MOLLUSCS. 



203 



cavity with the outer world (fig. 41). In some these 

 siphons may be greatly developed as long tubes and 





FIG. 41. Quahog (Venus mercenaria) , with foot and siphons extended. 



then strong retractor muscles to draw them back are 

 present. All of these muscles adductors, retractors, etc. 



-leave their impress on 

 the shell, so that the 

 student, with the shell 

 alone, may know of some 



/ 



of the structures of the soft 

 parts (fig. 42). 



Water is drawn into the 

 mantle-cavity by means 



of very minute hair-like FlG - 42. inside of bivalve shell 



ing muscular impressions of a, an- 



StrUCtlireS (Cilia} Which terior adductor; p, posterior adduc- 



tor; s, siphonal muscle. 



cover the gills and other 



parts. These cilia are in constant motion,* and thus 



currents of water are produced, flowing always in one 



* The teacher should demonstrate this ciliary action under the 

 compound microscope, 



