36o THE FRESH-WATER MUSSEL less. 



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. ap\ 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 (Fig. 93, Nphm), often called after its dis- 

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

 itself, opening at one end into the coelome {Nph. st), and at 

 the other on the external surface of the body {Nph. p) : it 

 has therefore the normal relations of a nephridium. The 

 epithelium of the bladder is ciliated, and produces an 

 outward current. 



It seems probable that an excretory function is also dis- 

 charged by a large glandular mass of reddish-brown colour, 

 called the pericardial gland or Keber's organ (Fig. 92, B, 

 k. o). 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 (Fig. 93, 

 Vent, and Figs. 94 and 96, v) and of right and left auricles 

 (au). The ventricle is a muscular chamber which has the 

 peculiarity of surrounding the rectum (Figs. 93 and 94) : 

 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. 94, a. ao) passing above, the 

 posterior aorta {p. ao) below the rectum. From the aortae 

 the blood passes into arteries (Fig. 96, art.,^ arf^) which 

 ramify all over the body, finally forming an extensive net- 

 work of vessels, many of which are devoid of proper walls 

 and have therefore the nature of sinuses. The returning 

 blood passes into a large longitudinal vein, the vena cava 



