Cnidarians in the class Hydrozoa go through a polyp and a medusa life stage, while Anthozoa go through only the polyp stage and Scyphozoa see only the medusa stage. Coral is a cnidarian that goes through only the polyp phase, while the Portuguese man-o-war experiences both. Organisms in a polyp stage have tentacles and a mouth that generally faces up. They are either connected to an object or affixed to a large group of other polyps. They are not free floating and are attached to one place. Adult sponges are very similar to cnidarians in this stage because they are also attached to one site and cannot move.
Sponges are simple organisms with many different species making up the phylum Porifera. Cnidarians comprise thousands of different organisms including hydroids, jellyfish, anemones and corals and are broken up into three groups: Anthozoa, Scyphozoa and Hydrozoa. There are some similarities and differences between the life cycle of a sponge and the life cycle of a cnidarian.
Cnidarians may also go through a life phase called the medusa stage. During the medusa stage a cnidarian has a mouth and tentacles that face down toward the water bottom. The organisms are no longer be attached to one place but become free floating. The best example of a cnidarian in the medusa stage would be a jellyfish. Sponges never have a free floating life cycle like cnidarians. Some species of sponge may be able to move slightly with water currents, but they never are able to float freely.
Sponges can reproduce both asexually and sexually. Sponges are hermaphrodites, which gives them the ability to produce eggs and sperm. Eggs or sperm are released at different times to allow for fertilization with other sponges. Different sponges release either sperm or eggs, and once fertilized, the larva eventually floats to the ocean bottom where it becomes an adult sponge. Sponges reproduce asexually by forming buds and having those buds break off, with the broken pieces eventually becoming a new sponge. Cnidarians also have the ability to reproduce asexually and sexually. Cnidarians reproduce asexually by budding, which normally occurs during the polyp life cycle. Sexual reproduction takes place in the medusa life stage.
Various species of sponges live for different periods of time. Some species live only for a year, while other species continually regenerate, making it difficult to estimate how long they survive in waters. The same thing is true with cnidarians. Coral can live for many years if it is not harmed by humans, while jellyfish do not live nearly as long.
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