Fresh-ivater Mussel. 197 



portion begins immediately posteriorly to the tendon of the 

 posterior retractor pedis, and extends as far back as the 

 entrance of the branchial nerves into the gill. It is bounded 

 below by the commissure of the two internal gills of the 

 two opposite sides of the body, and above, like the rest 

 of the canal, by the organ of Bojanus. Just beyond the 

 line of entrance of the gill-nerves, the canal thus made up 

 of three segments opens into a space, into which the ex- 

 ternal gilFs cavity also opens ; a canal having been left 

 along the dorsal attached border of that gill by the failure 

 of the dissepimental bands which connect the lower three- 

 foui-ths of its two lamellae together, to be developed there. 

 And as the rectiim m also opens into this space, it may be 

 called a ^cloaca/ Now it is easy to see how, under the 

 extruding action of the foot muscles, the ova will succes- 

 sively be pressed through the canal described into this 

 small cloacal space. When there, if the shell or the mantle 

 lobes are kept appressed posteriorly, or, as in the natural 

 position of the animal, superiorly, it is plain that they 

 must regurgitate, as additional relays of ova find their 

 way into the cavity, into the external gill-cavity from the 

 point c' forwards. 

 In this figure the nerve system is of a somewhat smaller size 

 than it is seen to possess, except when viewed in strict 

 profile, in nature ; and it has been owing to the necessity 

 for maintaining this position, which the demonstration of 

 the relations of the pericardium, and the two sacs of the 

 organ of Bojanus involved, that the distinction between the 

 muscular free border and the main mass of the mantle has 

 not been shown. The muscular portion of the foot is 

 figured in a condition of extreme contraction. 



For excellent figures of the nerve ganglia seen from below, with 



their brandies and commissural cords, see Duvernoy, Mem. 



Acad, des Sciences, tom. xxiv., 1854, pi. 7, fig. 2, pi. 8 and 9, 



figs. I and 2. 

 For a diagrammatic figure of the organ of Bojanus, see Lacaze 



Duthiers, Ann. Sci. Nat., Ser. iv., torn, iv., 1855, reproduced 



by V. Hessling, /. c, pi. v., fig. 6. 



