Squid and cuttlefish are both mollusks of the class Cephalopoda, which includes the squid, cuttlefish, octopus, and nautilus.
Both squid and cuttlefish have internal remnants of their ancient external shells, but these hard structures look quite different. Squid have a flexible, feather-shaped structure inside their bodies called the pen, where cuttlefish have a broader internal shell called the cuttlebone. The cuttlebone is porous and helps the cuttlefish stay buoyant underwater.
If you don’t want to dissect an unknown cephalopod to check its internal shell, just watch it move underwater. Squid are fast-moving predators, where cuttlefish are slower and move by undulating long fins on the sides of their bodies. You can also gaze into their eyes to tell them apart: squid have round pupils, where cuttlefish pupils are W-shaped.
And perhaps the easiest indicator of all? Squid have sleek, torpedo-shaped bodies, compared to the broader, stout body of the cuttlefish. (x)
The difference is cephalopod arms have suckers all the way along their length, but tentacles only have suckers at the end (except for nautilus tentacles, which have no suckers).
Arm:
Tentacle:
So octopus have 8 arms and no tentacles, squid and cuttlefish have 8 arms and 2 tentacles (except female cuttlefish, which have 6 arms), and nautilus have around 90 tentacles and no arms.
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