 Where the outer whorl of an ammonite shell largely covers the preceding whorls, the specimen is said to be "involute" (e.g., "Anahoplites").Where it does not cover those preceding, the specimen is said to be "evolute" (e.g., "Dactylioceras").A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers.