Nereis is a genus of polychaete worms in the family Nereidae. It comprises many species, most of which are marine, including the sandworm (Nereis virens) and the common clam worm (Nereis succinea). Nereis possess setae and parapodia for locomotion. They may have two types of setae, which are found on the parapodia. Acicular setae provide support. Locomotor chaetae are for crawling, and are the bristles that are visible on the polchaeta's exterior
Nereis are osmoconformers.
Nereids are dioecious and they release their haploid gametes into the water medium, a process called spawning. Moreover, Nereids upon fertilisation and mitotic divisions of the zygote, a larval stage forms which is similar to that of Molluscs, ie a Trocophore larva. This last paragraph was added by David Agius, St. Aloysius' College Malta