60 Our Food Mollusks 



lected material may, if it is desirable, be transported di- 

 rectly to the epibranchial chamber and out of the body; 

 but in all of our food mollusks, the gills can only collect 

 and transport to the palps any material, whether food 

 organisms or mud particles, that comes to them in mod- 

 erate quantities. If the mass of material is very great, 

 it may sometimes fall from the gill margins to the mantle 

 walls, instead of going to the palps. 



But in all cases in which the labial palps receive ma- 

 terial collected by the gills, they determine whether it 

 shall go into the digestive tract or be sent out of the 

 body. Their inner surfaces are seen to bear fine parallel 

 lines, the direction of which is indicated in Figures 3 and 

 14. These ridges are ciliated and the hairs lash in such 

 a direction that food crosses over them on its way to 

 the mouth. The palps being muscular and capable of 

 extensive movements, receive material from the gill edge 

 simply by placing their inner surfaces against the gill, 

 and the mass of mucus with its particles is easily lifted 

 off and carried onward. 



But on the lower edge of each palp, as shown in the 

 figures, is an unstriated margin very strongly ciliated, 

 that sweeps directly away from the mouth. Now when 

 large quantities of material are delivered by the gills, the 

 palps at once respond by moving these margins into such 

 a position that they, instead of the ridges, remove the 

 gill collection. Then, swinging down, with their loads 

 of accumulated waste, the palps cast the material off 

 from their free tips into the mantle chamber. Here the 

 undesirable collection is picked up by the mantle cilia, 

 and disposed of in the manner already described. This 

 ciliated margin of the palp, then, is the special organ for 

 switching the mud accumulation on to the outgoing track. 



