Chap. XVI.] THE FRESH-WATER MUSSEL. 311 



Thus in the region through which D. passes, there is a 

 common supra-branchial chamber for the inner and outer gills 

 of each side j in the region of C. this is sub-divided into three 



A 



pal. m. 



Fio. 91. MUSSEL TRANSVERSE SECTIONS : GILLS. 



A. Through the heart and posterior end of the foot. 



B. Through the middle of the foot near the anterior end of the gills. 



C. Through the posterior adductor muscle. 



D. Just behind the posterior adductor muscle. 



au. Auricle, c. 6. s. Central blood sinus. /. Foot. g. a. Gill axis. i. &. c. 

 Infra-branchial chamber, i. g. Imier gill. Ig. Ligament, n. c. Nerve com- 

 missure, o. g. Outer gill. p. c. Pericardial chamber, pal. Mantle, pal. m. 

 Pallial muscle, re. Renal organ, s. &. c. Supra-branchial chamber, sh. 

 Shell, ty. Typhlosole of rectum, ve. Ventricle, ves. Vestibule of renal 

 organ. 



In B. C. D. the cavity of the supra-branchial chamber is shaded. 



parts, one median, common to the two inner gills, and two 

 lateral, one for each outer gill : in the region of A. the median 

 chamber is (a) becoming sub-divided by the foot, and (b) is in 

 communication with the infra-branchial chamber. Finally, in 



