v.] THE FRESH-WATER MUSSEL. 323 



lobe as directed for Sect. B. ; lay open the pericardial 

 chamber and remove the auricle. The excretory organ 

 will now be visible as a blackish mass underlying the 

 floor of the pericardium; it becomes more marked 

 posteriorly, and extends behind the pericardium, termi- 

 nating immediately in front of the posterior adductor 

 muscle. 



Remove the anterior two-thirds of the outer gill- 

 lamina leaving its cut edge, and then dissect away 

 carefully a portion of the anterior third of the outer 

 wall of the inner lamina; there will thus be laid bare 



a. The excretory orifice (renal aperture); a small pore 

 situated immediately below and behind the anterior 

 end of the pericardium; it opens into the supra- 

 branchial chamber, immediately above the genera- 

 tive orifice. (Cf. Sect. B. d. y.) Insert a seeker 

 into it and wash the whole under a gentle current 

 until quite clean; the seeker will be found to 

 project into 



b. the excretory vestibule (non-glandular portion of 

 Bojanus); a thin-walled muscular sac, extending 

 back to the hinder end of the pericardium. Open 

 it up and gently raise its cut edge ; at its extreme 

 anterior end it will be seen \o be in communica- 

 tion with its fellow of the opposite side, by an 

 immense oval inter-reiial aperture. 



c. Underlying b. and visible through its thin wall, 

 there will now be seen the glandular seg?nent of the 

 organ. Open up the posterior portion of this, it is 

 tubular and its walls are plicated. 



Examine the cut edges of the whole with care; 

 note the transition from the glandular to the mus- 



21 — 2 



