| The opercula of various shells differ in the first place as to their chemical composition. |
| The third section contains the following genera of land shells with opercula. |
| The opercula of multivalve shells are composed of two or four pieces, which are called the opercular valves. |
| The figures represent some of the commoner types of opercula. |
| Acupuncture at a point commonly used to treat xerostomia is associated with activation of the insula and adjacent opercula. |
| These fish do not have gills or opercula like most bony fishes. |
