FRESH-WATER MUSSEL. 59 



shell. Remove the right valve. On Us inner 

 surface find: 

 9. The Hue of attachment of mantle to shell. 



10. Impressions of muscle-attachments. 

 Near the anterior end : 



(a) A large impression made by the anterior 



adductor and the anterior retractor mus- 

 cles. 



(b) A. small impression near'(a), that of the 



protractor muscle. 

 Near the posterior end : 



(c) A large impression, of the posterior adduc- 



tor and the posterior retractor muscles. 

 As the mussel lies on the left valve, find : 



11. The muscles whose impressions have been 



seen in 10. Observe : 



12. The mantle enclosing the other parts of the 



animal. 



13. The edges of the mantle closely applied ex- 



cept at the posterior extremity, where they 

 separate to leave two orifices ; 



(a) A smaller dorsal opening (through which 

 water passes out), the exhalent aperture. 



(b) A larger opening more ventral (through 



which water enters), the inhalent aperture. 

 Diagram ! 



Insert a bristle into each opening, and tlien raising 

 the flap of the mantle see : 



14. Inserted into the more ventral opening it 



