FOOD AND FEEDING IN FRESH-WATER MUSSELS. 
447 
tral margins of the mantles and expelled between the valves just beneath the 
inhalent siphon. It might appear at first thought that particles suitable for food 
were carried to the mouth and those unsuitable thrown off the palps and out of the 
mantle chamber. The matter is, however, not as simple as it would appear to be 
at first sight, aifd, as noted above, investigators disagree concerning both the prob- 
ability of selection or choice of substances ingested and the cause for and the mech- 
anism by which certain material is thrown off the palps. It will be noticed espe- 
cially that Allen (1914, 1921) and Kellogg (1915) hold diametrically opposed views 
regarding these points. Allen holds that the mussel exercises considerable choice 
in the ingestion of material, reversing the streams across the palps by altering the 
slope of the transverse ridges (Wallergren, 1905). Kellogg maintains that for 
lamellibranchs, in general, including fresh-water mussels, “there is no selection or 
separation of food organisms from other water-borne particles.” 
Kellogg further declares that if too much material is present it is all removed 
from the palp and nothing enters the mouth, and that the bivalve may actually 
starve under such conditions. Grave (1916) takes positive exception to this 
assertion of Kellogg, and states that the lamellibranch, especially the oyster, can 
select food material from the silt or debris and consequently is able to feed 
even though the water be heavily loaded. 
On account of these divergent views it seemed necessary to investigate again 
the mechanism of and factors involved in the ingestion of food in fresh-water 
mussels before proceeding further. Especially did it seem of advantage to under- 
take this study, in part at least, upon the very early juvenile stages, since the trans- 
parency of the valves allowed direct examination to be made of the entire feeding 
process under the binocular microscope. 
OBSERVATIONS AND EXPERIMENTS. 
MUSSELS 0.2 MM. LONG. 
When the mussel, after metamorphosis, leaves the gill of the fish it measures 
from 0.2 to 0.25 mm. in length. Its main anatomical features are represented in 
Figure 1. The gills consist of three papilla-like protuberances on each side, attached 
at their dorsal ends to the upper part of the visceral mass. These will later elon- 
gate and join at the free ends to form the lamellae of the inner gill, the outer gill 
developing much later. The outer palp appears as a ridge projecting downward 
from the side of the visceral mass. The inner palp has not yet begun to develop, 
and the siphons are not in evidence. 
Six specimens of Lampsilis anondontoides, which had been off the fish less than 
24 hours, were secured from the rearing troughs. They measured about 0.2 mm. 
in length. Almost no postglochidial shell had been formed. They were not very 
active. These mussels were placed in a watch glass with a little material from 
the trough and observed under the binocular microscope, light from a 75-watt 
electric bulb being reflected up through the culture. Since this treatment did not 
affect the feeding reactions of the mussels in the least, it was made the usual 
procedure in the following experiments. 
