26 MOLLUSC A. 



portion of the blood from the spongy masses of tissue 

 surrounding the stomach and intestine (Figs. 7, 12, z'), 

 from the palpi and from the great muscle, is collected 

 into tubes of greater or less size, and conveyed by them 

 to the median vessel which lies at the base of the two 

 inner laminae of the gills (Fig^. 5-8, x). From this 

 numerous tubes branch right and left through the par- 

 titions, and open into smaller tubes, which run in the 

 ridges along the bases of the gills on either side, and 

 (Jz) along the junction of the outer laminae with the 

 mantle (Figs. 6, r ; 7, /). From these vessels it flows 

 into the numerous vertical tubes of the gills (Figs. 7, 

 8, II, /'). From these it takes a horizontal course 

 into the depths of the semicircular chambers, and 

 thence, turning vertically again in both directions, 

 passes between the minute openings (Fig. 9, eo) of 

 the gills into the smallest of the return vessels. These 

 are in partitions (Fig. 10, r"+) which cut across the 

 folds of the gill, and open into the vertical tubes (/'), 

 which carry it upward to the larger reservoirs in the 

 partitions (Figs. 8, r"'+ ; 7, r'"), and force it onward 

 through the transverse connecting tubes to the venous 

 trunks previously described at the junctions of the 

 mantle and the gills (Figs. 6, 7,7). Here it mingles 

 with the return blood in the vessels jm, Fig. 6, from 

 the mantle. The leaves of the mande undoubtedly 

 also aerate the blood which is carried through them, 

 though probably much less effectually than the gills ; 

 and it is probable, also, that the great amount of sea- 

 water taken in by the water vessels also does some of 

 this work, and perhaps mingles more or less with the 

 blood itself. 



