THE SWAN MUSSEL 321 



exterior. 1 A pair of glandular bodies known as Xeber's organs 

 lie one on each side in front of the pericardium and pass into 

 it waste products which they excrete. They are derived from 

 the coelomic wall and, like the yellow cells of the earthworm, 

 represent a portion of its epithelium specialised for excretion. 

 The opening of the kidney has thick, yellowish lips. Immedi- 

 ately below it is a somewhat larger opening with thin lips. 

 This belongs to the gonad, which is a branched structure 

 lying in the upper part of the foot and alike in its general 

 structure in both sexes. 



rp.o- 



FIG. 229. — A diagram of the pericardium and kidney of the swan 

 mussel, from the left side. 



a.ao., Anterior aorta; aw., auricle; B, glandular limb of kidney; B', non- 

 glandular limb of the same ; k.o., opening of the same ; p.ao., posterior aorta ; 

 pm., pericardium ; rJ>.o. y renopericardial opening ; rm., rectum ; v., ventricle. 



The blood is colourless and contains white corpuscles. 

 The heart (Fig. 408) consists of a ventricle, 

 system" which forms a jacket around the rectum, 2 and 



two auricles, which are triangular, thin-walled 

 structures, one on each side of the ventricle. From the 

 front end of the ventricle an anterior aorta passes forwards 

 above the rectum, and from the hind end a posterior aorta 



1 See footnote to p. 335. 



2 The ventricle of the heart of the swan mussel is really a wide, 

 contractile part of a dorsal blood vessel, which is continued forwards 

 as the anterior aorta. In various animals related to the mussel it lies 

 altogether above the gut, but here it extends downwards at the sides 

 and encloses it. A similar vessel is found above the gut in Annelida 

 (e.g. the earthworm) and in Arthropoda (e.g. the crayfish and cockroach). 

 See p. 255. 



