NUTRITION AND FEEDING MECHANISMS 



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proceed posteriorly as an exhalant stream in the dorsal or exhalant 

 chamber. As the current slackens on entering the large inhalant chamber, 

 coarse particles fall out of the stream on to the mantle surface. Fine 

 particles become entangled in a mucus-sheet covering the gill surface. The 

 filtered particles are then carried to food grooves at the bases and free 

 margins of the gills and thence towards the mouth. 



In the majority of lamellibranchs (filibranchs, eulamellibranchs) the 

 gills are responsible for collecting food particles. Some species, e.g. 

 Ostrea and Pecten, are without siphons for regulating the passage of 

 water currents. In Ostrea the inhalant current is restricted to a ventral 

 area; in Pecten water is drawn into the mantle cavity along the whole 

 ventral and part of the anterior region, but chiefly in two restricted areas 

 (ventral and anterior). Water leaves the body posteriorly in an exhalant 



(a) -^ ^ (b) 



Fig. 5.11. Diagrams Showing the Organization of the Mantle Cavity in 

 (a) A Protobranch Nucula, and (b) a Typical Lamellibranch such as Ostrea 

 Transverse view across the body, with exhalant chamber above, and inhalant chamber 

 below the gills. Direction of water currents indicated by arrows passing through the 

 gills; direction of food streams by arrows on surface of gills; forwardly directed streams 

 to mouth indicated by x. (From Yonge, 1928.) 



region (Fig. 5.12 (b)). In many other lamellibranchs, in- and excurrents 

 enter and leave the mantle cavity by way of siphons at the posterior end of 

 the body, elsewhere the mantle folds being largely fused. 



Water currents are created by the lashing of lateral cilia on gill filaments 

 or leaflets, as illustrated in Fig. 5.13. In protobranchs the gills consist of a 

 series of flat leaflets on either side of the body. In other lamellibranchs the 

 gills on either side usually consist of two lamellae or demibranchs, each of 

 which is made up of a series of ascending and descending filaments united 

 at their free extremities. The lateral cilia cause a current to flow through the 

 narrow slits between the gill filaments or leaflets, from the inhalant into 

 the exhalant chamber. On the outer surface of the filaments are frontal 

 cilia which are chiefly responsible for collecting food particles. In proto- 

 branchs the frontal cilia transport particles to the mid-line, whence they 



