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-iamincB ; the inner one is free internally, 

 the supra and infra-branchial chambers being 

 thus in open communication. 



^. The excretory organ. Note the increase in 

 calibre of the glandular segment, as its posterior 

 end is neared; the vena cava is here interposed 

 between the muscular segments of opposite sides. 

 The cerebro-splanclmic commissures lie close 

 together beneath the vena cava. Cf. a. t,. 



y. The pericai'dial 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. 



^. 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 



