Radiate animals Cnidarians and Ctenophores

1. Chapter 7 Radiate animals Cnidarians and Ctenophores

3. Phylum Cnidaria • Over 9,000 species in the phylum Cnidaria • Equipped with specialized cells: cnidocytes • Contain a specialized stinging organelle, the nematocyst • Fossil specimens dated to over 700 million years ago Species • Most common in shallow marine environments • Some freshwater • None are terrestrial

4. Phylum Cnidaria Characteristics of Phylum Cnidaria • All are aquatic and mostly marine • Symmetry • Radial or biradial • Two body types • Free-swimming medusae • Sessile polyps • Diploblastic • Epidermis and gastrodermis • Mesoglea: extracellar matrix that lies between ectodermis and gastrodermis

5. Phylum Cnidaria • Four classes of Cnidaria • Hydrozoa-hydroids,Portuguese man of war • Scyphozoa-jellyfish • Cubozoa-cube jellyfish • Anthozoa-sea anemone and coral • A fifth class, Staurozoa,stalked jellyfish, has been proposed • No medusae in life cycle but polyp topped by medusa-like region

7. Ecological Relationship • Some ctenophores, molluscs and flatworms eat hydroids and use the stinging nematocysts for their own defense • Some molluscs and fish feed on cnidarians • Symbiotic relationship with crabs • Coral reefs

8. Figure 7_02a

9. Phylum Cnidaria Form and Function • Cnidaria have two basic body plans: polyp and medusa • Polyp • Hydroid form • Adaptation to a sedentary life • Tubular body with the mouth directed upward and surrounded by tentacles • Mouth leads into a blind gastrovascular cavity • Attach to substratum by pedal disc • Reproduce asexually by budding, fission, or pedal laceration

11. Phylum Cnidaria • In colonial forms • Polyps may be specialized for feeding, reproduction, or defense • In class Hydrozoa • feeding polyps (hydranths), and defense (dactylozooids) distinguished from reproductive polyps (gonangia) by absence of tentacles in gonangia

12. Figure 7_02b

13. Phylum Cnidaria • Medusa • Bell or umbrella‑shaped • Usually free-swimming • Mouth directed downward • Tentacles may extend down from rim of umbrella

14. Phylum Cnidaria • Colonial polyps are sessile. • Hydra can move freely across a substrate by gliding on their pedal disc aided by mucous secretions. • Sea anemone can move similarly on pedal discs • Medusae-contract the bell which expels water

15. Phylum Cnidaria Life Cycles • Polyps and medusae play different roles in the cnidarian life cycle • Typically, zygote develops into a motile planula larva • Planula settles, and metamorphoses into a polyp • Produce other polyps asexually • Polyps eventually produce a free-swimming medusa by asexual reproduction • Budding or strobilation

16. Phylum Cnidaria • Medusae • Dioecious-separate sexes in each individual • Reproduce sexually • True jellyfish (class Scyphozoa) • Medusa is large and conspicuous • Polyps typically very small • Most colonial hydroids • Feature a polyp stage and a pelagic medusa stage • Some hydrozoans (Physalia) form floating colonies • In Hydra, only stage is a small freshwater polyp

17. Phylum Cnidaria Feeding and Digestion Gastrovascular cavity acts as stomach. • Catch prey with tentacles and pass them to the gastrovascular cavity • Gland cells discharge enzymes to begin extracellular digestion • Intracellular digestion continues in the cells of the gastrodermis

18. Phylum Cnidaria Body Wall • Cnidarian body • Outer epidermis • Inner gastrodermis • Layers separated by mesoglea • Mesoglea • Gelatinous • Continuous in polyps, extending through body and tentacles • Supports body • Thicker in medusae • Thinner in hydromedusae • Gastrovascular cavity- Water enters due to beating cilia • Water serves as a hydrostatic skeleton

19. Phylum Cnidaria • Epitheliomuscular cells • Form most of epidermis and cause muscular contraction • Undifferentiated interstitial cells Develop into cnidoblasts, sex cells, buds, or nerve cells, but not epitheliomuscular cells. • Gland cells • On the adhesive disc secrete an adhesive and sometimes a gas bubble for floating

21. Phylum Cnidaria • In Hydra • Epidermis contains epitheliomuscular, interstitial, gland, cnidocytes, sensory, and nerve cells • Cnidarian bodies extend contract, bend, and pulse • No mesodermally derived muscle cells • Have epitheliomuscular cells • Form most of epidermis • Cover organism and function in muscle contraction

22. Phylum Cnidaria Cnidocytes • Many cnidarians are effective predators • Possible due to presence of a unique cell type, the cnidocyte • Cnidoctyes • Located in invaginations of ectodermal cells and some gastrodermal cells • Produces one of over 20 types of cnidae • One type of cnida (tiny capsule) is the nematocyst • End of cnida is covered by a little lid-operculum

23. Phylum Cnidaria • Nematocysts • Tiny capsules made of chitin-like material and containing a coiled filament • Filament may have tiny barbs or spines • Except in Anthozoa, cnidocytes equipped with trigger-like cnidocil (modified cilium) • Tactile stimulation cause nematocyst to discharge • After Cnidae discharge, cnidocyte is absorbed and another develops

