Jellyfish system. Nematocysts, which deliver the sting, are located mostly on the tentacles; true jellyfish also have them around the mouth and stomach. Jellyfish do not need a respiratory system because sufficient oxygen diffuses through the epidermis. They have limited control over their movement, but can navigate with the pulsations of the bell-like body; some species are active swimmers most of the time, while others largely drift. The rhopalia contain rudimentary sense organs which are able to detect light, water-borne vibrations, odour and orientation. A loose network of nerves called a "nerve net" is located in the epidermis. Although traditionally thought
Helmet jellyfish Helmet jellyfish The helmet jellyfish ("Periphylla periphylla") is a luminescent, red-colored jellyfish of the deep sea, belonging to the order Coronatae of the phylum Cnidaria. It is the only species in the monotypic genus "Periphylla." "P. periphylla" represents an exception, very rarely found in the phylum "Cnidaria": the medusae do not go through a polyp stage, thus presenting a "holopelagic" life cycle. The medusae strew fertilized eggs in open water and these develop directly into medusae, whose development rests entirely upon the egg's high yolk supply. The ephyra stage common among other jellyfish is not observed in "P. periphylla". The