Radial symmetry – means one half of the animal mirrors the other half. Body plan – Sac-like structure, has an internal cavity called the Gastrovascular Cavity, which has one opening (used as both the mouth and the anus) Diploblastic – body and tentacles have two cell layers. There is the endoderm and ectoderm and the mesoglea layer is found in between them. The mesoglea layer can just be there to hold the other two layers together, or it can make up the majority of the animal.
Polypoid Tentacles and mouth are face up The animal is attached to the ground (eg. Rock) Coral and anemones are Polypoid in structure Medusoid Mouth and tentacles are face down Free swimmers Jellyfish are Medusoid in structure
Many Cnidarian species can reproduce using sexual and asexual reproduction methods. Sexual reproduction is used by fertilizing gametes externally by spawning them in to the water. Sexual reproduction is done by adults with usually opposite sexes although some are found same sexes. Asexual reproduction is done by cloning with budding, fragmentation, and fission. True Jellies (Cnidarian) go through two body forms during the life stage. The asexual polyp and sexual medusa.
The adult jellyfish in the medusa form reproduce as male releases sperm in water and female collect the sperm to her mouth to hold her eggs. Fertilized eggs grow in to larvae and detach from the mom and drift through water, settling on to the sea bottom. Such organisms called polyps now reproduce asexually by diving or budding. Ephyra, cloned buds, swim away from polyp base and eventually grow in to adult medusa. After becoming the adult medusa, the two shape cycle repeats all over again.
Many polyps are found in shallow water, but anemones (also polyps) are found in deep water too. Medusae drift in currents, mostly in surface water of bays and coasts, but sometimes in the open ocean.
Nematocyts are “coiled thread-like stingers”. The nematocysts are found in cells called cnidoblasts, which are on tentacles, but may also be on other layers. The nematocysts can act as defense and offense. Nematocysts can paralyze and kill animals that are much bigger than the animal itself. There are small sensory hairs close to the nematocysts, which are sensitive to vibrations in the water. The nematocyst is triggered when a prey swims by. The nematocyst becomes straight and shoots out a “barb”, which injects the prey with venom, then a tentacle brings the prey toward the mouth.
Some of the chemicals that coral produce helps scientists study cancer and anti-cancer Cnidarians can be several tonnes in weight Cnidarians are carnivorous A Cnidarian is the longest animal in the world, can reach up to 120 feet in length
Includes sea anemones, coral Polypoid cnidarians Don’t have medusa stage, they release sperm and eggs that form planula, which attaches to substrate, and cnidarian grows. Some Anthozoans can reproduce asexually through budding Mostly feed on plankton, some fish Approx. 6,000 species
Coral are threatened by acidification in the ocean Coral are a rich source of food Coral provide protection for shores Most coral form colonies Tubular body with mouth and ring of tentacles (which are hollow) Long diverse fossil record extends back around 550 million years
“True jellyfish” Drifting and of the medusoid body form Size can range from 12 mm to 2 m across (Cyanea arctica is over 40 m long!) They have no head, skeleton, or special organs for breathing or excreting Scyphozoa have a more developed nervous system than other cnidarians; a nerve ring circling the edge of the bell-shaped part of the body connects with nerves all over the body, allowing for quick nerve signalling and creating movement through bell contraction and release.
The sting of a scyphozoan can cause rashes, cramps, and sometimes, although not very often, human death. Divided into 4 orders: Stauromedusae are cup-shaped and are not free swimming Corontae are deep-dwelling Semaeostomae are disc-shaped as well as the largest and most common Rhizostomae have no tentacles, but eight oral arms fused in the middle Cubozoa, box-shaped jellyfish, used to be an order, but are not a sister group to scyphozoa.
Most hydrozoans are marine but some cnidarians have adapted to fresh water. Most hydrozoans alternate between polyp and medusa stage. They spend lives as what it looks like “jellyfish” Many hydrozoans are colonial. They form massive colonies depending on specialized shapes and functions. Some examples or hydrozoan colonies include Hydractinia, forming crusts on shells occupied by hermit crabs and defend the crab from predators.
Scyphozoa (the “true” jellyfish) and hydrozoans differ when the polyp stage is left with the medusa absent. Usually the medusa (also known as a sporosarc in this case) never breaks away from the polyp, and the medusa stage is large when the polyp stage is small. The hydrozoan medusae have a muscular shelf known as velum. This is not found in “true jellies” or scyphozoans. Hydrozoans lack some cells in the mesoglea (jelly layer found between the basic cell layers
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