Squid (Rich Carey / shutterstock.com)

Squids are soft-bodied and generally have eight powerful arms and two long tentacles for catching prey. Like octopuses they have a highly developed nervous system. In contrast to octopuses, their mantle has two swimming fins along each side. Furthermore, the sucker rings of squids are armed with extra hooks that provide additional gripping power. Squids range from 2.5 centimeters (1 inch, Idiosepius sp.) to 20 meters (7.9 feet, giant squid) in length.

skull and crossbones
Some species venomous.

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This supplemental online resource accompanies the book Scuba Diving Hand Signals by Lars Behnke, available on Amazon.