Common squid and common NZ octopus - courtesy of http://www.ryanphotographic.com/

Colossal squid, giant squid and octopus 

What’s the difference between a squid and an octopus?

Both squid and octopus are cephalopods, a class of molluscs.
 
A typical squid has:
a streamlined body
a clear internal shell known as a gladius or pen
a head
a mantle (which fits like a hat over the main part of the body)
eight arms
two tentacles with hooks, or sucker rings, or both
two fins 

A typical octopus has:
a round, bulbous body
no internal shell 
a head
a mantle
eight arms with suckers (never hooks)
no tentacles and no fins

What’s the difference between a colossal squid and a giant squid?

Colossal squid (Mesonychoteuthis hamiltoni) are slightly shorter than giant squid (Architeuthis dux), but have a larger, heavier body. Te Papa’s colossal squid tips the scales at a massive 490 kg. In contrast, giant squid weigh up to about 275 kg.

All squid have sharp horny beaks made of material similar to human fingernails. The colossal squid has the largest beak of any squid, including the giant squid. The tips of the colossal squid’s tentacles (the clubs) are armed with two unusual rows of sharp, swivelling hooks, and two rows of tiny suckers. Other squid species also have tentacle hooks. The beak and the hooks are lethal weapons for catching and holding large fish like the toothfish. 

More information from the blog:

> Giant squid specimen
> Giant squid examination
> Measuring the tentacles of a giant squid
> Suckers at the end of the tentacles of a giant squid
> Giant squid brain
> Giant squid dissection
> Beak sizes

Mark Fenwick thawing a giant squid specimen Mark Fenwick thawing a giant squid specimen

Related resources