NATURAL HISTORY OF THE BAY SCALLOP 
585 
folds prove more effective and carry the material, including that in the grooves, to the 
outer edge of the demibranchs. It is probable that ordinarily the masses so carried, 
being heavy and no “food groove” being present in our scallop (although a shallow 
one is figured for P. maximus by Orton, 1912), drop off the gill. Under some circum- 
stances, however, as I have observed experimentally, the material carried there in 
long strings to the outer edge is not so heavy as to drop and is carried toward the 
palps. When not too abundant, apparently most material falls into the grooves and, 
unless it is sharp or irritating so that it greatly stimulates mucus secretion or causes 
the gills to “writhe,” is carried either to the base or tips of the filaments, where a 
strong ciliary current conveys it to the palps. 
A few observations on the rate of travel of particles along the frontal cilia paths 
were made in these investigations. Most of the observations were of the speed of 
particles or of small mucus strings conveyed transversely to the filaments, close to 
their tips and toward the palps. (Fig. Id, at top.) Rate of travel often could be 
seen to be very irregular. Sometimes a particle would hit a hump in the gill and 
stop or almost stop. At other times there would be obvious but less drastic slowing. 
However, there were many times when progression was more regular. Most of the 
speeds recorded fell close to 1 millimeter per second at about 21.5° C. (21.2° C.- 
21.8° C.). A few times notably higher speeds were noted (up to 2.3 millimeters per 
second) with small particles which went the distance without interference and evi- 
dently were in the most favorable current. Determination of rate of travel along the 
filaments was much more difficult because of conflicting currents and the “writhing” 
of the gills. Therefore, only a few readings were obtained. Speeds recorded were 
close to 0.4 millimeter per second. 
It may be remarked that the ciliary motion of lamellibranchs is not reversible, 
nor is there evidence of nervous control of the activity of cilia (Gray, 1928). This 
activity is, however, affected by temperature, hydrogen-ions, and other water condi- 
tions. (See the various papers by Gray, also Galtsoff, 1928 and 1928a.) 
BRANCHIAL MOVEMENT 
Although muscle fibers have been found in the branchial axis, the movements of 
the gill and of its parts indicate much more muscular tissue than would be supposed 
from morphological studies. Kellogg (1910) noted the writhing and swaying of the 
gills when much material was deposited upon them. Touching a filament with a 
needle causes contortions for a considerable distance along a lamella. Examined in 
more detail, motion is found to consist to a large extent of elongation, contraction, 
and pivotal movement of the connective spurs (see fig. 8, j and g), and of the 
extreme transverse movement of the principal filaments. Obviously one effect of the 
elongation and contraction of the spurs is to vary the interlamellar space, and this 
may be important for filtering (as by permitting large particles to pass through or by 
removing more effectively abundant fine silt). The transverse movement of the 
principal filament is remarkable. As shown in Figure 8, b and d, the frontal surface 
is highly concave with widely extending lateral edges which bear the large connective 
spurs. In this movement these edges may be brought close together or turned 
abfrontally until they and the spurs lie against the sides of the axis of the filament. 
Excised portions of principal and ordinary filaments have been seen to bend longi- 
tudinally to a marked degree and to respond to stimuli. Presumably the principal 
if not the sole purpose of branchial movement is to rid the gills of irritating sub- 
stances or objects. 
