xii PHYLUM MOLLUSCA 689 



open dorsally into a supra-branchial chamber (s. br. c.) continuous 

 posteriorly with the cloaca and thus opening on the exterior by 

 the exhalant siphon. 



The physiological importance of the gills will now be 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, 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 (portions of the true ccelome), 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 (Fig. 566, 

 kd.), and a thin- walled non-glandular part or bladder (bl.), which 

 communicates with its fellow anteriorly by a large oval aperture. 

 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 each 

 glandular part opens into the pericardium (r. p. ap.), and the 

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

 between the inner lamina of the gill and the visceral mass. Thus 

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

 Bqjanus, is simply a tube bent upon itself, opening at one end into 

 the coelome, and at the other on the external surface of the body : 

 it has thus the normal relations of a nephridium. The epithelium 

 of the bladder is ciliated, and produces an outward current. 



An excretory function is also discharged by a large glandular 

 mass of reddish-brown colour, called the pericardial gland or Kebers 

 organ (Fig. 568, B, .0.). It lies in the anterior region of the body 

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



The circulatory system is well developed. The heart lies in 

 the pericardium and consists of a single ventricle (Figs. 566, 568, 

 and 569, v.) and of right and left auricles (.). The ventricle is 

 a muscular chamber which has the peculiarity of surrounding the 

 rectum (Figs. 566 and 568, B): the auricles are thin-walled 

 chambers communicating with the ventricle by valvular apertures 

 opening towards the latter. From each end of the ventricle an 

 artery is given off, the anterior aorta (Fig. 566, a. ao.) passing 

 above, the posterior aorta (p. ao.) below the rectum. From the 

 aortse the blood passes into arteries (Fig. 569, art. 1 art. 2 ) which 

 ramify all over the body, finally forming an extensive network of 



