Squid

A Quick One: As a cephalopod, the squid is one of the largest, fastest and most intelligent of all invertebrates. Most squid bodies are torpedo-shaped; this helps propel them swiftly through the water. Squids swim by sucking water into their mantle cavity (a hollow space surrounding the internal organs) and quickly expelling it out of a siphon.

Unusual Anatomy: Unlike most mollusks, squids lack an outer shell; instead, they have an internal shell, or "pen," that supports the body and to which the muscles are attached. Squid have eight arms and two long, nonretractable tentacles that they use to catch prey.

Deep Sea Squid: Some squid species find safety in numbers, while others — especially those that live in the deep sea — hunt alone. A number of deep sea squid are bioluminescent; some even discharge bioluminescent ink clouds. The world's largest invertebrates — the giant squid and the colossal squid, which can reach 46 feet in length — live in deep ocean waters near Antarctica.