Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 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
Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 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,
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