EXCRETORY ORGANS AND GONADS 279 



of lower concentration than the coelomic fluid or the blood so 

 that solids must be reabsorbed in the kidney, and the mussel, 

 like other freshwater animals, is continually doing work to prevent 

 itself swelling with water. A pair of glandular bodies known as 

 Keber's organs which lie on each side in front of the pericardium 

 store excreta in their cells. They are derived from the coelomic 

 wall. The principal nitrogenous excreta are ammonia and amino 

 compounds. The opening of the kidney has thick, yellowish 

 lips. Immediately below it is a somewhat larger opening with 

 thin lips. This belongs to the gonad, which is a branched structure 



a.ao. 



*pjO'-' 



Fig. 207. — A diagram of the pericardium and kidney of the swan musseJ, 



from the left side. 



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

 k.o., opening of the same ; p.ao., posterior aorta ; pm., pericardial cavity; rm., rectum ; rp.o., reno- 

 pericardial opening ; v., ventricle. 



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

 in both sexes. 



VASCULAR SYSTEM 



The blood is colourless and contains white corpuscles. The 

 heart (Fig. 207) consists of a ventricle, which is a specialised part 

 of the dorsal aorta w^hich has extended round the rectum and 

 enclosed it, 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 passes backwards 

 below it. From branches of these the blood passes into spaces 

 between the organs. From the foot and viscera it is gathered into 

 a vena cava which lies below the pericardium between the kidneys. 

 Thence it passes outwards through the kidneys to the gills, where 

 it circulates in irregular spaces in the inner parts of the filaments. 

 From these it is returned to the auricles. The blood from the 

 mantle returns direct to the auricles. 



