580 
BULLETIN OF THE BUREAU OF FISHERIES 
but powerful exhalant currents occur during swimming and for the ejection of mate- 
rial rejected by the palps. 
An account of the action of the mantle during swimming will be found in the 
description of that process. 
GILLS 
In the Lamellibranchiata, gills are of unusual importance. Not only do they 
produce the water currents which bring food and oxygen and carry away carbon 
dioxide and other wastes, but they also separate food organisms from the inhalent 
current and convey them toward the mouth. It is even claimed that they absorb 
food directly from the water. Presumably, in spite of recent claims to the contrary, 
they are important organs of respiration. In addition to being of manifold func- 
tional importance, the gills provide morphological evidence of special value for classifi- 
cation, as previously noted. 
STRUCTURE OF THE GILLS 
The minute structure of the gills, especially the filaments, has been described or 
figured by Kellogg (1892), Drew (1906) and Dakin (1909). The present account 
will not enter elaborately into the histology and will depend upon illustrations for 
structural details and arrangements. Studies have been largely with living or fresh 
material. 
In the section dealing with classification it was noted that the filaments of the 
gills are reflected and are held one to another by spurs (not vascular connections). 
As now interpreted there are two gills (or ctenidia of branchiae) — one on the right 
and one on the left side of the body. Each gill (see fig. 7a) consists of two demi- 
branchs and a branchial or ctenidial axis from which they are outgrowths. In turn 
each demibranch consists of two lamellae (direct and reflected) — one composed of the 
direct limbs of the filaments, the other of the reflected limbs. The demibranchs are 
supported only by the axis, the inner lamella of each gill being free from the visceral 
mass, the outer from the mantle. The lamellae are not flat but accordion pleated or 
folded (plicate gills). This folding, which is steep next to the axis and relatively 
shallow and broad at the outer edge, is due to the arrangement of the branchial fila- 
ments, which are grouped in closely compressed folds at the axis and are united at 
thin plates at their tips. 
Two distinct types of filaments occur (heterorhabdic gills). At the bottom of 
each groove is a principal filament. Making up the convex folds between are 16 
ordinary filaments. (See fig. 7b.) As shown in Figure 3 the gills are roughly crescent 
shaped, being greatly curved and with the filaments shortened toward the ends of 
the axes. From the branchial axes and from the free edges of the reflected lamellae, 
the principal filaments spread fanwise and draw the ordinary filaments (which 
typically are held an approximately uniform distance apart by the spurs) nearer and 
nearer to their level and thus broaden and flatten the lamellar folds. 
The ordinary filaments are slender and relatively simple. (See figs. 7 and 8.) 
Around a thin-walled, flattened, chitinous tube is an epithelium which is thickest at 
and near the frontal face, whence arise, along its full length, the very numerous frontal 
cilia. Near the front on either side are the long and powerful lateral cilia. (See fig. 
7c.) The elongate latero-frontal cilia described for Mytilus, Ostrea, and various 
lamellibranchs have not been found in the scallop, nor have I succeeded in demonstrat- 
