EXAMINATION OF UNIO OR ANODONTA. 295 



forward into the glandular chamber may be made to pass 

 through a small opening at its anterior end into the peri- 

 cardium. 



(d.} The bristle which has been introduced into the 

 non-glandular part will, on the other hand, be found to 

 pass through an opening which communicates with the 

 cloacal chamber (&) of the inner gill. 



5. The relations of these various chambers should also 

 be examined in more anterior sections, especially one just 

 anterior to the heart. 



6. Suspended between the gills notice the large abdo.- 

 men (Fig. 148, u). 



(i.) The wall of this organ is a whitish integument 

 which is composed of an external layer of epithelium and 

 an inner layer of muscular fibres. 



(ii.) At the bottom or free end of the abdomen the 

 muscular fibres are more numerous, and form a muscular 

 foot. In the plane of this section the foot is quite small 

 or wanting, but further forward it is a conspicuous struc- 

 ture. 



(iii.) The cavity of the abdomen is traversed in all 

 directions by a loose white network of connective tissue, 

 and the meshes of the network are almost entirely filled 

 by the white or brownish reproductive organs. In various 

 parts of different sections of the abdomen, sections of the 

 various folds of the intestine will also be seen. 



7. Make a sketch of the section, showing the above 

 points. 



IV. Sections through the middle and the anterior por- 

 tion of the abdomen should also be examined and sketched, 

 although they will be readily understood without explana- 

 tion. 



1. In that through the middle of the abdomen the ex- 



