XI.] THE FRESH-WATER MUSSEL. 115 



part which lies within the non-glandular: the 

 bristle will be found to leave the passage in the 

 glandular portion by an aperture, which puts it 

 in communication with the non-glandular part, 

 and is situated on the upper side of the glan- 

 dular part, opposite the posterior end of the peri- 

 cardium. The glandular part extends back some 

 way beyond this point ; but it is imbedded closely 

 in the neighbouring tissues, and is not contained 

 in the loose non-glandular sac, which reaches back 

 no farther than the posterior end of the pericar- 

 dium. 



d. Examine the floor of the non-glandular part, at its 

 anterior end : in it will be found a small aperture ; 

 gently push a guarded bristle through this : then 

 turn the animal over, and detach the front end of 

 the inner gill on the same side, from the foot. 

 The bristle will be found to have passed out by 

 an aperture (external opening of the organ of- 

 Bojanus) which lies just above the attachment of 

 the gill to the body. 



6. The gills or branchiae. 



a. Cut out one of the gills and examine it ; it will be 

 found to consist of two lamellse united by their 

 ventral edges and enclosing a central cavity, which 

 opens into the cloacal chamber. The cavity be- 

 tween the lamellaB is subdivided by transverse 

 partitions, which pass from one lamella to the 

 other. 



6. Carefully cut out a bit of the wall of the gill-sac 

 on one side ; mount in water and. examine with 

 1 inch obj. The outer surface will be seen to be 



82 



