Remipedia

Remipedia is a class of blind crustaceans. They live in coastal aquifers.
So long as the water is salty, these little animals can be found.
They are found in almost every ocean basin, including in Australia, the Caribbean Sea, the Atlantic Ocean. They live in subtropical waters.
The first described remipede was a fossil called "Tesnusocaris goldichi" (early Pennsylvanian). Since 1979 at least 17 living species have been identified.
Genetic studies suggest that remipedes are the arthropods most closely related to insects.
Anatomy.
Remipedes are long. They have a head and a long trunk of up to forty-two similar body segments. The swimming appendages are on the sides of each segment, and the animals swim on their backs. They are generally slow-moving.
They have fangs connected to secretory glands. It is not known whether these glands secrete digestive juices or poisonous venom. They have a primitive body plan in crustacean terms, and may be a basal, ancestral crustacean group.
At least one species, "Godzilliognomus frondosus", has an organised brain, with a particularly large olfactory area. Species which live in dark environments need to detect scents in the water. The size and complexity of the brain suggested that Remipedia might be the sister taxon to Malacostraca, regarded as the most advanced crustaceans.