584 
BULLETIN OP THE BUREAU OP FISHERIES 
Although considerable respiratory function has been attributed to the pallial lobes, 
the gills are generally assumed to be the principal respiratory organs. The basis of 
this assumption is to be found in their typical, finely tubular structure and the fact 
that they are bathed as are no other suitable structure by the inhalant water current. 
In 1928 Pan tin wrote that “A Lamellibranch mollusk feeds with ‘gills,’ so called, 
which have no respiratory function.” Later, in a letter, he qualified this slightly, but 
admitted only a limited amount of respiratory function, as of any exposed surface. 
Referring to Dakin (1909) he stated that the mantle appears to be the chief organ of 
respiration owing to its extremely effective blood supply and probable slow metabo- 
lism, whereas the metabolism of the gill filaments is extraordinarily high, so that it 
is quite probable that all the oxygen absorbed by the gills is required for their own 
activity. He further refers to Dakin’s (1909) conclusion that the heart receives 
completely oxygenated blood from the mantle and “probably [the italics are mine] only 
incompletely oxidized” blood from the gills. Doubtless this makes a good case for 
questioning, but hardly for definitely denying that gills are organs of respiration. 
Considering the arguments advanced, it seems reasonable to continue to consider 
that the principal filaments with provisions for efficient vascular circulation and 
with structures so well suited for respiration as the branchial expansions, perhaps 
aided to an important extent by the interlamellar septa, are important respiratory 
structures. The extent of the branchial expansions alone are sufficient to constitute 
a very considerable gill. Indirectly the gills surely are important for respiration, 
for they produce the all-important oxygen bringing and C0 2 removing water current. 
CILIARY ACTION OF THE GILLS 
If there is some question as to whether or not the gills are important organs of 
respiration, there is none that they are important organs of feeling. The action of 
the lateral cilia, as shown by Wallengren in 1905 (see Yonge, 1926) and Orton (1912), 
create the inhalant-exhalant water current. From this current the gills filter out 
food organisms, often with much material that is undesirable, and pass them toward 
the mouth. For the filtering action the gill filaments of some mollusks are provided 
with long, latero-frontal cilia which interdigitate with those of adjacent filaments and 
beat slowdy with the effective stroke toward the center of the front of the filament. 
As previously noted no such cilia have been found for Pecten. However, incoming 
organisms or other particles are entangled by the mucus secreted by the filaments 
and thus effectively removed from the current. 
Particles caught in the mucus or otherwise brought to the frontal surface of the 
filaments come under the influence of the frontal cilia. Depending on whether they 
impinge on the filaments in the grooves (that is, on the principal filaments and those 
on either side) or on the tops of the folds (that is, on the filaments intermediate between 
the principal filaments) (see figs, lb and Id), they are carried by the longitudinal beat 
of the frontal cilia to the inner or outer edge of the lamella. At these places the 
frontal cilia beat transversely to the filaments in such a way as to convey the particles 
toward the palps. (Fig. Id.) As pointed out by Kellogg (1910, see also Kellogg, 
1915), this unusual condition provides a food-selective mechanism in the gills. If 
suspended particles are abundant, as when the water is heavily laden with silt, 
the material on the gills becomes imbedded in strings of mucus secreted in 
increased amounts, stretched across the filaments. The currents in the grooves tend 
to carry these strings toward the branchial axis; but the currents on the tops of the 
