Diese Präsentation wurde erfolgreich gemeldet.
Die SlideShare-Präsentation wird heruntergeladen. ×

ZL 121 Lecture 6 Cnidaria_Ctenophora(1)(1).ppt

Wird geladen in …3

Hier ansehen

1 von 61 Anzeige

Weitere Verwandte Inhalte

Diashows für Sie (20)

Ähnlich wie ZL 121 Lecture 6 Cnidaria_Ctenophora(1)(1).ppt (20)


Aktuellste (20)

ZL 121 Lecture 6 Cnidaria_Ctenophora(1)(1).ppt

  1. 1. LECTURE 6:PHYLUM CNIDARIA AND PHYLUM CTENOPHORA • Lecture outline • Part 1-Phylum Cnidaria • General Introduction • The body wall and Nematocysts • Alteration of Generations • Maintenance Functions • Reproduction in Cnidaria • Classification and evolutionary trends in Cnidaria • Summary
  2. 2. 1. Introduction • Cnidarians include jellyfish, sea anemones, corals, hydroids and siphonophores (e.g. Portuguese man-of- war) • The Cnidarians are the first animals to show body symmetry i.e. radial symmetry • Have symmetry = body parts organized about a central axis although they lack cephalization • They are like the a bicycle spokes in wheel, with multiple planes of symmetry. • Radial symmetry allows animals cnidarians to respond to stimuli from all directions.
  3. 3. • Cnidarians show a major evolutionary innovation as compared with sponges e.g. food is digested outside of cells in a gut cavity • Also show the beginning of cell differentiation (true tissues) • Cnidarians are diverse in form as evidenced by their ability to form colonies by asexual reproduction • e.g siphonophores in the Portuguese Man of War) • They also exhibit dimorphism-colonial or single polyps (asexual and sessile), or medusae (sexual and planktonic) • Cnidaria: General Introduction
  4. 4. • Polymorphism allows them to exploit different environments and resources. • Mostly marine, but some inhabit freshwater. • They are suspension feeders or opportunistic carnivores • Some have symbiotic relationships with algae (dinoflagellates) called zooxanthellae • Size variable: individuals from mm up to 2 m across; colonial forms such as corals may be much bigger • Cnidaria: General Introduction
  5. 5. • Like Ctenophorans, Cnidarians are diploblastic, i.e. their body wall consists of two germ layers of cells: ectoderm and endoderm. • The endoderm develops into true tissue, including tissue associated with the gut and associated glands. • The ectoderm gives rise to the epidermis, the nervous tissue, and if present, nephridia. • Simpler animals, such as sea sponges, have one germ layer and lack true tissue organisation. • The complex animals (from flat worms to humans) are triploblastic with three germ layers (a mesoderm, ectoderm and endoderm). • The mesoderm develop into true organs. Groups of diploblastic animals alive today include jellyfish, corals, sea anemones and comb jellies. Distinguishing Characteristics of Phylum Cnidaria
  6. 6. Distinguishing Characteristics of Phylum Cnidaria • Cnidarians exhibit a higher level of tissue organization (have true tissues). •The body plan demonstrates radial symmetry, oral ends terminates in a mouth surrounded by tentacles •Nervous system is composed of simple net with simple sensory structures in some members. •Cnidarians typically have a ciliated motile larva called planula, and are mostly marine but some fresh water forms exist.
  7. 7. Distinguishing Characteristics……… • Most cnidarians are carnivorous, trapping and killing their prey by means of special stinging structures called (cnidocysts) • These tentacles (cnidocytes) bear stinging organelles called nematocysts. • Cnidocytes are specialized cells used for defense and capturing prey. • Nematocyst: stinging organelle in cnidocyte that has a long filament coiled up inside it. • Often form colonies of individuals specialized for certain functions (polymorphism) • e.g. feeding polyps, defensive polyps and reproductive polyps all within the same colony.
  8. 8. Outer layer of cells – the epidermis Inner gastrodermis which lines the gut cavity or gastrovascular cavity 2. Body Wall and Nematocysts In between these tissue layers is a non-cellular jelly- like material called mesoglea. A sac-like body coelenteron serves as both mouth and anus
  9. 9. 3. Maintenance and Function • Polyps: hydrostatic support effected by water-filled coelenteron • It is constrained by circular and longitudinal muscles • Colonial anthozoans may incorporate bits of sediment and shell pieces into the column wall for further protection. •They have a flexible chitinous perisarc •In medusae the mesoglea provide support with jelly-like substances •It may sometimes have a cartilaginous consistency.
