XL] THE FRESH-WATER MUSSEL. 123 



TO. Keproductive organs. 



a. The animals are dioecious, but the reproduc- 

 tive organs are similarly constructed in both 

 sexes : they vary much in size with the season, 

 being large in winter and spring, but small at 

 other times. 



b. Close to the external opening of the organ of 

 Bojanus will be found another small opening 

 on each side, this is the generative opening. 



c. From the generative opening can be traced 

 back a duct, which divides into many caecal 

 branches which lie in the upper part of the 

 foot. 



ii. Muscular system. 



a. This is most readily dissected out in a spe- 

 cimen which has been hardened in spirit. The 

 chief muscles are : 



a. The anterior and posterior addtictor muscles 

 which pass directly from one valve of the 

 shell to the other. These have already been 

 seen. 



/?. The posterior retractor of the foot : this can 

 readily be found, on each side, running into 

 the foot from its attachment to the shell in 

 front of the posterior adductor muscle. 



y. The anterior retractor of the foot: this runs 

 from its attachment to the shell behind the 

 anterior adductor muscle, into the front of 

 the foot. 



B. The protractor of the foot arises from the 

 inner surface of the shell behind the organ 



