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BULLETIN OF THE BUREAU OF FISHERIES. 
* * * the inner surfaces of the labial palps, except their outer margins, are made up of 
minute vertical ridges or furrows. These constitute a quite complex mechanism for the sorting 
of material. * * *. 
Upon the ridges, as elsewhere, occurs a ciliated epithelium; but the ciliary currents are dis- 
posed in a unique manner. Upon the anterior slope of each ridge they are directed backward, 
while those on the posterior slope lead forward. This seeming conflict is not such in fact, because 
only one set of cilia comes into action at a time. The position of the ridges determines which set 
shall function at a given moment. Their normal position seems to be * * * a somewhat 
reclining one, overlapping one another toward the anterior. Thus, the after slopes are ordinarily 
brought uppermost, the ciliary currents leading to the mouth are upon the surface, while the cilia 
which lead from the mouth lie somewhat underneath the ridges. So long as no adverse stimuli 
are received, particles which lie between the palps are thought to be passed on forward from one 
ridge to another to the lips and mouth. 
In the event that distasteful matter reaches the palps a reflex erection of the ridges brings 
uppermost the cilia leading backward, and such material is returned from summit to summit to 
the edges of the palps and discharged into the mantle chamber. 
It is extremely difficult to observe the cilia which lie at the bottoms of the furrows. Wallen- 
gren (1905) ascribes to them the duty of carrying lengthwise of the furrow to the lower margin of 
the palps the minute particles that may fall between the ridges, but Siebert (1913) thinks they 
lead in the opposite direction. 
The present authors judge from Allen’s remarks, just quoted, and from his 
subsequent statements, that Allen himself did not observe this change of position 
in the ridges and the passage of material posteriorly across the palp from crest to 
crest of the ridges. It appears that he is summarizing Wallengren’s statements. 
We are not told that Wallengren states that he actually saw this behavior of the 
palps, and the fact that Allen says “* * * Wallengren has pointed out how 
selection and rejection may be made,” and “So long as no adverse stimuli are re- 
ceived particles which lie between the palps are thought to be passed on forward,” 
etc., conveys the impression that Wallengren had set forth, a theory rather than 
observations. Unfortunately the present authors did not have access to Wallen- 
gren’s paper while writing this report. Allen’s own observations are given on page 
133 of his 1914 paper: 
No one, to my knowledge, has succeeded in inducing a mussel to behave normally after the 
shock of removing parts of the shell and mantle in order to observe the palps at work; but I 
have repeatedly obtained the reactions which occur. When the palps lie in contact with either 
body, mantle, or gill their collections of material pass between the palps and mouthward. Oth- 
erwise such material is carried down by the several structures and discarded. 
As will appear later, these observations agree with those of the present authors. 
The word “down” is the significant one here; the material leaving the palp passes 
down off it in the vertical furrows and was never seen to be carried backward across 
its face from ridge to ridge. 
In general, Allen ascribes to the fresh-water mussel a high degree of ability to 
select the material ingested. This selection may be exercised at the incurrent siphon, 
the gills, the palps, and the mouth. The palps are, however, most important in 
this respect. Allen lists in the stomach contents of certain mussels 14 genera of 
diatoms, 16 of other algae, 9 of desmids, besides debris (organic and inorganic), 
mold, ova, spermatozoa, and spores. He especially stresses the point that the 
amount of inorganic debris was quite small, consisting of rather insignificant quan- 
tities that had accidentally passed the assorting mechanism. This fact is cited as 
