THE COMMON MUSSELS. 27 



accounts of his researches in several molluscs, points out and em- 

 phasizes the frequent substitution among molluscs of unwalled blood- 

 spaces for vessels with definite walls in various parts of the blood 

 circuit. It happens thus in M. latus that while the arterial system is 

 remarkably well developed the venous system is the reverse, and is, in 

 fact, not nearly so well defined as in M. edulis, whose arterial system 

 is not quite equal to that of M. latus. 



It will be well to take first Sabatier's (7) account of the venous 

 system of M. edulis, in which the veins are well defined, for, by 

 reference to this, the condition of M. latus can be more readily ex- 

 plained. In M. edulis the blood from the pallial arteries and other 

 arterial vessels is collected mainly in the large marginal sinus (m s, 

 Fig. 32), from which it finds its way either directly at the posterior 

 end of the mantle or else by means of small veins (ascending veins) 

 on the inner surface of the mantle to a horizontal venous sinus extend- 

 ing along the mantle below the level of the posterior adductor, and 

 called the horizontal vein (h v, Fig. 32) . From the upper side of this 

 horizontal vein numerous very fine vessels arise on the inner surface 

 of the mantle; these very fine veins divide and occupy the external 

 side of the so-called organes godronnes. These " organes godronnes " 

 are transverse bands very loose or lacunar in structure, which con- 

 nect the mantle and the base of the descending lamellse of the gills. 

 They are almost rudimentary in M. latus (Fig. 2), although well de- 

 veloped in M. edulis (o gd, Fig. 3) and magellanicus. These organs are 

 held by Sabatier to function as gills in the respiration of the blood. 

 In M. edidis, according to Sabatier, the blood conveyed upwards as 

 we have seen by very small vessels from the horizontal vein enters 

 the lacunar tissue of the " organes godronnes." Leaving them, the 

 blood may take slightly different courses. It may pass directly into 

 the longitudinal vein (Fig. 32), and from thence be conveyed directly 

 to the auricle by the oblique vein (o v, Fig. 34) : this happens 

 especially in the anterior half of the longitudinal vein. Or, leaving 

 the " organes godronnes," it may traverse a considerable part of the 

 organ of Bojanus (which surrounds the longitudinal vein in part, 

 and is interposed between it and the " organes godronnes"), and 

 thence enter the longitudinal vein to be conveyed to the auricle by 

 the oblique vein. This happens especially in the posterior part of 

 the longitudinal vein, where the masses of the organ of Bojanus so 

 obtrude themselves on the longitudinal vein as often to break up and 

 obscure the cavity of the vein. In the preceding cases the blood has 

 returned to the heart without passing through the gills. Behind the 

 posterior adductor, where the " organes godronnes " do not exist, a 