  10. 10. Feeding/Nutrition in Cnidarians • All cnidarians are carnivores feeding on live prey which they usually capture using nematocyst- laden tentacles. • They capture small animals with nematocysts and paralyze them with poison injection. • Tentacles push the prey into the gastrovascular cavity through the mouth. • Digestion is partly extracellular and partly intracellular. • Extracellular digestion takes place in the coelenteron through enzymes secreted by gastrodermal cells (gastrodermis) lining the cavity.
  11. 11. Functioning of Nematocysts • After receiving a signal, the operculum is moved back • This allows a hollow structure known as a tubule to fire. • This tubule may contain toxins (as is the case with nematocysts) adhesives that stick to the prey
  12. 12. Excretion, gaseous exchange and sensitivity in Cnidarians • Both the epidermis and the gastrodermis absorb oxygen for respiration directly from the environment or from the GV cavity. • Oxygen then diffuses to any underlying cells. • In the same manner waste products, such as carbon dioxide and ammonia (= ammonotelic) diffuse outward from cells to the body surface. • There are no specialized organs or body surfaces for gas exchange or the elimination of wastes. • The nervous system is a plexus of neurons serving sensory and motor systems (nerve net)
  13. 13. Nervous system •The nerve net is the simplest form of a nervous system found in multicellular organisms.. •Neurons found in nerve nets are found spread apart. •This nervous system allows cnidarians to respond to physical contact. •They can detect food and other chemicals in a rudimentary way
  14. 14. Locomotion in Cnidaria • Polyps are generally sessile; medusae are planktonic • Polyps such as anthozoans and some hydras have limited locomotion. • This is provided by the antagonistic circular and longitudinal fibrils (derived from myoepithelial cells) These work in conjunction with the gastrovascular cavity as a hydrostatic skeleton
  15. 15. 4. Alternation of Generations The Polyp and Medusa body forms Medusa form: • Bell- shaped or umbrella shaped body, with the mouth directed downward • Small tentacles directed downward • Possess a large amount of mesoglea • Motile, move by weak contractions of body • Polyp form: • Tubular form with the mouth directed upward • Around a mouth are a whorl of feeding tentacles • Only have a small amount of mesoglea • It is sessile
  16. 16. Generalized Cnidarian Life Cycle • The life cycle of cnidarians often includes an alteration of sexual and asexual generations, - a phenomenon that is referred to as metagenesis. • However fresh water hydra and some marine hydroids do not have a medusa stage. • When metagenesis occurs, the polyp is the asexual generation and the medusa is the sexual generation. • A generalized life cycle occurs as follows: • Medusae produce gametes which unite to form zygotes. • Each zygote divides repeatedly and develops into a free-swimming larval form called a planula larva • The larva then settles and grows into a polyp (asexual generation). •
  17. 17. Cnidaria reproduction cont.. • Each polyp then asexually produces medusae by budding to complete the life cycle. • This generalized life cycle is modified greatly in different groups • Either the polyp or the medusa stage may be reduced or even completely absent from the life cycle. • When the medusa stage is absent from the life cycle, the polyp reproduces both asexually and sexually.
  18. 18. Generalized Cnidarian Life Cycle
  19. 19. Strobilation in cnidarians • “Strobilation” = asexual reproduction consisting of the spontaneous transverse segmentation of the body. • The 2 processes occur simultaneously with segmentation to release new ephyras.
  20. 20. Cnidaria Classification • Four classes are recognized: • (a) The Hydrozoa to which Hydra and Obelia belong. • (b) Scyphozoa, the jellyfish, including Aurelia • (c) Anthozoa, the sea anemones and corals. • (d) Cubozoa, the box jellyfish & sea wasps)
  21. 21. Class Hydrozoa • Examples: Obelia, Portuguese Man-of-War, Hydra • Some species live as polyps, some as medusae, some alternate between forms. Hydra Portuguese man of war Fire coral
