214 WILLIAM RAY ALLEN. 



cilia of the style sac.) It involves little, if any, additional energy 

 to keep a stream of water coursing through it. That much food 

 material passes through without being digested is shown both by 

 the mass color of the feces and by the appearance of diatom and 

 other algal cells from the feces, when examined microscopically. 

 It has been argued that diatoms are accidental and not the real 

 food. Empty diatom tests are found in considerable quantity in 

 fecal matter, and there is no doubt that the contents have been 

 digested out of, at least, some of them. Furthermore, the freshly 

 formed style, in a mussel which has been starved and then fed 

 with diatomaceous matter, has the amber color of diatomin. In 

 this case digestion is prompt and rapid. The style tends to be- 

 come colorless as soon as the streaming of food through the style 

 sac is checked by the growing style. 



Since the ciliary mechanism is functioning constantly, and 

 since some material is digested and some is not, it may be con- 

 cluded that the demand for food on the part of the tissues affects 

 rather the secretion of digestive fluids and ferments than the con- 

 trol of ingestion. 



On several grounds it is seen, then, that a given particle may or 

 may not be digested on its passage through the alimentary tract. 

 Nelson's suggestion ('18), that the style sac serves as a means of 

 returning undigested particles to the stomach where they may be 

 exposed again to the digestive secretions, is very plausible. The 

 morphology of the structure fits such a function to a nicety. 

 Moreover, while a starved mussel is renewing its style much green 

 material is being threaded into its core. The fact that some 

 diatoms are digested while others are not might at first thought 

 find a sufficient explanation in Nelson's view. For some particles 

 would be diverted into the style sac and others continue on un- 

 digested. This would perhaps as readily explain the presence of 

 both normal diatoms and empty tests in the rectum as my sug- 

 gestion of uninterrupted feeding. There is no doubt that 

 Nelson's explanation is entirely adequate for Ostrcca and 

 Modiolus, for these forms interrupt siphoning and feeding with 

 every tide. Their styles are absorbed and renewed regularly 

 with the ebb and flood. Hence twice each day the stvle sac is 



