhalent siphon into the mantle cavity. The present on the inner mantle surface and labial palps also p in production of the water current. The current can easily be demonstrated by placing a few grains of powdered mine in the neighbourhood of the siphons. From the infra¬uchial chamber, the water enters the gills through the ostia. Within the gills, it flows up the water tubes to enter the supra-nchial chambers, which run one above each gill lamina and and posteriorly into the cloacal chamber. From here, the rent passes, through the exhalent siphon. It has been mated that water passes, through the gills of a mussel of rage size, at an average rate of 2 litres an hour.
Physiology of respiration
Respiration takes place through the walls of the gills. The blood, circulating in the foot water current outer gill-lamina mantle cavity branchial vessels in the interlamellar junctions of the gills, transfers CO₂ to the surrounding water and receives 0₂ from it. The water entering the mantle cavity carries with it not only oxygen but also the minute food particles, which are moved on by cilia towards the mouth. Similarly, the outgoing water carries with it not only the faecal matter but also the excretory and reproductive products. Probably the main function of the gills is to produce the food current rather than respiration which is shared more by the mantle.

Mantle. The surface of mantle is always bathed in water, and it is devoid of a cuticular covering which might hinder diffusion of gases. The mantle is also richly supplied with blood which is sent directly to the heart. It is, therefore, probable that it serves as an important supplementary respiratory organ.