406 THE MUSSEL CHAP. 



which sets in through the inhalant siphon into the pallial 

 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 of more importance 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, roi 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 immediately below the floor of the 

 pericardium : they communicate 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 and the visceral mass. Thus 

 the whole organ, often called, after its discoverer, the organ 

 of Bojanus, is simply a tube bent upon itself, opening at one 

 end into the ccelome (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 com- 

 municate 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 A'eber's organ (Fig. 103, /'.<?). It lies in the anterior region of 

 the body just in front of the pericardium, into which it discharges. 



