THE FRESH-WATER MUSSEL 123 



and remove part of the right half by cutting anteriorly from the 

 middle of the incurrent siphon about 5 mm. below the attached dorsal 

 edge. Taking care not to injure other structures, continue the cut 

 neatly below the anterior adductor muscle. The foot and the four 

 platelike gills, two on each side, are now exposed. Find the palps, a 

 pair of leaflike organs on each side of the foot and attached to the 

 mantle near the anterior adductor muscle. The mouth is a transverse 

 slit between the foot and the anterior adductor; the anus is located 

 at the termination of the intestine on the posterior face of the posterior 

 adductor muscle. 



(i) Using the dried right valve make an outline and draw the fea- 

 tures observed thus far, limiting the drawing to what can be seen on 

 the surface below the cut edge of the mantle. 



Exercise 4. — The Gills. 



(j) By looking into the uninjured excurrent siphon the posterior 

 surface of the posterior adductor muscle will be seen. Below this on 

 each side are two cavities extending anteriorly. Insert a tipped bristle 

 into the outer one of these and, using the bristle as a guide, cut 

 through the outer wall and expose the floor of the cavity. This is the 

 suprabranchial chamber of the right outer gill. It opens posteriorly 

 into the excurrent chamber, a region of the mantle cavity which is 

 continuous with the excurrent siphon. Carefully cut away the entire 

 outer wall of the excurrent chamber and of the suprabranchial 

 chamber so that their extent can be seen. The floor of the suprabran- 

 chial chamber is marked by a series of transverse partitions, inter- 

 lamellar junctions, which separate cavities, the water tubes of the gill. 

 If the gill is not too shrunken, pass a bristle down any one of the water 

 tubes to the ventral edge of the gill. How does the water tube end? 

 Remove a piece of the outer gill, and cut sections, with sharp scissors 

 or scalpel, at right angles to the water tubes. Examine under water 

 with a handlens. The water tubes and interlamellar junctions will be 

 seen cut transversely. It is in the water tubes that the zygotes of the 

 mussel begin development, and you may find the gill distended with 

 embryos. By looking forward from beneath the posterior adductor 

 muscle and gently exploring with a bristle, it will be seen that the 

 inner gill of this side and the two gills on the other side each have a 

 suprabranchial chamber and water tubes, as in the gill just examined. 

 Make a diagram ( X 2) , from the lateral view, of the suprabranchial 

 chamber and excurrent chamber as thus exposed. 



(k) You should now understand the course of the water current, 

 from w^iich the mussel obtains its food and oxygen and by which its 



