Jellyfish diameter; the tentacles and mouth parts usually extend beyond this bell dimension. The smallest jellyfish are the peculiar creeping jellyfish in the genera "Staurocladia" and "Eleutheria", which have bell disks from to a few millimeters in diameter, with short tentacles that extend out beyond this, which these jellyfish use to move across the surface of seaweed or the bottoms of rocky pools. Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope; they can reproduce asexually by fission (splitting in half). Other very small jellyfish, which have bells about one millimeter, are
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