V.] THE FRESH-WATER MUSSEL. 319 



orifices; two small apertures lying one above the 

 other and embraced postero-ventrally by a 

 common lip. (For further details see Sects.' E. 

 and L. a. .). 



8. The rectum; a yellowish thin-walled tube cours- 

 ing over the hind face of the posterior adductor 

 muscle. It opens by a terminal wide-mouthed 



anus. 



Introduce into the immediate vicinity of the 

 ventral siphon of the living animal some finely 

 divided colouring matter. It will be drawn into 

 this with the inhalent current induced by the 

 ciliary action at work, and ejected, a little later, 

 through the dorsal siphon with the exhalent one. 

 (Cf. pp. 308-9)- 



e. Note the position and general relations of the 

 pericardia! cavity. It lies in the middle line 

 immediately above the supra-branchial chamber, 

 wholly in front of the posterior adductor muscle. 

 Examine in relation to it 



a. the organ of Keber (pericardia! gland] ; a red- 

 brown thickening of the adjacent body-wall 

 and pallial-lobe; it is most conspicuous ante- 

 riorly, above the gills. 



ft. Lay open the cavity of the pericardium and note 

 the rectum; a yellowish tube, passing through it 

 on its way to the supra-branchial chamber. It 

 is surrounded at its middle by the yellow thick- 

 walled ventricle. (For further details see Sects. 

 F. and L.) 



Keber's organ can be advantageously seen 

 at this stage, if looked at from within 



