STUDIES ON BIOLOGY OF FRESHWATER MUSSELS. 235 



of the palps. When the palps were in a state of contraction, most 

 of the streaming carmine and carborundum were drawn to the 

 edge and into the mantle chamber. These mieager results tend to 

 corroborate previous observations on the reaction of the palps- 

 that food may be rejected through a greater or less erection of the 

 ridges. Negative results prove nothing, while ever so slight posi- 

 tive evidence may be taken as an indication that a reversal of the 

 ciliary streams can, and actually does, take place, through the 

 bringing of another set of cilia into play. 



The above shows clearly that the palps bear two sets of cilia 

 working at right angles to each other. Due to their interaction 

 materials often travel obliquely downward or obliquely upward. 

 The former materials are eliminated at the lower edge of the 

 palps, the latter reach the mouth. 



The effectiveness of the assorting mechanism is well brought 

 out by the measurements of the ingested particles. The largest 

 fragment I have ever secured from the enteron was found in the 

 intestine a pinnately branched alga 3.3 mm. in length, probably 

 Mv.roncina. The second largest particle was a bit of Oscillatoria, 

 1.5 mm. in length. It is unusual in Winona Lake mussels to en- 

 counter fragments of greater length than 500 microns. Starved 

 individuals, probably experiencing a sensation akin to hunger, are 

 observed to ingest freely much more large material than under 

 normal circumstances. 



Nelson ('18) has described the action of the food sorting 

 caecum of Modiolns, a diverticuluin of the stomach. Since 

 Modiolus ingests large quantities of sand in its periodic feeding, 

 Nelson's ccccitin affords a means of separating food from sand. 

 It has been shown elsewhere in this paper that feeding in the 

 Unionidse is a more constant function, and that little sand and 

 mud are taken into the stomach. The gills and labial palps are an 

 entirely sufficient assorting mechanism. 



(b) The Gills as an Assorting Mechanism. 



Little attention has been given to the gills as having a possible 

 food-sorting function. I find that they play no small part. Clots 

 of mucus taken from various parts of the gills and palps have 



