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How do Cryptodira turtles pull their heads into their shells?

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Illustrations of ten representative chelonians, including spiny softshell turtle, Galapagos giant tortoise, green turtle, alligator snapping turtle, Blanding's turtle, common map turtle, eastern box turtle, eastern painted turtle, spotted turtle, and eastern mud turtle, each labeled with its common and scientific name. A scale bar at the bottom shows size comparison. — turtle Turtles (order Testudines).

Turtles of the suborder Cryptodira—a name meaning "hidden neck"—retract their heads into their shells by bending their necks in a vertical plane.12

This movement creates an S-shaped curve, allowing the animal to pull its head directly backward and inward for protection. This mechanism distinguishes them from the suborder Pleurodira, or "side-necked" turtles, which instead fold their necks horizontally to tuck the head beneath the margins of the shell.23