MOLLUSCA 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 Arca 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 interlamellar concrescences. 
Neighbouring filaments are kept parallel to one another by an 
arrangement unique in the animal kinedom. 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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