  22. 22. Class Hydrozoa • Includes the solitary fresh water hydra (Hydra), Obelia, fire coral (Millipora) and the Portuguese Man o' War (Physalia), • Most are colonial and marine, with some fresh water forms. • Typical life cycle includes both asexual polyps and sexual medusa stages • However fresh water hydra and some marine hydroids do not have a medusa stage. • The polyp is sessile and usually colonial in that sexual budding can take place but the individuals so formed remain attached to each other.
  23. 23. Class Hydrozoa cont…… • The medusa is a free swimming member of the plankton and represents the sexually reproducing, dispersal phase, in a typical hydrozoan. • Medusa when present, with a velum, gonads forms in the ectoderm • Hydra is an exceptional member of this class in that it only occurs as a polyp, which can reproduce sexually. • Most colonies exhibit polymorphism in the polyp types
  24. 24. Obelia polyps • All polyps in the colony are interconnected • Two different kind of individuals that comprise the colony Feeding polyps or gastrozooids (C) Reproductive polyps or gonozooids (B)
  25. 25. Longitudinal section of Hydra
  26. 26. Typical structure of a Hydroid polyp
  27. 27. Hydra • Found in quiet ponds, lakes, streams. • Attach to rocks or water plants by means of sticky secretion by cells at base. • Can leave one location and move to another by secreting bubbles at base causing it to float to surface or can move by somersaulting motion • Reproduce asexually during warm weather by budding on side of body.
  28. 28. Life cycle of Hydroid (Obelia) • Starts with sessile, asexually reproducing polyp colony • Polyps release free swimming medusae (asexually, by budding). • The medusae reproduce sexually, releasing sperm and eggs that fertilize to form a zygote. • Zygotes morphs into a blastula, then a ciliated planula larvae which eventually settle to become polyp colonies.
  29. 29. Portuguese man-of-war, Physalia otricus • Is a hydrozoan cnidarian that has adopted the way of life characteristic of the jellyfish. • Are the most complex and specialized hydrozoans consisting of colonies of highly specialized individuals. • Pelagic colonial marine animals (polymorphic). • Body consists of both modified medusoid and polypoid individuals ‘zooids’ that are morphologically and functionally specialized. • Polyps specialized for feeding, digestion, or sexual reproduction • Gastrozooids take in and digest food and from here it is distributed through the whole colony
  30. 30. Class Anthozoa  Are sessile polyps with expanded and flattened oral disc.  Includes sedentary/solitary polyploid forms such as sea anemones, sea fans and colonials e.g. corals.  Only polyp forms, no medusa stage.  At one or both ends of the mouth is a ciliated groove called the siphonoglph; generates a water current and brings food to the gastrovascular cavity.
  31. 31. Class Anthozoa Are the most advanced members of the phylum Cnidaria Occur in both deep and shallow marine waters. Corals obtain much of their energy from microscopic photosynthetic green algae (zooxanthellae) or dinoflagellates that live symbiotically inside the cells of the corals.
  32. 32. Class Anthozoa - anemones and true corals -exist only as polyps - medusa stage is absent Capnella Soft Coral Anthozoa Colony
  33. 33. Cnidarian - zooxanthellae symbiosis • Because the zooxanthellae are plants, they need sunlight to photosynthesize • Some cnidarians are nearly completely dependent on zooxanthellae • Not all corals are dependent on symbionts • These live at great depths where there is low or no light zooxanthellae in the soft coral Xenia
  34. 34. Coral Reefs • Colonial, reef-forming corals depend on zooxanthalae; thus, reefs can only exist in shallow water. • Corals depend on algae to provide oxygen and to speed up accumulation of calcium from the sea water. • Algae depend on corals to supply vital nutrients.
  35. 35. Class Scyphozoa • Commonly known as jellyfish e.g Aurelia • 200 species. • “Cup animals”. Medusa is the dominant form of the life cycle - polyp stage is reduced.
  36. 36. Class Scyphozoa • In these animals the medusa is the dominant while polyp form is reduced (restricted to a small larva) or absent. • Polyp called a scyphistoma, undergoes strobilation to produce ephyra which are immature medusa stages. • Sexual reproduction involves gamete production by free-swimming medusa. • The medusae are large and contain massive amounts of mesogle • Solitary • Gonads form in the endoderm of medusa.
