072 



ZOOLOGY 



SECT. 



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 Bojanus, 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, but is of coelomic not ecto- 

 dermal derivation. 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 Keber's 

 organ (Fig. 578, B, k.o.). It lies in the anterior region of the body 

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



CLfl. 



FIG. 579. Diagram of the circulatory system of Anodonta. Vessels containing aerated 

 blood red, non-aerated blue, af.br.v. afferent branchial veins ; ao. aorta : art. 1, artery to 

 mantle ; art. 2, artery to body generally ; au. auricle ; ef.br.v. efferent branchial veins ; 

 nph.v. nephridial veins ; pc. pericardium ; v. ventricle ; v. c. vena cava. The arrows^ show 

 the direction of the current. 



The circulatory system is well developed. The heart lies in 

 the pericardium and consists of a single ventricle (Figs. 576, 578, 

 and 579, v.) and of right and left auricles (au.). The ventricle is 

 a muscular chamber which has the peculiarity of surrounding the 

 rectum (Figs. 576 and 578, 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. 576, a. ao.) passing above, the 

 posterior aorta (p. ao.) below the rectum. From the aortse the 

 blood passes into arteries (Fig. 579, art. I art.%) which ramify all over 

 the body, finally forming an extensive network of vessels, many 



