65 



forwards on its dorsal and ventral sides (val. fif^.sc), 

 From the end of the ventricle two vessels are given off. 

 The larger (av.p. figs. 10,56) runs forwards (figs. 28-30) 

 in the visceral mass, and passes ventral to and in front 

 of the posterior adductor, to bond over tha latter and en- 

 ter the mantle as the large dorsal or posterior pallial 

 artery. Tiiis I'uns as a single vessel in Xylotrya at the 

 right side of the anal canal and epibranchial cavity (figs. 

 28-34, d.a.) to the posterior end of the body, v;here it 

 divides into the two paired arteries of the siphons. This 

 aortal have just described is the morphological posterior 

 aorta, though its course at first is anteriorly. The sec- 

 ond aorta' leaving the ventricle runs posteriorly in the 

 visceral mass and is the morphological anterior aorta. 



The venous system consists of three important parts. 

 Blood from the viscera and anterior part of the body is 

 gathered into a system of afferent branchial veins con- 

 sisting anteriorly of large paired vessels (G.a. figs, 29 

 -31), which in the region of the visceral ganglion unite 

 to form the ver:/ large afferent branchial vein which mns 

 between the fused gills (figs, 10,32-34), Passing from 

 this vein through the gill lamellae, it enters tlie large 



