670 



ZOOLOGY 



SECT. 



ments or water-tubes (w.t.), closed ventrally, but freely open along 

 the dorsal edge of the gill. The vertical striation of the laminae is 

 due to the fact that each lamella is made up of a number of close- 

 set' gill-filaments (/.) : the longitudinal striation to the circumstance 

 that these filaments are connected by horizontal bars, the inter- 



FIG. 578. Anodonta cygnea. Three transverse sections, A, B, C, au. auricle ; bl. urinary 

 bladder ; ext. gl. external gill-lamina ;ft. foot ; i.l.i. inter-lamellar junction ; int. intestim' ; 

 int. gl. internal gill-lamina ; kd. kidney ; k.o. Keber's organ ; Ig. ligament ; m. mantle ; ?>.</</. 

 posterior adductor; pc. pericardium; ret. rectum; s.br.c. supra-branchial chamber; .>/ 

 shell ; ty. typhlosole ; t 1 . ventricle ; re. vena cava ; v. gn. visceral ganglion. (After Howes, 

 slightly altered.) 



filamentar junctions (i.f.j.). At the thin free or ventral edge of the 

 lamina the filaments of the two lamellae are continuous with one 

 another, so that each lamina has actually a single set of V-shaped 

 filaments, the outer limbs of which go to form the outer lamella, 

 their inner limbs the inner lamella. Between the filaments, and 



