1 6 MOLLUSC A. 



of cilia. The cilia move tip one side and down upon 

 the other of each of the tubes or minute folds on the 

 surface of the larger folds (Fig. 6'), so that all particles 

 are thrown towards the channels between the larger 

 folds, and retained there while being moved towards 

 the outer edge. The operation of feeding may be 

 watched by quite a number of pupils, and readily seen 

 by them. 



In nature the outer edges of the flaps are probably 

 joined so as to form a closed canal on either side lead- 

 ing from the gills to the mouth. 



In these disturbed specimens, however, the edges 

 are not often so closely fitted, and a considerable part, 

 if not the whole, of the food, brought forward along the 

 gills is carried too far out, or escapes from the incom- 

 plete bottom of the covered way, and being caught by 

 the cilia on the outer free edges of the flaps, called 

 palpi, is returned back from whence it came. 



A portion, however, may sometimes be followed on 

 its course ; and by opening the flaps and dropping a 

 little sea-water, dyed with indigo, the particles may be 

 seen as they are carried off by the cilia, all moving in 

 the direction of the mouth. 



The mouth itself is situated in the angle of the flaps 

 or palpi, and deeply buried between them (Figs. 2, 

 12, 15, om), so that it is difficult to get at, and cannot 

 be found unless great care is taken not to tear the 

 parts. Constant failures must be expected, even in 

 such simple operations, until the observer has learned 

 that nature yields nothing but a crop of mistakes to 

 hasty and thoughtless exploration. A single ill-planned 

 cut win generally destroy or mutilate just the part which 

 is being sought for. 



