102 THURLOW C. NELSON 



ment, when the end of the style is unopposed, is approximately 1 

 cm. per minute. 



We have already seen (p. 77) that the cilia on the stomach 

 walls in proximity to the gastric shield beat so as to rotate in 

 a clockwise direction any matter in contact with them. They 

 thus aid the movement of the style in keeping the food material 

 in quite rapid rotation. 



The tip of the style, enclosed in a mass of alimentary matter 

 is constantly dissolving away as it is pushed up against the 

 gastric shield. This dissolution is, however, confined to the 

 cortical layers, the central core of bubbly mucus remains undis- 

 solved and is wound up with the rest of the mass around the head 

 of the style. 



The causes for the dissolution of the style consist, first, in the 

 action of the juices of the stomach; second, the wearing against 

 the point of the shield, and, third, the tendency of the style to 

 dissolve spontaneously within a certain time after formation. 



Figure 12 shows the oesophagus, stomach and style-bearing 

 organs of Modiolus. In two favorable specimens I was able to 

 cut through the stomach wall and observe the movement of the 

 style with the surrounding alimentary mass, while strings of food 

 and mucus from the gills and palps were passing up the oesopha- 

 gus. It was therefore possible to observe the action of the 

 various parts while they were functioning in an approximately 

 normal manner. 



The food particles, with large numbers of sand grains, en- 

 tangled in mucus, pass up the oesophagus in a single string and, 

 bridging the groove at the entrance to the stomach, are carried 

 to the base of the gastric shield. Here they are caught in the 

 revolving alimentary mass and wound around it. The mass, in 

 turning, comes in contact with the spiral cavity described on 

 page 78 and shown in figure 12. As this mixture of mucus, 

 food particles, and foreign matter sweeps across the highly cili- 

 ated grooves of this structure, sand grains, together with much 

 of the newly arrived matter from the oesophagus, are caught 

 out of the mass and carried down the tract and into the right 

 side of the caecum (p. 78 and fig. 15). 



