Cnidaria Phylum PDF

Phylum: Cnidaria − from the Greek knide for nettle − represent one of the oldest group of animals on Earth − size of the body: tiny, but some species can have 1-2m − are diploblastic - belong to group Diblastica – have 2 germ layers: endoderm, ectoderm (but in blastocoel we can find...

Phylum: Cnidaria − from the Greek knide for nettle − represent one of the oldest group of animals on Earth − size of the body: tiny, but some species can have 1-2m − are diploblastic - belong to group Diblastica – have 2 germ layers: endoderm, ectoderm (but in blastocoel we can find some cells which has origin in mesoderm), between these layers is a jelly-like material called mesoglea − live in aquatic environments, mostly marine − radially symmetrical animals (radial symmetry = an arrangement of the body of an animal in which parts are arranged symmetrically around a central axis) − tissue level of organization – their cells create tissues (but not organs) − gastrovascular cavity (blastocoel) has a single exterior opening that serves as both mouth and anus, too − have typically a ring of tentacles surrounding the mouth region, they all have stinging cells (nematocyst) to harpoon and immobilise prey, cnidarians with these tentacles are capable even of penetrating human skin, sometimes producing a painful wound or in extreme cases, death − respiratory system – they breathe with the whole surface of body − nervous system – they have nerve net – it is a type of very simple nervous system, it consists of neurons, that are interconnected, but cnidaria don´t have any groups of neurons (ganglia) − mostly carnivorous otherwise filter feeders, they feed on protozoans, various worms, crabs, other cnidaria and even fish, some groups such as coral live symbiotically with algae − have 2 different forms exist: 1) medusa – bell shape form, with the mouth region directed downward, is adapted to a floating or free-swimming existence, at the edge of bell they have nerve and sensual cells 2) polyp - has a tubular shape, with the mouth region directed upward both body, sessile form, it reproduces usually asexually − these forms alternate and this alternation is called alternation of generations − both forms may have been a part of the life cycle of all cnidarians, because today we see an alternation of generations life cycle of these two forms in certain cnidarians, in life cycle, the polyp stage produces medusae, and the medusae, which produce eggs and sperm, disperse the species − are hermaphrodites − reproduction is: 1) asexual - budding 2) sexual – cnidarians have gonads, gametes are released by bursting of the wall of gonad, by releasing egg and sperm into the water, the eggs will be fertilized by sperm and develop into a larva called planula, the planula will then develop into a new polyp which will produce new, immature medusas called ephyra (this reproduction in special way, which is called strobilation = spontanous transverse segmentation, from each of this single segmented part a medusa is developed), medusa mature and it goes from the beginning)  − system of Cnidaria: they are divided into 4 main classes: Anthozoa (corals), Scyphozoa (jellyfish), Cubozoa (box jellies) and Hydrozoa Class: Hydrozoa − are primarily polyps, with only a brief medusoid life-stage − have huge regenerative capacity − live in oceans, but in fresh water, too − are filter feeders, feed on plankton − their medusae have velum (=a circular shelf-like rim attached to the margin of the umbrella and directed inward, it speeds up the movement) − members:  Hydra oligactis  Hydra viridis  Freshwater jellyfish (Craspedacusta sowerbyi) – you can find it static waters (for example in branches of Danube), it´s medusae can swim and it eats a lot of food Class: Cubozoa − have cube-shaped medusa − have long tentacles − their poisons are very dangerous (can cause death by human) − live in tropical oceans Class: Scyphozoa − are primarily medusoid in life style, with only a brief, insignificant polyp stage − by sexual reproduction they have planula and strobilation − have ocelli: light-sensitive organs that do not form images but are used to determine up from down, responding to sunlight shining on the water's surface − members:  Common jellyfish (Aurelia aurita) – can be found in European seas Class: Anthozoa − haven´t a medusa stage in their development − reproduction is sexual and asexual (budding), too − have nematocysts − have internal skeleton (made of needles located in mesoglea) and external skeleton (made of very hard, coral reefs and atolls are built from these skeletons) − live mostly in warm and clean oceans, in which they create large colonies (created by budding) − members:  Stony corals – Brain coral – build coral reefs  Sea anemones (= Sea flowers) - Beadlet anemone (Actinia equina) – haven´t external skeleton  Red coral (Corallium rubrum) – used in jewellery