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Phylum Cnidaria (Coelenterata) The “simplest” of the complex animals . . . Phylum Cnidaria Cnidarians Include Hydras, jellyfish, sea anemones & corals Radially symmetrical Acoelomate Two basic forms: –Polyp: Cylindrical form which attach bases to substratum –Medusa: Flattened, mouth down version of the polyp. Moves freely Forms of Cnidarians Types of Cnidarians Sea Anemone Jellyfish Hydras Sea Coral Simple Facts – Simple Creatures Over 10,000 living species – Both marine and freshwater Hydras in FW Corals, jellyfish, and anemones in marine Radial symmetry Tissue level of organization Body Plan Basic Body Plan: Has an outer epidermis Has an inner gastrodermis which forms a gastrovascular cavity Has a middle "jelly filled" layer called mesoglea Body Plan Two general body forms exhibited – POLYP Sessile Cylindrical body Ring of tentacles on oral surface – MEDUSA Polyp Form Medusa Form Flattened, mouth-down version of polyp Free-swimming Body Structure Basic body plan of ALL cnidarians Sac with a central digestive compartment Single opening serving as both mouth and anus Ring of tentacles on oral surface Passive Predators! All cnidarians are carnivores Tentacles capture and push food into mouth Tentacles are armed with stinging cells Cnidoblasts / cnidocytes Contain stinging capsules called nematocysts Nutrition •They use tentacles to capture prey and direct it toward the mouth so that it can be digested in the gastrovascular cavity via secretions from gland cells (extracellular digestion) • Some food is phagocytized by special cells and digestion occurs intracellularly •There is no system of internal transport, gas exchange or excretion; all these processes take place via diffusion Cnidocytes Cnidarians are predators which have tentacles possessing batteries of special cells called Cnidocytes Cnidocytes contain special stinging capsules called nematocysts. When triggered, these cells shoot out a discharge thread which can entangle and/or poison it's intended prey How does the stinging cell work?? The tentacle is stimulated – Pressure on “trigger” Nematocyst is discharged – Thread uncoils – Entangles prey Some species produce toxins – Injects toxin into prey, paralyzing it! Cnidocytes cover the length of tentacles Caught ya’!! … (now what?!?) Food is forced into the cavity Extracellular digestion begins Enzymes secreted into cavity Intracellular digestion completes process Partially digested food engulfed by endoderm cells I’m Feelin’ it! (Can Cnidarians feel their prey?) First true nerve cells in K. Anamalia – Nerve net sends impulses in all directions – Cells of epidermis and gastrodermis arranged into contractile fibers Do not have a brain to receive information! Reproduction in Cnidarians Varies among forms Budding in Hydra Hydras, anemones and corals – exist only in polyp form – Asexual reproduction (conditions good) Budding – Sexual reproduction (conditions unfavorable) Zygotes remain dormant until conditions improve Some colonial polyp forms have a medusa phase in their life cycle – Specialized reproductive polyps produce tiny medusas by asexual budding – Sexual reproduction by medusas produce ciliated larva planula – Planula settles and develops into new polyp What about jellyfish?!? Medusa and polyp stages – Separate sexes produce gametes – Fertilization occurs in GVC of female – Planula develops and is free-swimming – Planula settles, develops into polyp – Polyp produces new medusas by budding *Fig. 32-8, PG 690 The cnidarians may be a relatively small group comprising the Animal Kingdom, but they play a major role in any marine habitat. 3 Major classes – Class Hydrozoa: Hydras – Class Scyphozoa: Jellyfish – Class Anthozoa: Anemones and corals Class Hydrozoa: The Hydras Polyp phase is dominant Generally microscopic Generally freshwater Class Hydrozoa • Includes the solitary freshwater hydra; most are colonial and marine • Typical life cycle includes both asexual polyps and sexual medusa stages; however, freshwater hydras and some marine hydroids do not have a medusa stage Solitary Hydras • Freshwater hydras are found in ponds and streams occurring on the underside of vegetation • Most possess a pedal disc and mouth surrounded by 6-10 tenetacles • Mouth opens to the gastrovascular cavity • The life cycle is simple: eggs and sperm are shed into the water and form fertilized eggs; planula is by passed with eggs hatching into young hydras • Asexual reproduction via budding Class Hydrozoa cont. Colonial Hydrozoans - e.g., Obelia • Possess a skeleton of chiton that is secreted by the epidermis • All polyps in the colony are usually interconnected • Two different kinds of individuals that comprise the colony: feeding polyps or gastrozooids (C) and reproductive polyps or gonozooids (B) Class Hydrozoa cont. Life Cycle of Obelia • Gonozooids release free swimming medusae • Zygotes become planula larvae, which eventually settle to become polyp colonies • The medusae of hydroids are smaller than those of jellyfishes (C. Scyphozoa) Class Hydrozoa cont. Other Hydrozoans Portuguese man-ofwar: Single gas-filled float with tentacles Tentacles house the polyps and modified medusae of the colony Class Scyphozoa: The Jellies Generally referred to as jellies Medusa stage is dominant Come in a variety of forms Aggressive predators, and can be dangerous to humans Jelly Portugese Man-o-war Class Scyphozoa Jellyfish • The medusae are large and contain massive amounts of mesoglea •Possess four gastric pouches lined with nematocysts; these are connected with the mouth an the gastrovascular system Scyphozoan Life Cycle - Aurelia • Gametes develop in gastrodermis of gastric pouches; eggs and sperm are shed through mouth • Fertilized eggs develop into a planula larva; settles on substrate and develops into a polyp - scyphistoma • Scyphistoma produces a series of polyps by budding - strobila • The polyps undergo differentiation and are released from the strobila as free swimming ephyra • Ephyra matures into an adult jellyfish Class Anthozoa: Anemones & Corals Appear like plants but are animals. Polyp form is dominant. Corals build calcium shells to protect themselves Anemone – Have symbiotic relationship with algae. Can build extensive masses which can form land masses. Coral Polyps Class Anthozoa • Exclusively marine; there is no medusa stage • At one or both ends of the mouth is a ciliated groove called the siphonoglyph; generates a water current and brings food to the gastrovascular cavity • Possess a well developed pharynx • The gastrovascular cavity is large and petitioned by septa or mesenteries; increase surface area for digestion or support • Edges of the septa usually have threadlike acontia threads, equipped with nematocysts and gland cells Class Anthozoa cont. • Solitary anthozoans include sea anemones • Most anthozoans are colonial (e.g. corals) and secrete external skeletons composed of calcium carbonate. • Corals obtain much of their energy from microscopic photosynthetic green algae (zooxanthellae) or dinoflagellates that live symbiotically inside the cells of the coral Coral Reefs Clownfish living in An anemone