THE LAMELL1BRANCHIA 



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 (Tapes, Fig. 202, a.b\ 

 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 

 the mantle, for example, in Pecten. 



The essential respiratory organ , 



. J . (each composed of two lamellae); /, foot; 

 Of the Lamellibranchs IS a pair Of i, intestine; m, mantle-flap; p, p', peri- 



ctenidia. Each ctenidium is a SSSUSj*' (Prom Lankester> after 

 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, I/), but in the most archaic forms the gills still 

 occupy a relatively posterior' position (Figs. 230, 231, g), while in 



15 



FIG. 205. 



Transverse section through an Anodonta, 

 about the mid-region of the foot, a, auricle ; 

 br, outer gill-plate; b'r 1 , inner gill-plate 