26. Phylum Cnidaria • Mechanism of Nematocyst Discharge • When stimulated, water to rush into the capsule • The operculum opens and rapidly launches the filament • Barbs inject poison into prey • Only a few jellyfish and the Portuguese man-of-war can seriously harm humans

27. Phylum Cnidaria Nerve Net • Nerve net of cnidarians one of the best examples of diffuse nervous system • Two nerve nets, one at the base of epidermis and one at the base of gastrodermis, interconnect • Nerve action potentials transmitted across synapses by neurotransmitters • Unlike higher animals, • Nerve nets have neurotransmitters on both sides of the synapses • Allowing transmission in either direction • No myelin sheath on axons

28. Phylum Cnidaria • Nerve cells synapse with both slender sensory cells and epitheliomuscular • Association often called a neuromuscular system • The nerve net pattern is also found in annelid and human (nerve plexus) digestive systems

29. Phylum Cnidaria Class Hydrozoa • Most marine and colonial with both polyp and medusa forms • Freshwater Hydra is not typical • Colonial Obelia is more exemplary • Typical hydroid has a base, a stalk, and one or more terminal zooids (individual polyp animals) • Base is a rootlike stolon, or hydrorhiza • Gives rise to stalks called hydrocauli • Living part of the hydrocaulus is a tubular coenosarc • Hydrocaulus covered by a non-living chitinous sheath, the perisarc

30. Phylum Cnidaria • Hydra • Found on the underside of aquatic leaves and lily pads in clean fresh water • Worldwide distribution (16 species in North America) • Body is a cylindrical tube • Bottom end has a basal or pedal disc for attachment • The mouth (oral end) on a conical elevation, the hypostome • Ring of 6–10 hollow tentacles encircles mouth

33. Phylum Cnidaria • The mouth opens to a gastrovascular cavity • Buds may project from the side, each develop a mouth and tentacles • Hydras feed on a variety of small crustaceans, insect larvae, and worms • Mouth is located on a raised hypostome, and opens into the gastrovascular cavity • Food organisms brush against the tentacles are captured by nematocysts

34. Phylum Cnidaria • Hydras reproduce sexually and asexually • Asexual reproduction • Budding • Most hydraare dioecious-separate sexes • Temporary gonads appear in autumn, stimulated by lower temperatures or stagnation • Eggs and sperm shed externally • Cyst forms around embryo • Encysted form endures the winter, then young hydras hatch in the spring

35. Phylum Cnidaria • Hydroid Colonies • Individual zooids are attached to the base • Hydranths (gastrozooids) are feeding polyps with circle of tentacles surrounding mouth • Colonial hydroids bud off new individuals • Individuals may be new hydranths or medusae buds • In Obelia, the medusae buds are formed by a reproductive polyp called a gonangium

37. Phylum Cnidaria • Hydroid medusae • Usually smaller than schyphozoan medusae • Margin of the bell projects inward as a shelf-like velum • Mouth opens at the end of a suspended manubrium • Mouth connects to a stomach and four radial canals • Radial canals connect to a ring canal that runs around the margin of the bell and connects with the hollow tentacles • Bell margin has many sensory cells • Typically also bears statocysts, specialized sense organs that function in equilibrium, and light-sensitive ocelli

38. Hydroid-Gonionemus

40. Phylum Cnidaria • Other Hydrozoans • Orders Siphonophora form floating colonies • Contain several types of polyp individuals-feeding polyps, reproductive polyps, nematocysts, jelly polyps • In Physalia, the float, pneumatophore • Thought to have expanded from the original larval polyp • tentacles can stretch up to 165 feet below the surface of the water • it is able to deflate itself and descend to escape the threat • Other hydrozoans secrete calcareous skeletons resembling true corals and are the hydrocorals

43. Phylum Cnidaria Class Scyphozoa • Most of the larger jellyfishes belong to this class • Nearly all float in open sea • One order is sessile, attached to seaweeds by a stalk • Bells vary in shape and size • Composed mostly of mesoglea • Mesoglea contains ameboid cells and fibers • Lack shelf-like velum found in hydrozoan medusae • Margin of the umbrella has indentations, each bearing a pair of lappets

45. Phylum Cnidaria • Between lappets is a equilibrium sense organ called a rhopalium • Mouth located beneath the umbrella • Manubrium forms four oral arms • Capture and ingest prey • Tentacles, manubrium, and often entire body may have nematocysts • Feed on small organisms from protozoa to fish • Cilia on gastrodermis circulates food and oxygen throughout the animal

47. Phylum Cnidaria • Sexes are separate • Fertilization is internal in the gastric pouch of the female-sperm is carried to gastric pouches • Zygote develops into a ciliated planula larva • Attaches and develops into a scyphistoma • Scyphistoma undergoes strobilation • Form buds called ephyrae that break loose to form jellyfish medusae

49. Phylum Cnidaria Class Staurozoa • Commonly called stauromedusans • No medusa stage • Solitary polyp body that is stalked • Uses adhesive disk to attach to seaweeds, and objects on sea bottom • Polyp top resembles a medusa with eight extensions (“arms”) ending in tentacle clusters surrounding mouth • Reproduce sexually