NATURAL HISTORY OP THE BAY SCALLOP 
583 
spurs bear active cilia. The ordinary filaments originate nearer the center of the 
face of the branchial axis than do the principal filaments, and connect only with the 
efferent branchial vessel. 
The principal filaments are much greater in diameter than the ordinary filaments 
and stiffened more with chitin. (See fig. 8.) In addition they are much more com- 
plex. The connective spurs are much enlarged and frontal, rather than abfrontal, in 
position. The basal or branchial expansion and interlamellar septum occur on every 
principal filament, but on these only. The principal filaments, so numerous they lie 
almost one against another along the branchial axes, provide nearly all the transverse 
support for the gills. The interlamellar septa are important in maintaining the re- 
flected lamellae in position. Vascular connection is not only with the efferent but also, 
along the interlamellar edge and the branchial expansion, with the efferent branchial 
vessel. The presence of abfrontal cilia is indicated in my experiments by the move- 
ment of fine carborundum along the abfrontal edge of the branchial expansions toward 
the branchial axis. 
Filamentary nerves of ordinary and of principal filaments are described by Dakin 
(1909), who figures a small nerve at the interlamellar edge of an ordinary filament. 
Kellogg (1892) figured no filamentary nerves, Drew only those of the principal fila- 
ments. The behavior of the filaments indicates nervous structures in both types. 
The branchial axis is decidedly tough and firm and consists largely of connective 
tissue, but with a good supply of muscle fibers (Dakin, 1909). It contains the 
branchial nerve and afferent and efferent vessels. The epithelium is ciliated. The 
suspension or attachment of the axis evidently is adapted to the animal’s existence with 
the right side down. (See fig. 7a.) Anteriorly each axis is attached for a very short 
distance to the lateral edge of the body mass (surrounding the stomach) and to the 
adjoining mantle. For the left axis this is over the pericardium. The next attach- 
ment is to the sheath of the adductor muscle rather close to the pallial lobe. This 
attachment of the left lobe continues to the posterior limit of attachment, somewhat 
overlapping the rectum. For the right axis conditions are more complicated. The 
simple direct attachment to the adductor sheath continues only a short distance, 
about half the length of the kidney. Ventrally and posteriorly to this, the support 
may be said to be twofold. On the one hand the axis is attached to the mantle lobe, 
on the other to the sheath of the adductor. (Fig. 7a.) Attachment to the adductor 
is by two connective tissue flaps which serve to raise the gill above the pallial lobe. 
The anterior and larger of these flaps is roughly triangular. Considered as an isos- 
celes triangle, the base lies along the branchial axis, one side (attached) along the 
dextro-posterior margin of the kidney (or the course of the branchial nerve outside of 
the gill), the apex near the right urinogenital opening and the other side (free) along 
the course of the fourth lateral pallial nerve. At the posterior limit of attachment 
the second flap extends from the mantle along the adductor sheath securing the axis 
thereto at a considerable distance from tbe lobe. Each axis continues around the 
adductor considerably beyond the posterior limit of attachment. 
FUNCTIONS AND ACTIVITY OF THE GILLS 
Respiration .—- 1 The functions of the gills have been briefly alluded to in the para- 
graph introductory to the discussion of the gills. It was there noted that recent 
denial of important respiratory function has been made. Hitherto it has been almost 
universally held that the gills of lamellibranchs were important for respiration. 
10441—31 3 
