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
1. nematocysts are only in the epidermis
2. gametes are epidermal and released to the
outside of the body rather than into the
gastrovascular cavity
3. 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.

20. Thank You…!