Coelenterates have a comparatively simple physiology. They are basically a sac of jelly which contains their internal organs. The sac has an opening their mouth, this is surrounded by tentacles. The tentacles have stinging structures called nematocysts, which are used to stun or paralyse their prey. After stinging, the tentacles pull the prey into the mouth. Each individual is known as a polyp. Coelenterates that are found on Cornish shores include the sea anemones, true jellyfish and the hydroids.
Coelenterates (Phylum Coelenterata or Cnidaria) include jellyfish, anemones, corals, and hydras. The phylum is characterized by a gelatinous body, tentacles, and stinging cells called nemadocysts. Most species are found in marine waters, but some occur in brackish or even fresh water.
Coelenterates may be either sessile or free swimming, depending on the species and/or stage of the life-cycle. Standard methods of introduction include ship fouling or transportation in ballast water of ocean-going vessels. Potential impacts include competition with native species for suitable substrate or food, negative effects on organisms to which they attach, utilizing native species as a food source, and "swarming", where thousands of jellyfish mass in an area and can affect the local fauna. An example of an introduced coelenterate is the Black Sea jellyfish, Maeotias inexspectata. These small purple jellyfish were introduced via ballast water from Russian ships into San Francisco Bay in 1993. They under went a population explosion and infested tributaries of the bay. This outbreak caused considerable public alarm and inspired news headlines like "Jellyfish Jam the Petaluma River".
Phylum Coelenterata like many animals, get their name from a description of their bodies. Coelenterates have a single body opening which leads into a hollow gut -- coel means "hollow" and enteron means "gut". Their body plan is circular, and the gut opening is surrounded by tentacles. Coelenterates have soft bodies and consist of two layers of cells sandwiching a jelly-like substance. In many species this jelly layer is thin, but in jellyfish it is very thick and fills most of the body.
Largest -- the jellyfish species Cyanea arctica can grow to 11.8feet 3.6m) in diameter and have tentacles 118 feet (36m) long. With 39-foot (12 m) tentacles, Portuguese men-0'-war are also huge.
Picture Of Portuguese Man-of-War
The Portuguese Man-of-War is a jelly-like marine animal but they are not jellyfish. They are actually a small colony of various creatures working together and functioning as one animal. The beauty of its long, black threads are its hunting tentacles. The tentacles (used to secure small fish for food) can grow up to be 40 feet long and can inflict an extremely painful sting on anyone who touches them, even days after the animal has died. Fortunately, the antidote is a simple mixture of vinegar and meat tenderizer and works very quickly.
Picture of Cyanea arctica
This is the largest jellyfish, found in the North Atlantic Specimens have been measured up to 7 feet 6 inches across the bell with a tentacle of 120 feet.
Most dangerous -- people have died from the venomous stings of the sea wasps Chironex fleckeri and Chiropsalmus quadrigatus, found in tropical waters off Southeast Asia.
Picture Of Chiropsalmus quadrigatus
Picture of Chironex fleckeri Pronunciation - (Ki-ron-ex fleck-er-eye)
This species is often known by the common names of: sea wasp, big stinger, sea stinger and the northern Autralian box jellyfish. It is possibly the worlds most venomous animal. This species occurs throughout the tropical waters of the Indo-Pacific. This species is almost transparent, milky white colour. In larger specimens, the base of the tentacles appear deep blue, brownish or yellow. C. fleckeri can reach up to 220mm across the bell and large specimens can have as many as 15 thick ribbon shaped tentacles hanging of each pedalium. Tentacles in this species are capable extending to more than 3 meters in length.