Cnidocytes contain specialized structures called nematocysts, which look like miniature light bulbs with a coiled thread inside. As the polyp grows, it begins developing branches of other feeding individuals, thus forming a new generation of polyps by asexual budding. At least four hypotheses have been proposed:, Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles when capturing a brine shrimp. Juveniles of all groups are generally planktonic, and in most species, resemble miniature adult cydippids, gradually developing their adult body forms as they grow. Sponges do not have nervous, digestive or circulatory systems. There are four types of larvae, but all are balls of cells with an outer layer of cells whose flagellae or cilia enable the larvae to move. , In ctenophores, bioluminescence is caused by the activation of calcium-activated proteins named photoproteins in cells called photocytes, which are often confined to the meridional canals that underlie the eight comb rows. , Within the Cnidaria, the Anthozoa (sea anemones and corals) are regarded as the sister-group of the rest, which suggests that the earliest cnidarians were sessile polyps with no medusa stage. In the genome of Mnemiopsis leidyi ten genes encode photoproteins. They are nearly all marine with only a few known freshwater species of Cnidarians. Like those of cnidarians, (jellyfish, sea anemones, etc. Several more recent studies comparing complete sequenced genomes of ctenophores with other sequenced animal genomes have also supported ctenophores as the sister lineage to all other animals. Obelia has a worldwide distribution except the high-arctic and Antarctic seas. In species for which suspended food particles are important, the tentacles and oral arms often have rows of cilia whose beating creates currents that flow towards the mouth, and some produce nets of mucus to trap particles. The Nuda contains only one order (Beroida) and family (Beroidae), and two genera, Beroe (several species) and Neis (one species).  The larvae, called planulae, swim or crawl by means of cilia. All sponges are sessile aquatic animals. There are two known species, with worldwide distribution in warm, and warm-temperate waters: Cestum veneris ("Venus' girdle") is among the largest ctenophores – up to 1.5 meters (4.9 ft) long, and can undulate slowly or quite rapidly. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea. If they contact another sponge of the same species, the water flow carries them to choanocytes that engulf them but, instead of digesting them, metamorphose to an ameboid form and carry the sperm through the mesohyl to eggs, which in most cases engulf the carrier and its cargo. More commonly, the mesohyl is stiffened by mineral spicules, by spongin fibers or both. Like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. A few species of sponge that live in food-poor environments have become carnivores that prey mainly on small crustaceans. However a few also have massive external skeletons made of aragonite, another form of calcium carbonate. However, it is unclear how the other groups acquired the medusa stage, since Hydrozoa form medusae by budding from the side of the polyp while the other Medusozoa do so by splitting them off from the tip of the polyp. Unlike other animals, they lack true tissues and organs, and have no body symmetry. The animals produce large quantities of antioxidants to neutralize the excess oxygen.  As such, the Ctenophora appear to be a basal diploblast clade. Figure 7.3: Grantia longitudinal section. Polymorphism refers to the occurrence of structurally and functionally more than two different types of individuals within the same organism. Their bodies consist of a mass of jelly, with a layer two cells thick on the outside, and another lining the internal cavity. Biologists are especially interested in Hydra because of their regenerative ability — they do not appear to die of old age, or indeed to age at all. ", "Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China", "Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA", "The unique skeleton of siliceous sponges (Porifera; Hexactinellida and Demospongiae) that evolved first from the Urmetazoa during the Proterozoic: a review", "Cladistic analysis of Medusozoa and cnidarian evolution", "First Complete Mitochondrial Genome Sequence from a Box Jellyfish Reveals a Highly Fragmented Linear Architecture and Insights into Telomere Evolution", "Synopsis of edible jellyfishes collected from Southeast Asia, with notes on jellyfish fisheries", "Jellyfish Responsible for Irukandji Syndrome". If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the mesoglea to increase its bulk and decrease its density, to avoid sinking. Some have no polyp stages and some (e.g.  Fringing reefs just below low-tide level also have a mutually beneficial relationship with mangrove forests at high-tide level and seagrass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes of silt, fresh water and pollutants.
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