Chap. XVI.] THE FRESH-WATER MUSSEL. 307 



shell to the other, is seen the mouth. The vent opens into the 

 supra-branchial chamber close to its exhalent aperture. 



With the handle of a scalpel the mantle may now be separated 

 from its attachment to the left valve of the shell by a thickened 

 ridge, the pallial muscle. The anterior adductor muscle must be 

 cut close to the shell, and a similar muscle (posterior adductor] 

 must be cut at the hinder end. In cutting these muscles, other 

 smaller muscles will also be severed. By cutting the ligament 

 the left valve of the shell may now be removed without diffi- 

 culty. 



Its inner surface is white and glistening, with a play of iri- 

 descent colours. The scars or impressions of the adductor 

 muscles (Fig. 89, B., a. ad. i., p. ad. i.) will be readily seen, and 

 smaller impressions of the anterior retractor (a. r. i, confluent 

 with a. ad. i.), posterior retractor (p. r. i.), and protractor pedis 

 (p. p. i.). From each of these impressions there runs towards 

 the umbo a fainter tapering impression which marks the gradual 

 shifting of the muscles during the growth of the shell. The 

 impression of attachment of the pallial muscle of the mantle 

 is seen as the pallial line (pal. I.). 



If a thin section of a fragment of the shell be ground down 

 and examined under the microscope, the three layers of which 

 it is composed will be readily made out. Externally is the 

 horny periostracum ; beneath this is a prismatic layer of pallisade- 

 like prisms ; internal to this is a laminated nacreous layer. The 

 nacreous layer is secreted by the whole external epithelium of 

 the mantle. The outer layers are formed only at the free edge 

 of the mantle. 



The mussel now lies exposed to view in its right valve. The 

 left lobe of the mantle should be cut away close to its origin, 

 and the underlying parts exposed as in Fig. 90, A. The cut ends 

 of the muscles ( a. ad., p. p., a. r. ra., p. r. m., p. ad.), the mouth 

 (m.), foot (/.), and gills (i. g., o. g.}, the right mantle lobe (pal.), 

 and the inhalent and exhalent siphonal apertures (in. si., ex. si.) 

 will be readily made out. A probe (pr.) has been passed 

 through the supra-branchial chamber into the infra-branchial 

 chamber, between the left inner gill and the foot. Two labial 



