406 THE MUSSEL CHAP. 



the pallia! cavity, through the ostia into the water-tubes, 

 thence into the supra-branchial chamber, and out at 

 the exhalant siphon. The in-going current carries with 

 it not only oxygen for the aeration of the blood, but also 

 diatoms, infusoria, and other microscopic organisms, 

 which are swept into the mouth by the cilia covering 

 the labial palps. The out -going current carries with it 

 the various products of excretion and the faeces passed 

 into the cloaca. The action of the gills in producing 

 the food current is more important than their respiratory 

 function, which they share with the mantle. 



The excretory organs are a single pair of curiously 

 modified nephridia, situated one on each side of the body 

 just below the pericardium. Each nephridium consists 

 of two parts, a brown, spongy, glandular portion or kidney 

 (Figs. 101 and 103, kd), and a thin-walled, non-glandular 

 part or bladder (bl). The two parts lie parallel to one 

 another, the bladder being placed dorsally and immedi- 

 ately below the floor of the pericardium : they com- 

 municate with one another posteriorly, while in front the 

 kidney opens into the pericardium (r. p. a), and the 

 bladder on the exterior by a minute aperture (r. ap) 

 situated between the inner gill-lamina and the visceral 

 mass. Thus the whole organ, often called, after its dis- 

 coverer, the organ of Bojanus, is simply a tube bent upon 

 itself, opening at one end into the coelome (see p. 401), 

 and at the other on the external surface of the 

 body : it has therefore the normal relations of a 

 nephridium (p. 340). The two bladders communicate 

 anteriorly (Fig. 101, x), and their epithelium is ciliated, 

 producing an outward current. 



It seems probable that an excretory function is also 

 discharged by a large glandular mass of reddish-brown 

 colour, called the pericardia! gland or Keber's organ (Fig. 103, 

 k. o). It lies in the anterior region of the body just i-n 

 front of the pericardium, into which it discharges. 



