194 GENERAL ZOOLOGY 



There are four of these gills and they hang like double 

 curtains along the right and left sides of the body (Fig. 101). 

 Oxygen, dissolved in the water, passes through the incurrent 

 tube of the siphon into the mantle cavity and over the gills. 

 The gills are thin and soft, and are thus adapted for the 

 ready passage of oxygen to the blood vessels inside. 



In the blood the oxygen combines with hsemocyanin, a 

 substance analogous to haemoglobin (see page 122). At the 

 same time the waste carbon dioxide in the blood is given 

 off to the water in the mantle cavity. The mantle folds, as 

 well as the gills, take part in respiration. It is possible for 

 them to do so because of their rich supply of blood vessels. 



Returning from the gills and the mantle, the blood, freed 

 of carbon dioxide, is carried to the right (Fig. 101) and left 

 auricles of the heart. These thin-walled, sac-like reservoirs 

 force the blood into the ventricle of the heart. The heart 

 contracts and forces the blood both forward and backward 

 through arteries. The anterior artery lies above the intes- 

 tine, and the posterior artery lies below the rectum. Both 

 arteries branch into smaller arteries in all parts of the body. 

 The blood flows from the open ends of the smallest arter- 

 ies into blood spaces, from which it is once more collected 

 and carried to the purifying organs in the manner already 

 described. 



The Siphon. Besides the rows of cilia which carry food 

 from the region of the incurrent tube to the mouth, there 

 are rows of cilia on the body and along the mid-ventral line 

 of the mantle, and it is known that these cilia wave toward 

 the incurrent tube. Food that has been rejected, or waste 

 that has accumulated, may be carried by the out-waving 

 cilia to the base of the incurrent tube. By muscular con- 

 traction of the siphon at its base these substances may be 

 expelled through the tube. The excurrent tube of the si- 

 phon, however, is the customary path of exit for substances 



