MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
399 
the water the valves close and the water in the branchial chamber is discharged 
principally through the branchial siphon, so that the membrane must have been raised 
in order that this might occur; 
If the valves of the shell should suddenly close, much of the water in the branchial 
chamber would escape ventrally. While this is going on, the siphons are being drawn 
within the shell, and of necessity against this pressure in the branchial chamber, 
caused by the closing of the shell. If, instead of being closed, the branchial siphon 
should be opened, and thus allow the imprisoned water to rush out, it would allow 
their retraction to be more easily and quickly accomplished. 
The siphons of Venus, however, are small, and do not meet the difficulty in retrac- 
tion encountered by the enormously developed siphons of My a arenaria , the “long- 
necked clam,” especially as the branchial chamber is here closed below over nearly its 
entire extent, and allows little water to escape between the mantle edges. When the 
siphons are contracted — and the process is always comparatively a very slow one — a 
stream of water is discharged from both, but mainly from the branchial. Though 
finally brought within the shell, their outer ends are somewhat exposed, as the shell 
in their region is expanded and its valves can not meet behind them. The branchial 
membrane is not here present, but may perhaps be represented by part of the thick- 
ening in the partition between the siphons (Fig. 94, PI. xciii, brm). If this is so, the 
organ may have been lost because of its interference, though it may have been slight, 
in the laborious process of withdrawing the siphons into the shell. 
Just what advantage may be subserved in the forms where the branchial mem- 
brane is so greatly developed is not apparent to me. 
The series of figures 18 to 22 represents this posterior region of the body of Venus 
in vertical section. Pig. 18 has been cut just in front of the branchial membrane (br m) 
and the posterior adductor is seen above. The mantle edge has become muscular and 
very thick at m, to form, farther back, the walls of the siphons. The bases of these 
have been cut across in Pig. 19, and it may be here seen how their walls are gradually 
constricted off from the mantle at x. A fold of the mantle (m) extends across under 
the lower siphon and is also present in Fig. 20. In this latter figure the mantle fold 
is entirely separated from the siphon walls, except dorsally, and these walls are seen to 
be very thick and muscular, especially those of the cloacal siphon. In Fig. 21 they 
have assumed a uniform thickness. In Fig. 22 the siphons are cut across where 
they have protruded backward beyond the mantle edge. The basal part of the lumen 
of the upper or cloacal siphon (Fig. 19, ns) is somewhat triangular in section, while 
that of the branchial siphon is more nearly circular. Toward their outer end they 
appear as slits elongated dorso-ventrally, though not to so great an extent in the 
living animal. 
The anatomy of the siphons of Mya is much the same as in Venus. Fig. 29 is a 
thick section just before the bases of the siphonal openings. The cloacal chamber is 
cut across at cl, showing the gills at their posterior ends, separating the cloacal from 
the branchial chamber. The posterior end of the partition between the siphons is 
seen at br m . The muscles, which are to become the siphon walls farther back, are 
shown at ws. These, from this region to the ends of the siphons, are covered by a 
thick, gelatinous, semitransparent tissue, ct. Still farther back, as in Fig. 30, in which 
section the right side has been cut deeper than the left, are seen the siphon walls on 
