76 THURLOW C. NELSON 



lie close to the basal membrane. Most striking of all are the 

 great bristle-like cilia on these cells. Of such size are they that 

 one gains the impression that the very ends of the cells them- 

 selves have been drawn out into the fiber. The distal portion 

 of the cells is very rich in protoplasmic material which stains 

 deeply, and forms a distinct region extending from the groove 

 on the dorsal side, entirely around the style sac to the edge of the 

 major typhlosole (fig. 2). A better conception of the cilia on 

 these cells may be gained from figure 10, which shows a section 

 through the style sac at right angles to the transverse folds. 



The only cells of the epithelium of the stomach which need 

 concern us are, first, those on the folds of the walls and, second, 

 the cells which lie beneath the gastric shield. The former, ac- 

 cording to Sabatier ('77), are much more heavily ciliated than 

 the surrounding furrows. The latter, following Gutheil ('11), 

 are the only columnar cells of the entire alimentary tract which 

 are devoid of cilia. Their function is to secrete globules of a 

 clear fluid which hardens to form the gastric shield. 



The ciliary mecJianism 



Nowhere in the animal kingdom is ciliary activity brought 

 to the high degree of development and complexity that it attains 

 in some of the molluscs. Not only has it replaced muscular 

 peristalsis in many of these forms, but, as is shown in the excel- 

 lent works of Allen ('14) and Kellogg ^'15), the lamellibranchs 

 have developed very efficient and delicately balanced modes of 

 feeding, entirely by the use of cilia. 



Allen found that the quality of the food permitted to enter 

 the mouth is controlled by bringing into action one or the other 

 of two opposed sets of cilia. These are situated on opposite 

 faces of the transverse folds of the palps. According as these 

 folds lie in their normal position or are erected by reflex mus- 

 cular action, the food matter, entangled in mucus, is directed 

 into or away from the mouth. By means of this mechanism, 

 streams of food material which contain too large a percentage 

 of foreign particles are directed backwards, and enter the re- 



