52 



The blood whicli does not reach the heart through 

 the minute channels of the nuchal mantle, or rid the 

 left kidney, goes out to the mantle skiii 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 the pallial gills 

 and mantle edge, blood is returned to the large ])allial 

 vein by small veinlets projecting on the ventral surface 

 of the mantle skirt (tig. 7). Those from the edge (W. fig. 

 7) unite with those from the gills (G.\'. fig. 7), and the 

 channels thus formed turn outwards to open info tlie 

 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, wliich runs 

 completely round the mantle skirt just external to the 

 attachment of the pallial gills (tig. o). The completeness 

 of the circle is broken at one point directly anterior to 

 the left front end of the shell mus(do ; here the two sides 

 of the vein bend inwards and fuse into one trunk which 

 runs along the inner side of (he shell niuscde to the left 

 front c(U'ner of the pericardium. 



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

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

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

 nature of a lacuna. 



(JcELOMic System. — The extensive development of the 

 blood system has entailed a corresponding reduction of 

 the ccelom, of Avhich there remains practically nothing 

 except the pericardium. The form and position of the 

 pericardium have already been described, and it only 

 remains to say now that the pericardial gland seems to 

 be absent. The pericardium communicates with the 

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

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



