66 



paired efferent branchial veins which enter the auricles. 

 Blood from the posterior part of the body is gathered into 

 an afferent renal vein (a.r.v, figs. 7)3,34) v/hich runs for- 

 ward and enters the pericardial spaces ,at the posterior 

 ends of the kidneys. 



The description I have just given ajplios to X. fim- 

 briata and T, navalis. In T, norvegica, while the rela- 

 tions arc somewhat different, the homologies remain the 

 same. In this species, the principal part of the visceral 

 mass has remained more anteriorly and the posterior part 

 of the body is longer in proportion. In following the 

 gills, the heart has become much more elongated, and this 

 elongation has taken place in the aorta-like structure 

 which runs forv/ard from the more thickened portion of the 

 ventricle. In this species, the pericardial cavity ex- 

 tends much further forwards than in X. fimbriata, passing 

 under and anterior to tJie posterior adductor muscle as a 

 long canal, to end under the aesophagus. In it, the ven- 

 tricle runs to the anterior side of the posterior adduct- 

 or, to dip into the visceral mass. Valves mark the end of 

 the very long ventricle, from which tv/o vessels pass for- 

 wai'ds. The larger, after, giving off branches in its 



