Fresh-water Mussel. 195 



ganglia. It passes between the fibres of the retractor pedis 

 anterior and those of the protractor through the upper part 

 of the footj just internally or inferiorly to the generative 

 orifice, t ; then through the glandular portion of the organ 

 of Bojanus, s ; and across the tendon of the retractor pedis 

 posterior just where it bifurcates for insertion into either 

 shell, to end in the parieto-splanchnic ganglion. 



/. Parieto-splanchnic ganglion. The two ganglia of the two 

 sides of the body are closely apposed, so as to form a trans- 

 versely oblong mass, which however still retains an in- 

 dication of its morphological duality in the bilobed con- 

 formation, which is somewhat exaggerated in this figure, 

 and is much less definite than that of the pedal centres 

 between j and /. Two nerves are figured in connection 

 with it, one, a parietal nerve, going to the mantle, the 

 other, a splanchnic nerve going to the gill. 



m. Eectum ending in the anal compartment of the mantle, a 

 little beyond the posterior free edge of the posterior ad- 

 ductor. A delicate nerve is figured by Duvernoy, I. c, as 

 passing to it from the parieto-splanchnic ganglion. 



n. Heart ; the letter pointing to the slit left by removal of the 

 left auricle. The rectum passes through the heai't, having 

 commenced by emerging from the foot-mass, in close con- 

 nection with the aorta anterior, which lies above it and 

 below the mantle raphe, at a point corresponding to the 

 vertical plane of the orifices of the generative gland and 

 of the organ of Bojanus. 



0. Pericardial space into which open the glandular portions of the 

 organ of Bojanus, as also certain orifices belonging to the vas- 

 cular system; see V. Hessling, ' Die Perlmuscbeln/ p. 239. 



p. Opening by which the excretory portion of the organ of 

 Bojanus communicates with the branchial cavity. 



q. Opening by which the excretory portion of the organ of 

 Bojanus communicates with the secretory. 



r. Wide opening by which the excretory portion of the organ 

 of Bojanus of one side communicates with that of the other. 

 This opening does not exist in Unio margaritlfer. The 

 two secretory sacs are similarly connected at a deep level 

 in the same plane. 



o a 



