NUTRITION AND FEEDING MECHANISMS 



217 



are conveyed anteriorly in two streams towards the mouth. In other 

 lamellibranchs the frontal cilia usually beat towards the lower free margin 

 of the lamellae where there is a food groove in which particles move 

 anteriorly towards the mouth (Figs. 5.13, 5.14). In addition, the gill 

 filaments of many lamellibranchs possess large latero-frontal cilia, which 

 strain the feeding currents and throw particles on to the frontal face of the 

 filaments where they come under the influence of the frontal cilia. 



A certain amount of mechanical selection of food particles takes place 

 on the gills. In Pecten the gill lamellae are thrown into folds, and the frontal 



Gill <3.x is 



Fig. 5.14. 



Cuter 

 derm branch 



Transverse Section through Gill Filaments 

 of Pteria hirimdo 



Solid circles, orally directed currents. The arrows indicate directions of currents 

 created by frontal cilia : ventral currents are due to coarse cilia ; dorsal currents to fine 

 cilia. (From Atkins, 1936.) 



cilia in the grooves between the folds beat towards the base of the lamellae, 

 where there are forwardly-directed ciliary tracts. On the ridges of the folds 

 the frontal cilia beat normally towards the free extremities of the filaments, 

 again provided with ciliated tracts. But when particulate matter becomes 

 very heavy it evokes a profuse secretion of mucus, and the mucous strings 

 which result are conveyed by frontal cilia on the ridges to the ventral ends 

 of the lamellae, to be dropped off on the mantle wall. Muscular movements 

 of the gills are also used in getting rid of large or irritant particles. Such 

 movements cause the gill grooves in Ostrea and Pecten to contract, and 

 throw their contained particles upon the ridges, whence they drop upon 

 the mantle surface. Heavy particles also tend to fall out of the ventral food 

 grooves upon the surface of the mantle, whence they are removed. 



