306 MOLLUSCA [CH. 



of Bojanus, are dark coloured bodies situated beneath the floor 

 of the pericardium on either side of the vena cava. Each consists 

 of a U-shaped tube lying horizontally, with one liinb placed ver- 

 tically above -the other and the bend directed backwards. The 

 deeper limb is the active part ; it has numerous folds projecting 

 into it which are covered with dark cells. It opens into the peri- 

 cardium in front by a curved slit lined with powerful cilia which 

 produce an outward current. This of course is the re no-peri- 

 car dial duct such as has been already described in the snail. 

 The outer and upper limb is wide and smooth-walled arid opens 

 into the deeper limb beneath the posterior adductor. In front it 

 opens to the exterior through a pore with thick lips placed just 

 above the place where the upturned ends of the inner row of 

 filaments are attached to the foot (Fig. 139). 



The kidney, since it is a tube lined with excretory cells and 

 communicating internally with the body-cavity, has been compared 

 with the nephridium of Lumbricus, but it is developed as an out- 

 growth from the coelom not as an ingrowth from the ectoderm like 

 the nephridium of Lumbricus. It must rather be compared to the 

 coelomiduct of that worm which serves as genital duct. The 

 function of the internal opening is to convey to the exterior the 

 fluid in the body-cavity, which contains excretory matter thrown 

 out by the cells lining the coelom. The anterior end of the peri- 

 cardium of the mussel has a brownish red colour and is produced 

 into numerous little pockets lined by peculiar cells, which are 

 excretory in function (Fig. 138). This portion of the pericardial 

 wall is called Keber's organ, and the excreta thrown out by it pass 

 down the reno-pericardial canal. 



The heart consists of a ventricle which surrounds the rectum, and 

 two flat triangular auricles, the broad bases of which are inserted 

 into the wall of the pericardium just over the place where the 

 bent-up ends of the outer filaments of the ctenidium are attached 

 to the mantle. From the ventricle blood is driven forwards by an 

 anterior aorta dorsal to the rectum, and backwards by a posterior 

 aorta ventral to the rectum. From these arteries it finds its way 

 into a multitude of irregular spaces in the foot and the other portions 

 of the body, and eventually reaches a vessel, called the vena cava, 

 lying under the floor of the pericardium in the middle line, between 

 the right and left upper limbs of the two kidneys. From the vena 

 cava the blood streams out through many channels in the wall 

 of the kidney and reaches the axis of the ctenidium, whence it 

 makes its way into the filaments, especially those of the outer 



