640 ZOOLOGY SECT. 



to this arrangement it will be seen that the water tubes all open 

 dorsally into a supra-branchial chamber (s. br. c.} continuous pos- 

 teriorly 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 pallia! 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 

 mcso-nqiJ/ridia, 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. 528, Jed.}, and a thin- walled 

 non-glandular part or bladder (bl.}. 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 the kidney opens into the 

 pericardium (r. p. 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 dis- 

 coverer, 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. The epithelium of the bladder is ciliated, and 

 produces an outward current. 



It seems probable that an excretory function is also discharged 

 by a large grandular mass of reddish-brown colour, called the 

 pericardial gland or Keber's organ (Fig. 530, B, Jc. 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 (Figs. 528, 530, 

 and 531, T.) and of right and left auricles (w.). The ventricle is 

 a muscular chamber which has the peculiarity of surrounding 

 the rectum (Figs. 528 and 530, 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. 528, a.ao.} passing 

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

 aortse the blood passes into arteries (Fig. 531, art. 1 art: 2 } which 



