MOLLUSC A 193 



a vena cava in the floor of the pericardium, and is thence sent 

 through the nephridia to the gills and returned to the 

 auricles. The circulation is partly lacunar, the blood being 

 contained in irregular splits in the tissues and not in distinct 

 vessels. The blood contains amoeboid corpuscles, and is usually 

 colourless ; two species, however, Solen legumen and Area Noe, 

 contain haemoglobin in their corpuscles. 



The gills consist primitively of an axis, which is fused to 

 the body for the greater part of its course ; this contains an 

 efferent and an afferent blood-vessel. The axis gives off two 

 series of filaments, which hang down parallel to one another, 

 thus forming two lamellae. The filaments of both series may 

 be bent up, forming V-shaped structures, those of the outer 

 series having their free ends external and next to the mantle, 

 whilst those of the inner series have their free ends internal 

 and next to the foot, so that each series forms a gill with an 

 outer and an inner lamella. In Mytilus and some others the 

 outer and inner limbs of each filament are connected by 

 certain pieces of tissue termed inter! amellar concrescences. 

 Neighbouring filaments are kept parallel to one another by an 

 arrangement unique in the animal kingdom. Each filament 

 bears certain patches of ciliated cells, and the cilia of two opposite 

 patches are interlocked, in the same way as a couple of brushes 

 when put together. In more complex genera these ciliary 

 junctions are replaced by interfilamentous concrescences, and 

 in Anodonta the interlamellar and interfilamentous concres- 

 cences are developed to such an extent as to leave but narrow 

 passages through which the water circulates. The free ends 

 of the filaments of the outer lamella of the external gill, and 

 of the inner lamella of the internal gill, very frequently fuse 

 with the contiguous organs, the mantle, or the foot. 



Between the lamellae of each gill a certain space is de- 

 veloped which is more or less continuous with that of the 

 other gills. This epibranchial space often serves to lodge the 

 developing ova, it communicates with the dorsal siphon, 

 through which the waste products leave the animal. 



Each gill filament contains a blood-vessel, and it is often 

 stiffened by two rods of a chitinous material. Its outer 

 epithelium bears cilia, which serve to create a current of 



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