GENERAL ANATOMY OF A LAMELLIBRANCH. 269 



granular, and continuous with a thin layer of granular 

 protoplasm which invests the nucleus (e). 



d. The latter is united by a short thread of granular 

 protoplasm (d) to a ganglion cell (e), from the opposite 

 end of which a nerve fibre (f) originates and runs down 

 into the auditory nerve, and so to the third thoracic gan- 

 glion of the central nerve cord. 



XXV. THE GENERAL ANATOMY OF A LAMELLI- 

 BRANCH. 



THE following description is strictly applicable only to 

 the fresh-water genus Anodonta, but any of the Unionidaj 

 may be used for laboratory work, or if these are not to 

 be had, the common long clam (Mya) or the round clam 

 (Venus) may be used instead. Mya and Venus may be 

 obtained of the fish-dealers in most of our cities, and Unio 

 and Anodonta may usually be found in abundance in most 

 ponds, lakes and streams. Either fresh or preserved 

 specimens may be used. The valves of the shell of a 

 living specimen are usually so tightly closed that some 

 difficulty may be found in opening them. The best plan 

 is to place them in warm water about fifty-five or sixty 

 degrees centigrade for a few minutes. The muscles 

 will then relax enough to allow the blade of a scalpel to 

 be introduced to cut their attachment to the inside of the 

 shell. After the specimen has been opened it should be 

 placed in a dish of water, or water and alcohol, and all 

 the dissecting should be performed while the specimen is 

 submerged. The addition of alcohol to the water is a 

 great help, since transparent parts are rendered opaque 

 and visible by it, and it also coagulates the slime which 

 covers the body, arid thus facilitates the work. 



