390 THE MUSSEL ckAP. 



obvious. By the action of their cilia a current is produced 

 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 

 fajces 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 

 iiepliridia, situated one on each side of the body just below 

 the pericardium. Each nephridium consists of two parts, a 

 hxawnjU^yowgy, gla/idular portion or kidney (Figs. 93 and 95, 

 kd), and a thin-walled, non-glandular part or //ladder [id). 

 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 delome, and at the other on the external 

 surface of the body : it has therefore the normal relations of a 

 nephridium (p. 331). Two bladders communicate anteriorly, 

 and their epithelium is ciliated, producing an outward current. 



It. SL-cms i")ruljal»lc tlial an cxcruUiry funclion is al.so discliarged l)y a 

 ]ai"ti;e L^landular mass of rcdflish-lirow n colour, called \\\^ pci'i,ardial 

 g/aiiil KiA'clici's nr!;aii (Fig. 95, t.o). Il lies in the anterior region of the 

 Ijodyjust ill front of the |jcricardiuin, into which it discharges. 



