52 



The hlood Avliieh does not rearli the heart through 

 the minute channeLs of the nuchal mantle, or via the 

 left kidney, goes out to the mantle skirt and pallial 

 gills through channels running between the fasciculi 

 of the shell muscle, and, afterwards, in the substance of 

 the mantle skirt. After oxygenation in tiie ])allial gills 

 and mantle edge, blood is retiiriicd to the large pallial 

 vein by small veinlets projecting on the ventral surface 

 of the mantle skirt (hg. 7). Those from the edge (A'l. fig. 

 7) unite with those from the gills {(jr.\'. tig. 7), and the 

 channels thus formed turn outwards to open into the 

 large pallial vein. The veinlets from the mantle edge 

 can, therefore, be seen at intervals crossing over the 

 ventral surface of this large pallial vein, which runs 

 completely round the mantle skirt just external to the 

 attachment of the pallial gills (fig. •!). The completeness 

 of the circle is broken at one point directly anterior to 

 the left front end of the shell muscle; here the two sides 

 of the vein bend inwards and fuse into (me trunk which 

 runs along the inner side of the shell muscle to the left 

 fr(nit corner f)f the pericardium. 



It must be remembered that, though the name " vein " 

 is applied to them, none of the blood channels in the mantle 

 are true vessels, even the great pallial vein being of the 

 nature of a lacuna. 



C(ELOMic System. — The extensive development of the 

 blood sj^stem has entailed a corresponding reduction of 

 the ccelom, of which there remains practically nothing 

 except the pericardium. The form and position of the 

 pericardium Jiave already been described, and it only 

 remains to say now that the ])ericardial gland seems to 

 be absent. The pericardium commnnicates with the 

 large right kidney, and, as we can positively state, 

 Atith the small left one as well, but this matter will 



