V.] THE FRESH-WATER MUSSEL. 337 



b. Through the middle pericardial region, across the 

 heart and auriculo-ventricular valves. Compare 

 generally with a, and work over, in addition. 



a. The gill-lamina ; the inner one is free internally, 

 the supra and infra-branchial chambers being 

 thus in open communication. 



/8. The excretory organ. Note the increase in 

 calibre of the glandular segment, as its posterior 

 end is n eared; the vena cava is here interposed 

 between the muscular segments of opposite sides. 

 The cerebro-splanchnic commissures lie close 

 together beneath the vena cava. Cf. a. . 



y. The pericardial chamber and heart. The former 

 is here at its maximum of development. Observe 

 the fleshy wall of the ventricle and the thin auri- 

 cles; the auriculo-ventricular valves are pocket- 

 shaped in section, their mouths being directed 

 towards the ventricle. 



Note the course of the rectum; it traverses 

 the ventricle, but is in no way bound down to it. 



c. Through the middle of the posterior -adductor 

 muscle. 



a. The attachments of the adductor muscle; it tra- 

 verses the body and pallial lobes, and is inserted 

 directly into the substance of the valves of oppo- 

 site sides. 



fi. The rectum; lying in the dorsal middle line 

 immediately above the adductor muscle. 



y. The supra-branchial chamber; completely shut 

 off, by the confluence and attachments of the 

 gills, from communication with the infra-bran- 



M. 22 