  37. 37. Life cycle of a scyphozoan (jellyfish) • They eventually undergo multiple transverse fissions through a process called “Strobilation” >a process by which new medusae are produced' • The resulting Strobila sheds immature jellyfish (Ephyra) at the oral (tentacular) end. Ephyra eventually mature into adult jellyfish • Depending on the species, a single polyp may produce one or many ephyrae all at once, over a period of time, or at different intervals. • The ephyra subsequently develops into a mature medusa over a period of weeks to months
  38. 38. Generalized life cycle of a scyphozoan jellyfish, Aurelia.
  39. 39. Scyphozoan feeding strategies • Some species of jellyfish do not use their nematocysts for food capture • Instead feed on suspended zooplankton and organic matter, drawn into the coelenteron by currents of water generated by flagellae of the gastrodermal cells. • Others use nematocysts, which can be charged with venom. • The most potent is found in the tropical sea wasps or box jellies (Cubozoans). • A sting from a sea wasp can cause death in humans within twenty minutes! • Scyphozoans and siphonophores are predators of fish and small invertebrates
  40. 40. Class Cubozoa • Box jellyfish and sea wasps belong to this class. • Both polyp and medusa forms are present (medusa = dominant, polyp = reduced) • Polyp transforms directly into medusa stage without strobilization • Solitary • Medusa without a velum, but with a velarium • Medusa = sexual generation, gonads forms in the endoderm • All are marine
  41. 41. Summary on Cnidaria • Members of Phylum Cnidaria are radially or biradially symmetrical • They also possess diploblastic tissue level of organization • Cells are specialized for food gathering, defence, contraction, coordination, digestion and absorption • Hydrozoans differ from other cnidarians in having extodermal gametes • Also have mesoglea without mesenchyme cells and nematocysts only in their epidermis
  42. 42. Summary on Cnidaria • Most hydrozoans have well developed polyp and medusa stages • Class Scyphozoa contains the jellyfish • The polyp stage of scyphozoans is usually very small. • Members of Class Cubozoa live in warm, tropical waters, some posses nasty nematocysts • The Anthozoa lack the medusa stage. They include sea anemones and corals
  43. 43. Part 2: PHYLUM CTENOPHORA General Introduction • Ctenophorans are commonly called Comb jellies • About 90 species with worldwide and marine distribution. • Ctenophora means “comb-bearer” (Greek word) and these animals are named for their eight rows of comb- like plates composed of fused cilia arranged along the sides of the animal • There are nematocysts in one group of ctenophores (Euchlora rubra) (order Cydippida) • This shows that ctenophores may have arisen from a Medusa ancestor. • However, the relationship between the 2 phyla is uncertain.
  44. 44. PHYLUM CTENOPHORA • The interesting behavior in comb jellies is their bioluminescence (cold light) • The light comes from the gastrodermis cells in the walls of the meridional canals.
  45. 45. –Bioluminescence: production of light. Often observed in large swarms near surface at night.
  46. 46. 2. Distinguishing Characteristics of Phylum Ctenophora • Ctenophorans are diploblastic i.e have two embryonic germ layers • The outer layer (epidermis) and inner layer (gastrodermis) by jell-like material called mesoglea • Have biradial(radial + bilateral) symmetry with an oral- aboral body axis. • They lack nematocysts (except Euchlora rubra) but have 2 tentacles with specialized adhesive exocytotic structures called colloblasts • These are “glue cells” used to capture prey (feeding). • Have only one “body cavity” gastrovascular cavity in form of canals.
  47. 47. Distinguishing Characteristics of Phylum Ctenophora…cont…. • No specialized gas exchange, circulatory or excretory systems. • Sub epidermal nerve network (nerve net) is best developed under the comb rows and more specialized than in Cnidarians • Body support is through elastic mesenchyne (gelatinous, makes up most of the body mass; mostly water) with muscle cells • Planktonic, neutral buoyancy maintained through passive osmotic adjustment.
  48. 48. 3. Classification of Ctenophora • At least 100 species have been described. • They are classified on the basis of their tentacles, body form, oral lobes and body compressions. • There are 2 classes and 6 orders in the phylum. • Class Tentaculata (with tentacles)
