Nervous system
Nervous systems are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.
Pelagia noctiluca
"P. noctiluca" is a member of the cnidarian class Scyphozoa and adapted to a pelagic mode of life. Whereas most scyphozoan jellyfish (also called scyphomedusae) have a complex life cycle with both the pelagic (swimming) jellyfish, or medusa, stage and a bottom-living polyp stage, "Pelagia" has adapted in such a way that the polyp stage appears absent, thus direct development exists. The male and female jellyfish spawn respectively sperm and eggs, which develop directly into young (male or female) "Pelagia noctiluca" jellyfish. These organisms have a well-developed manubrium, a proboscis-like structure bearing the mouth and four long oral arms. Also the mesoglea, or jelly, is relatively thickened and well developed in this species. Sense organs, known as rhopalia in the scyphomedusae, are located around the umbrella margin in notches and alternate between tentacles. Cnidae are present in the epidermis and gastrodermis of the umbrella, as well as on the tentacles.