66 Our Food Mollusks 



it transfers its mass to the outer, or casts it off into 

 the mantle chamber. The outer gill often touches the 

 mantle, the cilia of which relieve it of its burden and 

 carry it away. 



Thus the gills as well as the palps of Pecten reject 

 material when it is too abundant, but the process in this 

 case is purely automatic. The course taken by foreign 

 matter is determined by its volume, and so certainly that 

 the experimenter is soon able, when allowing carmine 

 particles to settle on the gill, to predict which path they 

 will follow on reaching its surface. 



There are few known mechanical contrivances of 

 animal bodies more wonderful than this self-operating 

 mechanism of the Pecten gill. Cilia, in all animals 

 above the Protozoa, or single celled forms, lash only in 

 one direction. Yet here is a ciliated surface that auto- 

 matically selects from the water what usually is suitable 

 for food, and rejects that which is not suitable, carrying 

 the first toward the mouth, and the other in the opposite 

 direction, the matter being determined wholly by the 

 quantity of the material. 



It is possible here to make only the brief statement 

 that some bivalves develop special organs the ciliated sur- 

 faces of which are constructed to cope with peculiar 

 conditions of the environment. Such, for example, is a 

 filmy membrane that grows out from the posterior sur- 

 face of the body wall in a species of Pholas in the 

 Gulf of Mexico. This organ rolls itself into the form 

 of an inverted trough, and, collecting mud from the sides 

 of the body, carries it under cover, directly against the 

 incurrent stream, out into — and perhaps entirely 

 through — the lower siphon tube to the exterior. This 

 special apparatus is apparently necessary in this creature 



