Lecture 4 coelentrates and cnidarians
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Cnidarians are a phylum of aquatic animals that include jellyfish, corals, sea anemones and hydroids. They are radially symmetrical with a mouth surrounded by tentacles, and have stinging cells called nematocysts. Many cnidarians alternate between sessile polyp and motile medusa body forms. Their body wall is composed of two tissue layers with a jelly-like mesoglea in between. They have a nerve net and gastrovascular cavity for digestion and gas exchange. The five classes are Hydrozoa, Scyphozoa, Staurozoa, Cubozoa and Anthozoa.
Lecture 4 coelentrates and cnidarians
- 2. Phylum Coelenterate • Coelenterata is scientifically classified from the Animalia kingdom and Eumetazoa sub-kingdom. • The term Coelenterata is no longer recognized as scientifically valid, as the Cnidaria and Ctenophora. • This phylum includes about 9000 species , which are mainly marine ; few are fresh water . • The name of this phylum came from the stinging cells or cnidoblasts present on the ectoderm of tentacles and the body of these carnivorous animals. • This phylum includes aquatic (marine and fresh water), solitary or colonial forms
- 3. Characteristics of Phylum Cnidaria • Radially symmetrical animals • No anterior or posterior ends. • Directions are based on the position of the mouth opening • Mouth containing end is the oral end • Opposite end is the aboral end. • Radial symmetry is advantageous for sedentary animals because sensory receptors are evenly distributed around the body. These organisms can respond to stimuli from all directions.
- 4. • Gelatinous mesoglea between the epidermal and gastrodermal tissue layers • Gastrovascular cavity • Nerve cells organized into a nerve net • Specialized cells, called cnidocytes, used in defense, feeding, and attachment • Diploblastic Characteristics of Phylum Cnidaria
- 5. The Body Wall and Nematocysts • The ectoderm gives rise to epidermis, while endoderm to gastro dermis. Both perform different functions • Protection • Food gathering • Coordination • Movement • Digestion • Absorption • Jellylike mesoglea is present.
- 6. Cnidocytes produce structures called cnidae (fluid-filled, intracellular capsule enclosing a coiled, hollow tube) The cnidocyte usually has a modified cilium, called a cnidocil. 30 kinds of cnidea, 6 or more present in one individual The Body Wall and Nematocysts
- 7. Alternation of Generations • Many cnidarians possess two body forms in their life histories • The polyp is usually asexual and sessile. It attaches to a substrate at the aboral end, and has a cylindrical body, called the column, and a mouth surrounded by food-gathering tentacles. • The medusa is dioecious and free swimming. It is shaped like an inverted bowl, and tentacles dangle from its margins.
- 8. Generalized Cnidarian Life Cycle
- 9. Maintenance Functions • The gastrodermis of all cnidarians lines a blind-ending gastrovascular cavity. • This cavity functions in digestion, the exchange of respiratory gases and metabolic wastes, and the discharge of gametes. • Food, digestive wastes, and reproductive stages enter and leave the gastrovascular cavity through the mouth.
- 10. Digestive system Role of Nematocysts in digestion • Feed small crustaceans and small fishes, Entangle ad paralyze prey • Contractile cells in tentacles shorten ad food moves inward Gastrodermal gland • Mucus and enzymes (food to soupy broth) Nutritive muscular cells Phagocytize partially digested food into food vacuoles Circularly oriented contractile fibers that help move material into and out of gastrovascular cavity by peristaltic contraction Expelling undigested food through mouth
- 11. Hydrostatic skeleton • Gastrovascular cavity act as hydrostatic skeleton • Epithelio-muscular cells are contractile and help in movement • By contraction of Epithelio-muscular cells polyp bends toward that side and mouth is opened, water escapes and polyp collapses • If mouth do not open the polyp elongates
- 12. Locomotion Polyp • Somersaulting • Inch worm fashion • Glide ( base attach on substrate and tentacles move) Medusae • Floating (water currents ad wind for horizontal movement) • Swimming (contraction of circular and radial Epithelio-muscular cells cause rhythmic pulsation of the bell propelling medusa through water
- 13. Nervous System and Sensory Receptors • Nerve cells under epidermis and near mesoglea interconnect to form two dimensional nerve net • Nerve net conducts impulses around the body in response to localized stimulus • Receptors for perceiving touch and chemicals distributed throughout the body • Have large surface area to volume ratio • All cells are at a short distance from body surface • Oxygen, Carbon dioxide and nitrogenous wastes are exchangd with the environment by diffusion
- 14. Class Hydrozoa • Nematocysts are only in the epidermis • Gametes are epidermal and released to the outside of the body rather than into the gastrovascular cavity • The mesoglea is largely acellular • Hydra is a common freshwater hydrozoan • Large oceanic hydrozoans belong to the order Siphonophora • Medusae usually with a velum; many polyps colonial; mostly marine with some freshwater species. • Examples: Hydra, Obelia, Gonionemus, Physalia
- 15. Class Hydrozoa
- 16. Class Scyphozoa • Medusa prominent in the life history, Polyp small • Gametes gastrodermal in origin and released into the gastrovascular cavity • Cnidocytes present in the gastrodermis as well as epidermis • Medusa lacks a velum Mesoglea with wandering mesenchyme cells of epidermal origin • Example: Aurelia Mastigius spp. Aurelia aurita
- 17. Structure of a Scyphozoan Medusa
- 19. Class Staurozoa • Medusae absent • Develop from benthic planula larvae • Eight tentacles surrounding the mouth attachment to substrate by adhesive disk • Sexual reproduction only • Marine • Haliclystis.
- 20. Class Cubozoa • Medusa prominent in life history • Polyp small • Gametes gastrodermal in origin • Medusa cuboidal in shape with tentacles that hang from each corner of the bell • Marine • Chironex
- 21. Class Anthazoa • Colonial or solitary polyps • Medusae absent • Cnidocytes present in the gastrodermis • Cnidocils absent • Gametes gastrodermal in origin • Gastrovascular cavity divided by mesenteries that bear nematocysts • Internal biradial or bilateral symmetry present mesoglea with wandering mesenchyme cells tentacles solid • Marine. • Anemones and corals. Metridium.