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Nudibranchs are a group of sea slugs characterized by having no shell, thus having their gills exposed. The name in latin, nudibranchia means precisely that: nudi (nude, exposed) + branchia (gills).
Instead of the protective shell, the nudibranchs use other intricate defense mechanisms and that's one of the reasons they are such an interesting group to study. Not to mention their exquisite appearance.

More info about this species:
www.seaslugforum.net/find/pruvpsel
hypselodoris.blogspot.com/2009/09/pruvotfolia-pselliotes-...

Sacoglossans are a group of sea slugs characterized by feeding on algae in a peculiar way: their radular teeth cut the algae cells open and the slug sucks the interior. The name of the group derives from the shape of the radular teeth: saco (scythe) + glossa (tongue).
The most peculiar thing about these animals is that they don't digest the chloroplasts from the algae and actually integrate them in the animal's tissues. Even more amazingly, the chloroplasts keep performing photosynthesis and these animals can survive for months relying only on solar power. This ability is unique among animals.

More info about this species:

www.seaslugforum.net/showall/elysviri
hypselodoris.blogspot.com/2011/03/elysia-viridis-montagu-...

Cephalaspideans or headshield slugs are a group of sea slugs characterized by having a broad head often used to plow under the sediment and a headshiled which prevents the sand from entering the mantle cavity. They are often fierce predators of other marine invertebrates such as nudibranchs.

More info about this species:
www.seaslugforum.net/find/philaper

Cephalaspideans or headshield slugs are a group of sea slugs characterized by having a broad head often used to plow under the sediment and a headshiled which prevents the sand from entering the mantle cavity. They are often fierce predators of other marine invertebrates such as nudibranchs.

More info about this species:
www.seaslugforum.net/find/aglatric

Armina loveni or A. tigrina or A. neapolitana