13. class hydrozoa with clear explanation only on slide share free

13. class hydrozoa in full explanation

1. CLASS HYDROZOA By SYED SHAKEEL SHAH Department of Zoology University of Narowal

2. Introduction Hydrozoans are small, relatively common cnidarians. The vast majority are marine, but this is the one cnidarian class with freshwater representatives. Most hydrozoans have life cycles that display alternation of generations; however, in some, the medusa stage is lost, while in others, the polyp stage is very small.

3. Hydrozoans Differs From Other Cnidarians 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 never contains amoeboid mesenchyme cells

4. Most hydrozoans have colonial polyps in which individuals may be specialized for feeding, producing medusae by budding, or defending the colony.

5. Obelia In Obelia , a common marine cnidarian , the planula develops into a feeding polyp, called a gastrozooid . The gastrozooid has tentacles, feeds on microscopic organisms in the water, and secretes a skeleton of protein and chitin, called the perisarc , around itself. FIG: Obelia Structure and Life Cycle.

6. Growth of an Obelia colony results from budding of the original gastrozooid . Rootlike processes grow into and horizontally along the substrate. They anchor the colony and give rise to branch colonies. FIG: Obelia Structure and Life Cycle.

7. The entire colony has a continuous gastrovascular cavity, body wall, and perisarc , and is a few centimeters high. Gastrozooids are the most common type of polyp in the colony; however, as an Obelia colony grows, gonozooids are produced. FIG: Obelia Structure and Life Cycle.

8. A gonozooid is a reproductive polyp that produces medusae by budding. Obelia’s small medusae form on a stalklike structure of the gonozooid . When medusae mature, they break free of the stalk and swim out an opening at the end of the gonozooid . Medusae reproduce sexually to give rise to more colonies of polyps. FIG: Obelia Structure and Life Cycle.

9. FIG: Obelia Structure and Life Cycle. Obelia alternates between polyp and medusa stages. An entire polyp colony stands about 1 cm tall. A mature medusa is about 1 mm in diameter, and the planula is about 0.2 mm long. Unlike Obelia , the majority of colonial hydrozoans have medusae that remain attached to the parental colony, and release gametes or larval stages through the gonozooid . The medusae often degenerate and may be little more than gonadal specializations in the gonozooid .

10. Gonionemus Gonionemus is a hydrozoan in which the medusa stage predominates. It lives in shallow marine waters, where it often clings to seaweeds by adhesive pads on its tentacles. The biology of Gonionemus is typical of most hydrozoan medusae . The margin of the Gonionemusmedusa projects inward to form a shelflike lip, called the velum. A velum is present on most hydrozoan medusae but is absent in all other cnidarian classes.

11. The velum concentrates water expelled from beneath the medusa to a smaller outlet, creating a jet-propulsion system. The mouth is at the end of a tubelike manubrium that hangs from the medusa’s oral surface. The gastrovascular cavity leads from the inside of the manubrium into four radial canals that extend to the margin of the medusa. An encircling ring canal connects the radial canals at the margin of the medusa .

12. In addition to a nerve net, Gonionemus has a concentration of nerve cells, called a nerve ring, that encircles the margin of the medusa. The nerve ring coordinates swimming movements. Embedded in the mesoglea around the margin of the medusa are sensory structures called statocysts .

13. A statocystconsists of a small sac surrounding a calcium carbonate concretion called a statolith . When Gonionemus tilts, the statolith moves in response to the pull of gravity. This initiates nerve impulses that may change the animal’s swimming behavior.

14. Gonads of Gonionemusmedusae hang from the oral surface, below the radial canals. Gonionemus is dioecious and sheds gametes directly into seawater. A planula larva develops and attaches to the substrate, eventually forming a polyp (about 5 mm tall). The polyp reproduces by budding to make more polyps and medusae .

15. FIG: A Hydrozoan Medusa. (a) A Gonionemus medusa. (b) Structure of Gonionemus .

16. Hydra Hydra is a common freshwater hydrozoan that hangs from the underside of floating plants in clean streams and ponds. Hydra lacks a medusa stage and reproduces both asexually by budding from the side of the polyp and sexually. Hydras are somewhat unusual hydrozoans because sexual reproduction occurs in the polyp stage. Testes are conical elevations of the body surface that form from the mitosis of certain epidermal cells, called interstitial cells. Sperm form by meiosis in the testes. Mature sperm exit the testes through temporary openings.

17. Ovaries also form from interstitial cells. One large egg forms per ovary. During egg formation, yolk is incorporated into the egg cell from gastrodermal cells. As ovarian cells disintegrate, a thin stalk of tissue attaches the egg to the body wall. After fertilization and early development, epithelial cells lay down a resistant chitinous shell. The embryo drops from the parent, overwinters, hatches in the spring, and develops into an adult .

18. Order Siphonophora Large oceanic hydrozoans belong to the order Siphonophora . These colonies are associations of numerous polypoid and medusoid individuals. Some polyps, called dactylozooids , possess a single, long (up to 9 m) tentacle armed with cnidocytes for capturing prey. Other polyps are specialized for digesting prey. Various medusoid individuals form swimming bells, sac floats , oil floats , leaflike defensive structures, and gonads.

19. Physalia physalis Physalia physalis , commonly called the Portuguese man-of-war, is a large, colonial siphonophore . It lacks swimming capabilities and moves at the mercy of wind and waves. Its cnidocyte -laden dactylozooids are lethal to small vertebrates and dangerous to humans.

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