ANODONTA 63 



fusion of adjacent filaments, increased development of the 

 inter-lamellar junctions, and fusion of the upper ends of the 

 ascending hmbs (outer lamellre of the outer gill-plates, inner 

 lamellffi of the inner plates) with the mantle or body-wall, or 

 gill of the opposite side, as the case may be, brings about the 

 condition found in Anodonta. 



The colourless and corpusculated blood circulates in a 

 number of irregular lacunar spaces in the gills. From thence 

 it is returned by numerous vessels running for the most part 

 in the interlamellar junctions to a large pair of vessels running 

 along the bases of the outer lamellfe of the outer gills. These 

 efferent branchial veins carry the blood to a pair of wide, thin- 

 walled triangular sacs, the auricles of the heart, and these in 

 turn empty the blood into a median muscular ventricle of 

 which the position has already been noted. The two auricles 

 lie right and left of the ventricle, and traverse a considerable 

 space, the pericardial cavity, in which the ventricle also lies. 

 The ventricle is wrapped round the rectum, and gives off an 

 anterior and a posterior arterial vessel, the branches of which 

 carry the blood to the different parts of the body. The blood 

 from the foot, the visceral mass and the hinder part of the 

 body is collected into a large vessel lying in the middle line 

 below the floor of the pericardial cavity. Thence it is con- 

 ducted by aflferent branchial vessels to the gills and from the 

 gills back to the heart again. The blood from the mantle 

 does not pass through the gills, but is returned direct to the 

 efferent branchial vessels, and so to the heart. This indicates 

 that the mantle is an auxiliary respiratory organ. 



The wide pericardial cavity in which the heart and rectum 

 lie represents the perivisceral ccelom in the adult animal. In 

 the remainder of the visceral mass, in the foot and gills, the 

 perivisceral coelom has disappeared and its place is taken 

 partly by the viscera including the large digestive glands, 

 partly by the extension of lacunar blood-spaces running 

 between the viscera and the muscle fibres of the foot. The 

 pericardial cavity, however, has no connection with the gen- 

 erative organs, and therefore represents only a part of the 

 coelomic space. The remaining part is represented by the 

 gonads, which will be described later. The coelomic nature 

 of the pericardial cavity is shown both by its not containing 

 blood and by its relations to the excretory organs. 



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