INVERTEBRATA. 331 



most part into a large longitudinal vein ("vena cava") 

 just ventral to the pericardial cavity. From this it 

 passes right and left, through the excretory organs (to be 

 presently described), to the gills. The blood, thus purified, 

 first of nitrogenous waste and then of carbon dioxide, 

 passes into the auricles and so to the ventricle. The gills, 

 however, are not the sole organs of respiration : the mantle 

 is also adapted for this function, and the blood from it 

 returns directly to the heart without passing through 

 nephridium or gill. 



In fig. 171 we have this circulation compared with that of 

 the dogfish. It will be seen that whereas in the latter 

 the heart contains deoxygenated blood, which it sends to 



HEART 



TfSS C/fS 



Fig. 171. COMPARISON OF CIRCULATION IN ANODONTA (LEFT) AND 

 SCYLLIUM (RIGHT). 



the gills, the heart of Anodonta receives oxygenated blood 

 from the gills. This difference is expressed by calling the 

 heart of Scyllium a branchial heart, and that of Anodonta 

 a systemic heart. The blood of Anodonta is colourless and 

 contains colourless amoeboid corpuscles. 



7. The Renal-Excretory Organs of Anodonta have 

 none of the characteristics of nephridia, but are coelomic 

 ducts with excretory functions. They are often spoken 

 of as the organs of Bojanus (fig. 172). They lie side by 

 side just ventral to the pericardium, being partly sepa- 

 rated by the vena cava, and partly by the posterior 

 retractor muscle. Each is a tube doubled upon itself: 

 the two limbs of the tube communicate at their posterior 



