FEESHWATER MUSSEL 95 



(e) Dorsal and anterior to the preceding is the pos- 

 terior retractor. 



(f) The mantle muscle is attached along the pallial 

 line. 



2. Note the right and left mantle lohes and their relation 

 to one another. 



8. Note the inhalcnt and exhalcnt apertures and the mar- 

 gins of the mantle in this region. 



4. Fold back the left mantle lohe and distinguish the two 

 gills just beneath the mantle ; the foot, a heavy muscular struc- 

 ture ; the visceral mass, the enlarged region dorsal to the foot ; 

 the lahial palpi, two small triangular flaps at the anterior end of 

 the body. 



5. Make an outline drawing of the left side with the shell 

 and mantle removed, to show the relation of parts. Label all 

 parts. 



6. Pass the probe into the exhalent aperture and note the 

 cavity, the cloaca, which lies above the bases of gills and pos- 

 terior to the posterior adductor muscles. Above the muscles is 

 the anal opening of the intestine. The portion of the intestine 

 leading to the anus is the rectum,. 



7. Insert a probe into the mouth, which lies between the 

 anterior adductor muscle and the anterior edge of the foot. De- 

 termine the relation of outer and inner pair of palpi to the 

 mouth. 



8. Determine the relation of the two pairs of gills to one 

 another. The gills ma^' be distended with eggs and larvae in 

 some specimens. Locate the supra-brancliial chamber and de- 

 termine its relation to the exhalent aperture. 



III. Examination of Cross Sections 



1. Cut a section, about i/4 i^^ch thick, obliquely through the 

 anterior part of the pericardial cavity and the foot. Distinguish 



