FOOD AND FEEDING IN FRESH-WATER MUSSELS. 
443 
one proof of his contention that the mussels exercise choice of the material ingested. 
Another proof, he thinks, is the observation that the mussels with which he worked 
did not ingest carmine grains. Furthermore, he never found sand in the alimentary 
tract of the mussels, and states that no one has reported finding it there. He 
thinks that quite probably “much of the stuff which Evermann and Clark call 
‘mud’ is organic” (Allen, 1921, p. 237). It will be noted below that Coker, Shira, 
Clark, and Howard (1921) list “silt” in the stomach-contents of certain mussels 
examined. 
The senior author (1915 and 1916) endeavored to test histologically the con- 
tention of Putter (1907 and 1908) that material in solution formed a large part of 
the food of many aquatic invertebrates and that this was to some extent absorbed 
directly by the cells of the outer body wall. It was found that fresh-water mussels 
could make use of nutriment in solution in the water, and that in the case of fat at 
least, some of this could be absorbed directly by the outer epithelial cells of the body, 
especially those of the gills. 
In natural conditions probably the concentration of dissolved material is not 
sufficiently great to admit of its forming a very important part of the nutriment 
of lamellibranchs. Martin’s (1923) statement, however, on page 152 of his paper, 
that “It must be regarded not only as unproved, but as improbable, that dissolved 
organic substances play an appreciable part in the nutrition of the oyster” is too 
sweeping, to say the least. Since the only experiments with lamellibranchs (those 
of Churchill and Mitchell) that put the possibility of using such material to the 
test gave affirmative evidence, the matter can not be dismissed in favor of the 
negative with no experimental evidence whatever. While probably such material, 
as just said, is not a very important item in the dietary of lamellibranchs, it can 
not he said that it does not “play an appreciable part.” 
Kellogg (1915) worked out with great care the details of the arrangement and 
movements of the cilia of the mantle cavity, gills, and palps in some 30 species of 
lamellibranchs, including 2 fresh-water mussels — TJnio ( Symphynota ) complanata 
and Anodonta sp. In regard to the fresh-water mussels he states that material is 
caught on the gills, carried to the palps (substantially as described by Allen), and 
is moved across these by forward-beating cilia on the transverse ridges. He states 
that there is no reversal in the direction of the beating of the cilia, and that “there 
is no selection or separation of food organisms from other water-borne particles. ” 
He found that the cilia in the bottoms of the grooves on the palps beat downward 
and that on occasions the palps “elongate,” thus causing the grooves between the 
ridges to widen and expose more freely the cilia therein. Material is removed from 
the palps by these downward-beating cilia. Kellogg concluded that volume alone 
determined whether or not the “collected foreign matter that reaches the palps 
shall proceed to the mouth or be removed from the palps” and that “a lamelli- 
branch is able to feed only when waters are comparatively clear.” In turbid 
waters all material passes off the palps and the animal might actually be star ving 
in the midst of plenty. (See Grave and Nelson below). In fresh-water mussels 
the material thrown from the palps into the lower part of the mantle chamber is 
moved backward by cilia on the mantles and passed out between the valves at a 
