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© James Wood

The Sepiida are the cuttlefish, which have short, broad, flattened bodies. They are good swimmers, although not as fast as the more streamlined oceanic squid. The cuttlefish shell, although internal, still functions in providing buoyancy. The spaces between the septa contain fluid and air. By regulating the proportion of these, buoyancy can be regulated. Light plays an important role in this, with buoyancy decreasing when the animal is exposed to light and increasing again in the dark. This means that the animal lies buried in the sand during the day and rises in the water column at night, becoming active and searching for food. Like both Nautilus and the squid, cuttlefish are predatory, grabbing prey with their tentacles and tearing it apart with their jaws.

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