Necks in The Animal Kingdom
The neck appears in some of the earliest of tetrapod fossils, and the functionality provided has led to its being retained in all land vertebrates as well as marine-adapted tetrapods such as turtles, seals, and penguins. Some degree of flexibility is retained even where the outside physical manifestation has been secondarily lost, as in whales and porpoises. A morphologically functioning neck also appears among insects. Its absence in fish and aquatic arthropods is notable, as many have life stations similar to a terrestrial or tetrapod counterpart, or could othewise make use of the added flexibility.
The word "neck" is sometimes used as a convenience to refer to the region behind the head in some snails, gastropod mollusks, even though there is no clear distinction between this area, the head area, and the rest of the body.
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