XI PHYLUM MOLLUSCA 275 



ering the labial palps. The outgoing 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 (Fig. 

 162, kd) t and a thin- walled non-glandular part or bladder 

 (bf) communicating with one another posteriorly, while in 

 front the kidney opens into the pericardium (r. /. ap), and 

 the bladder on to 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 coelom, and at the other on the external 

 surface of the body. 



The circulatory system is well developed. The heart lies 

 in the pericardium and consists of a single ventricle (Figs. 

 162 and 164, v) and of right and left auricles (au). The 

 ventricle is a muscular chamber which has the peculiarity 

 of surrounding the rectum (Fig. 162) ; 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. 162, a. ao) passing above, the posterior aorta 

 (p. ao) below the rectum. From the aortae the blood passes 

 into arteries (Fig. 164, art. i,art. 2) which, ramifying all over 

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

 of which are devoid of proper walls and have therefore the 

 nature of sinuses. 



The nervous system is formed on a type quite different 



