Tentacles vs arms

You might have come across confusing statements like "octopuses have arms, squids have tentacles" and wondered what's the difference. I did and here's the (unsatisfactory) answer.

Cephalopods (squid, octopus, nautilus and a number of other aquatic creatures, which are a class of mollusc) have a number of muscular "limbs". Traditionally experts make the distinction between arms and tentacles. For example:

• Octopuses have eight arms and no tentacles
• Squid and cuttlefish have eight arms and two tentacles
• Nautiluses have around 90 (suckerless) tentacles

What's the difference?

• Arms occur around the mouth
• Arms have suckers all the way along, while tentacles have them only at the tips.
• Arms have finer control
• Tentacles have an elongated shape and are longer than arms
• Tentacles are mainly used to catch prey with the arms assisting and grasping
• The internal structure of tentacles and arms differ.

Easy? Let's confuse the issue:

• Variation and anomalies are common within cephalopods, including missing or extra arms, forking limbs, etc.
• The tentacles of squid and cuttlefish appear to be evolutionarily derived from arms
• It's not clear (to me anyway) that what's is called a tentacle in a nautilus is anything to do with a tentacle on a squid.
• It's also unclear that a cephalopod "arm" is in any way related to the arms or limbs found on better known animals like mammals.
• Biologists are prone to getting lazy and using the words interchangeably.