  49. 49. Class Tentaculata (with tentacles) • All in this class have two tentacles; a characteristic species is Hormimorpha plumosa. • Some in this class are flattened into a ribbon-like shape along the plane of the tentacles and have an undulatory motion. i.e. Cestium sp.
  50. 50. Class Nuda (without tentacles) • These do not have tentacles at any stage in their lives • The characteristic species is Beroe punctatus. • Have a wide, flexible mouth on their bell shaped bodies. • Look hollow & can eat prey larger than themselves. • Have hooks on their gullet to allow them to hold their prey • Also have glands that secrete a poison that paralyzes their prey • Half of combjelly spp belong to Class Nuda
  51. 51. 4. Ctenophora Feeding and Nutrition • All ctenophores are predatory (carnivorous), feeding on zooplankton including copepods, small crustaceans, larval fish and fish eggs . • Comb jellies either actively seek their prey or wait in ambush for it. • Prey is captured by long tentacles equipped with “sticky cells” colloblasts • When the tentacles touch prey, the sticky cells burst and discharge a strong, sticky material. • Then the tentacle is pulled up to the mouth and wiped off. •
  52. 52. Feeding and Nutrition………….. • The food starts to be digested extracellularly in the pharynx. • Partially digested food is then distributed throughout the body by complex systems of gastrovascular canals. • These canals are lined with endodermal cells that complete digestion of food materials intracellularly. • The nutrients are absorbed via pinocytosis and phagocytosis
  53. 53. 5. Reproduction in Ctenophora • Ctenophora reproduce sexually, with the exception of species of the order Platyctenida. • These reproduce asexually by regeneration of an entire organism from a small piece of the adult body. • Most species are hermaphroditic, but some gonochoristic. • In most, gametes are shed through mouth. • Most comb jellies shed their eggs and sperm into the sea water. • Fertilization and development are external.
  54. 54. Reproduction……cont…. • In some species, fertilization takes place inside the body (internal fertilization) • Self-fertilization is somewhat rare and is known only to appear in the genus of Mnemiopsis. • The larvae (cydipid) swim freely during their transformation into adults.
  55. 55. Ctenophore life cycles • Ctenophores carry out their entire life cycle in the plankton, so they are considered to be "holoplanktonic". • Most, but not all, species of ctenophores are hermaphroditic, meaning that a single ctenophore carries both male and female gonads. • Ctenophores spawn these eggs and sperm freely into the sea, where the sperm must first find the eggs for fertilization • Fertilized eggs develop into a larvae called a Cydippid larvae. • This free swimming larvae grows naturally into a new Comb jelly.
  56. 56. Locomotion in Ctenophora • Neutrally buoyant, but swim using comb rows with “ctenes” (transverse bands of partially or wholly fused cilia beating synchronously. • May have two long tentacles or no tentacles
  57. 57. Circulation, Gaseous exchange and sensitivity in ctenophorans • No circulatory system • Gaseous exchange is by simple diffusion. Occurs at gastrovascular or epithelial surface. • Primitive Nervous system – “no brain”, just a network of neurons (Nerve net). • Have sense organ (statocyts) situated apically - near the anus containing a calcareous statolith. • These specialized cells that help them maintain vertical position & orientation when swimming e.g. jellyfish. • Some have chemical sense organs around their mouth, e.g class Nuda use chemicals to detect presence of other Ctenophores.
  58. 58. 7. Summary on Ctenophora • Members of Phylum Ctenophora are biradially symmetrical and diploblastic • Bands of cilia called comb rows characterize ctenophore • The Cnidaria and Ctenophora are distantly related phyla. • Within the Cnidaria, the ancient hydrozoans are believed to be the rock from which modern hydrozoans and other cnidarians evolved.
  59. 59. 8. Differences between Ctenophora and Cnidaria Ctenophora  Cydippid larva  No nematocysts except in Euchlora rubra, instead have colloblast cells  Typically hermaphrodite  Multiciliated cells  No alternation of generations  Have apical organs:  Bi-radial symetrical Cnidarians  Planula larva  Have nematocyts  Typically gonochoristic  Monociliated cells  Alternation of generations occurs  No apical organs  Radial symetrical

Hinweis der Redaktion

  • Types of cell found in cnidaria nematocyst,intestial cell,cnidocyst,epithelia muscular cell,enzymatic cell