1. Instructor: Almonther I. Alhamedi
cnidaria
Instructor: Almonther I. Alhamedi
The Islamic University of Gaza
Department of Biology
Web page :

2. General characterstics
Radial Symmetry
Diploblastic – 2 embryonic tissue layers.
Belong to the subkingdom Metazoa.
Tissues – derived from two embryonic germ layers
Cnidocytes – special cells with stinging organelles called nematocysts.
The basic body plan of a cnidarian is a sac with a central digestive compartment, the gastrovascular cavity.
A single opening functions as both mouth and anus.
Cnidarians have an outer tissue layer, the epidermis, derived from ectoderm, and an inner gastrodermis, derived from endoderm, with jellylike mesoglea in between.

3. Phylum Cnidaria
Cnidarians have diversified into a wide range of both sessile and floating forms including jellies, corals, and hydras.
Polymorphism – some species exist as both polyps and medusae during their life cycles.

4. Reproduction
Polyps can reproduce asexually by budding, fission, or pedal laceration.
small pieces of the pedal disc break off and regenerate into small anemones

5. Reproduction
Cnidarians, typically medusae, can also reproduce sexually.
A zygote usually develops into a motile planula larva.
Some species only exist as polyps, others only as medusae, others alternate between the two.

6. Feeding Cnidarians are carnivores that use tentacles to capture prey.
The tentacles are armed with cnidocytes – unique cells that function in defense and the capture of prey.
Nematocysts contain toxins used for prey capture and defense.

7. Feeding Nematocysts are used to inject a toxin. Variable in form.
May be used for prey capture or defense.

8. Feeding
Extracellular digestion begins in the gastrovascular cavity, but is completed within the cells of the gastrodermis.
Some cnidarians supplement their diet with nutrients collected from algal symbionts (zooxanthellae).

9. Classification The phylum Cnidaria is divided into four major classes:
Class Hydrozoa Class Scyphozoa Class Cubozoa Class Anthozoa

10. Classification

11. Class Hydrozoa
(Gr. hydra, water serpent)
polyps and medusa stage, although polyp stage is dominant.
gut cavity of polyp is simple, lacking a pharynx .
tetramerous (four-part) radial symmetry.
gonads are ectodermal (found in the epidermis).
medusa stage may possess specialized balance organs called statocysts and photosensitive organs called ocelli.
solitary or colonials; some colonial forms highly polymorphic.
Hydras have nematocysts that: 1) penetrate prey and inject poison; 2) recoil and entangle prey; and 3) secrete an adhesive for locomotion and attachment
includes hydroids (“stinging limu”), fire coral, pink coral, siphonophores and obelia

12. A B
A- Hydra with developing bud and ovary.
B- Hydra catches an unwary water flea with the nematocysts
of its tentacles. This hydra already contains one water flea eaten previously.

13. Class Hydrozoa - Polyps
Typical hydroids have a base, a stalk, and one or more terminal zooids (individual polyp animals).
Thecate – presence of a protective cup around the polyp.
Obelia
Athecate – no such protection.
Ectopleura

14. Class Hydrozoa - Medusae
Hydroid medusae are usually smaller than those in the class Scyphozoa.
Gastrovascular cavity is continuous from mouth to tentacles and is lined by gastrodermis.
Velum, inward projection of the bell, is present.
Specialized organs:
Statocysts – equilibrium
Ocelli – light sensitive
Bell medusa, Polyorchis penicillatus, medusa stage of an unknown attached polyp.

15. Structure of Gonionemus. A, Medusa with typical tetramerous arrangement. C, Portion of a tentacle with its adhesive pad and ridges of nematocysts. D, Tiny polyp, or hydroid stage, that develops from the planula larva. It can produce more polyps by budding (frustules) or produce medusa buds.

16. Life cycle of Obelia, showing alternation of polyp (asexual) and medusa (sexual) stages. Obelia is a calyptoblastic hydroid; its polyps as well as its stems are protected by continuations of the perisarc.

17. Hydrozoan colony … Obelia

18. Hydrozoan Medusa

19. Hydrozoan Colonies

20. A Portuguese man-of-war colony, Physalia physalis (order Siphonophora, class Hydrozoa). Each colony of medusa and polyp types is integrated to act as one individual. As many as 1000 zooids may be found in one colony. The nematocysts secrete a powerful neurotoxin.

