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The animal on the cover of Squid: The Definitive Guide is a giant squid (Architeuthis dux). Of the class Cephalopoda, which means “head foot,” the giant squid holds much fascination for humans, part of which has to do with the fact that it has never been observed alive in its natural habitat. Scientists have only been able to study specimens that have been caught or found washed up on beaches. This invertebrate can grow to 60 feet in length and weigh as much as a ton. It’s a deep-sea dweller (660-2,300 feet) that is found throughout the world’s oceans.

A giant squid consists of seven parts. Its head houses a complex brain. Its eyes are the largest in the animal kingdom-up to 10 inches in diameter. (Most deep-sea animals have very large eyes so they can gather the small amounts of light available in the depths of the ocean.) Its fins are relatively small and help it to balance and maneuver as it swims. Its main body is called a mantle: it’s a muscular sac that contains most of the organ systems. Its eight arms are studded with two rows of suckers; it also has two much longer feeding tentacles, the ends of which also have suckers and are called clubs. Finally, its funnel is a multipurpose tube used to breathe, squirt ink, lay eggs, ...

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