XIII] ANODONTA 303 



moves only when for some cause the water becomes unsuitable for 

 its purposes. 



Since the upturned ends of the inner rows of filaments of both 

 ctenidia are united behind the foot, a bridge is formed dividing the 

 mantle cavity into an upper or epibranchialandalower or hypo- 

 branchial division. The gaping opening between the mantle lobes 

 at the posterior end is similarly divided into an upper portion, the 

 dorsal siphon, and a lower, the ventral siphon. Since it is the 





18 



FIG. 139. Right side of Anodonta mutabilis, dissected to show the viscera 

 x about 1. From Hatschek and Cori. 



1. Cerebro-pleural ganglion. 2. Cerebro-pedal commissure. 3. Oeso- 



phagus. 4. Anterior protractor muscle. 5. Liver. 6. Stomach. 

 7. Aorta. 8. External opening of organ of Bojanus or kidney. 



9. Internal opening of the same. 10. Pericardium. 11. Eight 



auricle. 12. Posterior end of ventricle passing into posterior aorta. 



13. Rectum. 14. Glandular part of kidney. 15. Anus. 



16. Opening of epibranchial chamber. 17. Ventral siphon. 18. Edge 

 of shell. 19. Cerebro-visceral commissure. 20. Intestine. 21. Foot. 

 22. Reproductive organs. 23. Pedal ganglion of right side. 24. Mouth. 

 25. Opening of the reproductive organ. 



outer lower surfaces of the filaments which are clothed with cilia, 

 it is into the ventral siphon and hypobranehial chamber that the" 

 current passes. The lips of both siphons especially the ventral 

 siphon are plentifully beset with small papillae, which are sensitive 

 to light and shade. If the shadow of the hand be allowed to pass 

 over them the mantle edges are instantly drawn together and the 

 siphons thus closed. In the Scallop (Pecteri) similar papillae are 

 developed into well-formed eyes. Part of the water passes through 