21. By-the-Wind-Sailor A Floating Colony of Polyps

22. A B
These hydrozoans form calcareous skeletons that resemble true coral. A, Stylaster roseus (order Stylasterina) occurs commonly in caves and crevices in coral reefs. These fragile colonies branch in only a single plane and may be white, pink, purple, red, or red with white tips.
B, Species of Millepora (order Milleporina) form branching or platelike colonies and often grow over the horny skeleton of gorgonians, as is shown here. They have a generous supply of powerful nematocysts that produce a burning sensation on human skin, justly earning the common name fire coral.

23. Fire Corals NOT TRUE CORAL - Hydrozoan colony
TRUE CORAL - Anthozoan colony

24. Class Scyphozoa (Gr. skyphos, cup)
life cycle with both polyps and medusae, but medusae dominate with polyp stage reduced or absent.
polyp stage (scyphistoma) goes through strobilization (transvers fission ) to produce young medusa.
bell margin lacks a velum(amemmbrane on the subumbrella surface of the jelly fish).
tetramerous (= four-part) radial symmetry.
gut divided into a complex system of radial canals.
some with a simple single mouth, but many with thousands of microscopic “mouths” at the ends of oral arms.
gonads endodermal (found in the gastrodermis).
specialized sense organs called rhopalia(club-shaped sense organ) with ocelli & statocysts .
includes some 200 marine species.
"true" sea jellies.
Stage in gelly fish cycle when fixed

26. Sea Jellies

27. Life cycle of Aurelia, a marine scyphozoan medusa.

28. B A
A- Thaumatoscyphus hexaradiatus (order Stauromedusae, class Scyphozoa). Members of this order are unusual scyphozoans in that the medusae are sessile and attached to seaweed or other objects.
B- Moon jellyfish Aurelia aurita (class Scyphozoa) is cosmopolitan in distribution. It feeds on planktonicorganisms caught in mucus on its umbrella.

29. Class Cubozoa
(Gr. kybos, a cube)
polyps and medusae stages, but medusae dominate with polyp stage reduced.
polyp stage develops directly into medusa.
bell margin with a velarium(shelf-like extension of the sub umbrella edge that increase swimming efficiency).
tetramerous (= four-part) radial symmetry; bell cube-shaped with tentacles arising from each corner.
gonads endodermal (found in the gastrodermis).
specialized sense organs called rhopalia with ocelli & statocysts .
includes some 15 marine species.
includes box jellies and sea wasps.

34. Class Anthozoa lack medusa stage entirely (polyp forms only)
(Gr. anthos, flower)
lack medusa stage entirely (polyp forms only)
mouth with a tubular pharynx that projects inward into the gut
large gut cavity divided by mesenteries that radiate inwards from the body wall
hexamerous (6-part) or octamerous (8-part) radial symmetry or biradial (modified radial symmetry that limits the number of planes that can divide the body into equal halves
includes sea anemones, “true” stony corals, sea fans, sea pens, organ pipe coral, precious black coral.
Sea anemones usually move by gliding slowly along on their pedal discs.

35. sea pens

36. sea fans
sea fans

37. sea anemones
precious black coral
Sea anemone attach to rocks using their pedal disc . usually move by gliding slowly along on their pedal discs.
Feed on fish or any other food of suitable size.

39. Mutualisms
Sea anemones sometimes harbor zooxanthellae (photosynthetic protists) like hard corals do.
 The algae gives off oxygen and other nutrients that the coral polyp needs to live and in return the polyp gives the algae carbon dioxide and other substances the algae needs. That is why coral reefs grow so near the surface of the water where it is the sunniest--the algae need sunshine for photosynthesis.
Some crabs will decorate their shells with anemones.

40. Mutualisms
Some damselfish (anemone fishes) form associations with large anemones.
In a symbiotic mutualistic relationship, the clownfish feeds on small invertebrates that otherwise have potential to harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the sea anemone.
The clownfish is additionally protected from predators by the anemone's stinging cells, to which the clownfish is immune.

41. Class Anthozoa – Hard Coral
Hexacorallia – 6-fold symmetry
Precipitate Ca3(CO3)2 from sea water to produce skeletal structures that become coral reefs
Contain zooxanthella
Fig

42. Class Anthozoa – soft corals
Octocorallia – 8-fold symmetry
Form complex tube-like skeletal structures
Lack zooxanthella
Fig

43. Class Anthozoa – Soft Corals
Fig

44. Reefs Shallow tropical waters
Hermatypic corals (Class Anthozoa, subclass hexacorallia, order scleractinia)
Contain symbiotic dinoflagellates
Precipitate CaCO3 to form “exoskeleton”
Fig a

45. Zooanthella
Green algae

46. Atoll
Fig b