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THE LAMELLIBRANCHIA 225 
The foot, the mantle, and the siphons derived from the latter 
are gorged with blood when their muscles are relaxed, and their 
sudden contraction often produces a reflux of arterial blood towards 
the heart. In Lamellibranchs with a well-developed foot and 
siphons, the return of blood into the ventricle is prevented by 
valves situated at the origins of the aortae, and a sphincter is also 
often to be found at the root of the posterior aorta, and sometimes 
a valve in the siphonal artery. In addition, highly developed 
aortic bulbs, separated from the ventricle by one of the above- 
mentioned valves, are frequently present, generally on the posterior 
aorta, where a large bulb may be seen, within the pericardium, in 
many Siphonates, particularly in the Veneridae (Zapes, Fig. 202, a.0), 
Petricolidae, Tridacnidae, Mactridae, etc. A bulb or aortic dilata- 
tion also occurs on the anterior aorta, inside the pericardium in 
Pecten and the Mytilidae, outside the pericardium in Anodonta. 
The arterial blood forced back towards the heart by the contraction 
of the foot or mantle or siphons enters and fills these various bulbs. 
The blood carried to the different parts of the organism by the 
ultimate ramifications of the arterial trunks finally enters the 
venous sinuses, of which the most 
important are the pallial sinuses, 
the pedal sinus, and the great 
median ventral sinus. The last 
named is situated between the 
pericardium and the foot, and is 
separated from the pedal sinus by 
the valve of Keber, which prevents 
the foot from emptying itself of 
blood when in movement. It is 
from this great unpaired median 
sinus that the greater part of the 
blood is derived that passes through 
the kidneys and thence goes on to 
the gills. But a certain quantity 
of blood is carried to the auricles 
without having passed through the 
gills: this blood is brought from BGs 202: 
the mantle, for example, in Pecten. gpout the mid-resion of the foot, a, auricle? 
The essential respiratory organ br, outer gill-plate; b’r’, inner gill-plate 
3 G : (each composed of two lamellae); f, foot; 
of the Lamellibranchs is a pair of 4, intestine; m, mantle-flap; p, p’, peri- 
ctenidia. Each ctenidium is a Genesee CGhae PATE 
lateral pallial offset, occupying a 
longer or shorter space between the mantle and the posterior part 
of the visceral mass. It may extend as far forward as the labial 
palps (Fig. 241, 07’), but in the most archaic forms the gills still 
occupy a relatively posterior position (Figs. 250, 231, g), while in 
15 
