THE RIVER MUSSEL. 145 



of the body. Observe that they are thin, ribbed and per- 

 forated folds of membrane extending longitudinally beside 

 the body, a pair on either side. Between the two pairs 

 observe the soft, white, muscular abdomen, extending 

 downward and forward, and terminating insensibly in the 

 retracted and minutely wrinkled foot. 



Examine the surface of the gills with a lens. 



Lift with forceps the lower edge of one of the gills (if 

 the outer gill appear dark and distended, lift the inner 

 one), stick a sharp knife or scalpel through it, and make 

 a vertical slit out to the edge. Then look at the cut edge 

 of the piece held in the forceps, and see that the gill 

 membrane is double and V-shaped in cross section, and is 

 suspended by the arms of the V from above. Insert the 

 point of a knife blade into the end of the piece held in the 

 forceps, and split it lengthwise. Then take a small piece 

 of the single gill membrane thus obtained, and mount, and 

 examine it (without cover glass) with low power. Note 

 the shape and arrangement of the openings in the mem- 

 brane, the location and action of the cilia. 



Pass a bristle or other probe into the cloacal chamber 

 from its siphonal orifice. Observe that this chamber lies 

 entirely above the gills, and that it has no communication 

 with the branchial chamber below, except through the gills. 



Lay open the cloacal chamber for half an inch by a 

 single cut with scissors through the wall of the cloacal 

 siphon. Observe the minute openings leading from the 

 floor of the cloacal chamber down into the gills. Pass 

 a bristle into one of these, and observe that it ends blindly 

 at the lower edge of the gill. 



Make a short longitudinal cut through the posterior 

 part of the attachment of both gills to the cloacal cham- 

 ber. Then lift with forceps the cut edge of the outer 

 layer of the outer gill, and observe the numerous septa 

 or partitions extending transversely between the outer 



NEED. ZOOL. 10 



