CH. Vl] THE FRESH-WATER MUSSEL 169 



animal are very sluggish, and are accomplished in a manner 

 very different from the gliding motion of a gastropod. The 

 rate of progression of Unio margaritifer has been recorded 

 by Boycott and Howell as about 15 feet a day, or roughly 

 a mile a year. I have noticed that specimens kept in 

 aquaria do not, as a rule, move during the daytime, but 



12 13 



15 Ima ^ atm , rmll . 



^^ 



6' 



16 



Fig. 31. Bight side of Anodonta mutabilis with the mantle cut away and 

 the right gills folded back x about 1. From Hatschek and Cori. 



1. Mouth. 2. Anus. 3. Cerebro-pleural ganglion. 4. Anterior 

 adductor muscle. 5. Anterior protractor muscle. 6. Retractor 

 muscle. 7. Dorsal siphon. 8. Inner labial palp. 9. Foot. 

 10. External opening of nephridium or organ of Bojanus. 11. Open- 

 ing of genital duct. 12. Outer right gill-plate. 13. Inner right 

 gill-plate. 14. Ventral siphon. 15. Epibranchial chamber, 



the inner lamellae of the right and left inner gills having been slit 

 apart. 16. Posterior protractor muscle. 



become active a few hours after sunset, and further that 

 the activity of one individual almost invariably appears 

 to stimulate its companions to exert themselves, even 

 though they may be situated several inches apart. 



The organ of locomotion is the "foot," a strong muscular 

 and bluntly hatchet-shaped projection of the mid- ventral 

 surface. When at rest the foot is concealed within the 



