XI.] THE FRESH- WATER MUSSEL. 119 



a. Tease out a bit of the liver in water, and ex- 

 amine with J- obj. It is composed of branched 

 caecal tubes lined by a layer of brownish epi- 

 thelial cells. 



10. Reproductive organs. 



a. The animals are dioecious, but the reproductive 

 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. 



11. Muscular system. 



a. This is most readily dissected out in a specimen 

 which has been hardened in spirit. The chief 

 muscles are : 



a. The anterior and posterior adductor muscles 

 which pass directly from one valve of the shell 

 to the other. These have already been seen. 

 /3. 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. 



7. 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. 



