54 Our Food Mollusks 



served, though there are several variations in the method 

 of directing food to the mouth. The fact that palps in 

 all forms also possess ciliated tracts leading away from 

 the mouth, and that the sides of the visceral mass and 

 the walls of the mantle possess complicated ciliated sur- 

 faces, has been very generally overlooked. 



In some bivalves certain gill faces or lamellae carry 

 the captured organisms to the base, instead of to the 

 margin of the gill, where they also are borne along 

 ciliated paths leading to the palp surfaces ; and in one 

 case among our food mollusks, that of the scallops, the 

 transportation on the gill faces may at one time be to the 

 bases, at another to the edges of the gill, and without any 

 reversal of the ciliary action. The figure of the oyster 

 also shows a tract at the gill base that leads to the palps. 



One who carefully observes oyster beds or clam flats 

 at different seasons, notices that there are often great 

 changes in external conditions. Even where tide cur- 

 rents are strong, thus mixing waters, there is consider- 

 able variation in temperature during the summer, and 

 the average difference between summer and winter tem- 

 peratures on the north Atlantic coast of course is great. 

 Continued cold checks the growth of diatoms, and so les- 

 sens the amount of bivalve food. Though it has not 

 been carefully studied in most forms, it has recently been 

 shown that scallops and little neck clams do not grow at 

 all during the winter in northern waters. Whether this 

 is true of warm southern waters is not known, but prob- 

 ably it is not. It is a safe prediction that the growth of 

 all bivalves living on the shore of the Gulf of Mexico 

 will be found to be uninterrupted. There are also great 

 changes in the salinity of water, especially near the 

 mouths of rivers entering the sea, and oysters par- 



