274 MANUAL OF ZOOLOGY 



scopic 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, 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. 

 150, kd.\ and a thin-walled non-glandular part or bladder 

 (</.), communicating with one another posteriorly ; while in 

 front the kidney opens into the pericardium (r. p. a.), and 

 the bladder on to the exterior by a minute aperture (r. ?/>.), 

 situated between the inner gill and the visceral mass. Thus 

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

 discoverer, is simply a tube bent upon itself, opening at one 

 end into the ccelome, 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. 

 150 and 152, v.) and of right and left auricles (au.). The 

 ventricle is a muscular chamber which has the peculiarity of 

 surrounding the rectum (Fig. 150) : 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. 150, a. ao\ passing above, the posterior aorta 

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

 into arteries (Fig. 152, art. 1 art. 2 ) which ramifying all over 

 the body, finally forming an extensive network of vessels, 

 many of which are devoid of proper walls and have there- 

 fore the nature of sinuses, 



