Flatworm or nudibranch/slug?
How to tell them apart?
updated Apr 2020

These two kinds of animals are often mistaken for one another. They are both rather flat and many are brightly coloured. There are also all kinds of other animals that resemble slugs and flat worms.

Phylum Platyhelminthes
Nudibranchs and slugs
Phylum Molusca
Flatworms are really REALLY flat. Nudibranchs and slugs are not so flat.
Flatworms tend to move faster. Nudibranchs and slugs are not so fast.
Most of the flatworms we see on our shores don't have 'real' tentacles on their head. Most only have pseduotentacles made out of folds of the body margin Nudibranchs and slugs have 'real' tentacles. But these can be retracted into the body, or may be small and obscured.
Flatworms don't have flowery gills on their backs. Some nudibranchs have flowery gills on their backs. But these can be retracted into the body. Slugs and some other nudibranch, however, do not have external flowery gills.
Flatworms are delicate and tear easily.
Don't pick them up.
Nudibranch are not as delicate, but it's still best not to pick them up.
Flatworms are unsegmented worms belonging to Phylum Platyhelminthes. Nudibranchs and slugs belong to Phylum Mollusca.

More comparisons

Some flatworms pseudo tentacles made out of folded edges of the body margins. Some may also have tiny bumps sticking out of their bodies.

In others, the pseudotentacles are
not so obvious

Nudibranchs have
'real' tentacles

The Leaf slug has tentacles
but no external gills.

Onch slugs also have tentacles
but no external shell.

The Hoof-shield limpet has a shell but
its body is much larger than the shell!

Ribbon worms are flat but very long.

Some bristleworms are flat,
but they have lots of bristles!

Tiny flatfishes are sometimes
mistaken for flatworms!

Cowries often cover their shells with a
'hairy' mantle and are thus mistaken for slugs.

Limpets when covered with 'furry' algae
may sometimes be mistaken for slugs.

Moon snails often cover their shells with
their mantle so that they appear slug-like.

how to tell apart hairy slugs and snails
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