218 



THE BIOLOGY OF MARINE ANIMALS 



In protobranch bivalves the gills are relatively small and function chiefly 

 in the creation of a water current, food being collected principally by the 

 large labial palps (Fig. 5.12 (a)). These are provided with a long appendage 

 or proboscis which is grooved and ciliated; as this moves over the substra- 

 tum it collects fine particles which are carried along the ciliated groove to 

 the base. Here they are subjected to sorting by the palp lamellae, suitable 

 material being carried by cilia to the mouth. 



In other lamellibranchs the labial palps sort out food material passed 

 to them from the gills. There is a pair of palps in front of the gills on either 

 side of the mouth. The inner surface of the palps is ridged and ciliated, and 

 conveys food particles from the ciliated tracts on the demibranchs to the 

 mouth (Fig. 5.15). The ciliation of the palps is complicated, the whole 



Upper margin 

 of palps 



Mouth 



Proximal 

 oral groove 



Lateral 

 oral groove 



Base of 

 gill 



face of^ 

 palp 



Fig. 5.15. View of Labial Palps and Junction with the Gills 

 in the Oyster Ostrea edulis ( x 5\) 



Arrows indicate directions taken by particles ; X, point where material is rejected from 

 the palps. (From Yonge, 1926.) 



forming an extremely efficient sorting mechanism. A tract of especially 

 large cilia on each labial fold drives small particles across the palps towards 

 the mouth. Large particles or mucous masses tend to be drawn down into 

 furrows on the palps by other ciliary tracts. The cilia in the furrows beat 

 towards the upper or anterior margins of the palps, where there are pos- 

 teriorly directed ciliated tracts carrying material to the mantle to be 

 rejected. Muscular action is also of importance in the degree of sorting 

 which takes place, retraction of the palps, for example, opening up the 

 furrows and causing particles to fall into the outwardly directed ciliary 

 tracts. 



Habits and modes of feeding of lamellibranchs are many and varied. 

 Most are dependent upon fine detritus, bacteria and plankton. Mytilus and 

 Ostrea filter particles down to a lower limit of about 2//. The optimal 

 particle size for most efficient filtering is 7-8^ in Mytilus, and particles 



