THE COMMON MUSSELS. 31 



bases of the gills and neighbouring organ of Bo j anus (o gd, Figs. 

 2, 3, 35, and 37) . TYhile tliey are large and noticeable organs in M. 

 edulis and magellanicus , they are very small and inconspicuous in M. 

 latus. They are mentioned by Sabatier as existing only on the outer 

 side of the gill (o gd, Fig. 3) in M. edulis ; but I think the transverse 

 bands occupied by the organ of Bojanus on the inner side of the gill 

 anterior to the mesosoma in M. edulis are exactly comparable to the 

 bands on the outer side of the gill opposite the mesosoma, which are 

 alone termed " organes godronnes " by Sabatier. In M. latus the 

 descending lame! Ia3 of the two gills are attached along one line 

 between the mantle and the mesosoma (Figs. 2 and 35), as is the case 

 in M. edulis, but the further arrangement of the gills differs slightly. 

 In M. latus the descending lamellae do not hang directly downward 

 from the line of attachment, but diverge from each other ; and a 

 little from their base is another point of attachment (Fig. 35) 

 in the case of the inner gill to the body and of the outer 

 to the mantle. The means of attachment are small triangular 

 vertical plates exactly answering to the plaited organs of M. edulis 

 and magellanicus, one passing to each filament. These small plates 

 exist along the whole course of the gills from their anterior 

 end to where the gills become free at the posterior adductor. The 

 continuation of these transverse connections the whole length of the 

 gills in the case of both lamellae is my reason for considering the 

 plaited organs of Sabatier and the transverse bars on the anterior 

 part of the organ of Bojanus in M. edulis and magellanicus as exactly 

 similar structures, as they are quite homologous with these small 

 vertical plates of M. latus, which are uniform throughout and differ 

 only in size from the above organs. Sabatier ascribes to the plaited 

 organs an important part in the aeration of the blood, and in this I 

 think he is right; but, since we have seen that the organ of Bojanus 

 takes an important part in this operation, then the transverse bands 

 in M. edulis on the inner side of the descending lamellae may well be 

 considered as plaited organs that happen to be occupied by the organ 

 of Bojanus. Sabatier, while giving very great prominence to the 

 plaited organs on the outer side of the gill, has made but slight 

 mention of these bars on the ventral side of the gill, and has not 

 noted their resemblance to the structure to which he has restricted 

 the term plaited organs, or "organes godronnes. " In M. edidis these 

 organs are not developed on the outer side anterior to the mesosoma, 

 nor on the inner side behind the anterior end of the mesosoma (o gd, 

 Fig. 3). 



