1. Phylum Cnidaria (Coelenterata)
The “simplest” of the complex animals . . .

2. Phylum Cnidaria
Cnidarians Include Hydras, jellyfish, sea anemones & corals
Radially symmetrical
Acoelomate
Two basic forms:
Polyp: Cylindrical form which attach bases to substratum
Medusa: Flattened, mouth down version of the polyp. Moves freely

3. Forms of Cnidarians

4. Types of Cnidarians
Sea Anemone
Jellyfish
Sea Coral
Hydras

5. Simple Facts – Simple Creatures
Over 10,000 living species
Both marine and freshwater
Hydras in FW
Corals, jellyfish, and anemones in marine
Radial symmetry
Tissue level of organization

6. Body Plan Basic Body Plan: Has an outer epidermis
Has an inner gastrodermis which forms a gastrovascular cavity
Has a middle "jelly filled" layer called mesoglea

7. Body Plan Two general body forms exhibited
POLYP
Sessile
Cylindrical body
Ring of tentacles on oral surface
MEDUSA
Flattened, mouth-down version of polyp
Free-swimming
Polyp Form
Medusa Form

8. Body Structure Basic body plan of ALL cnidarians
Sac with a central digestive compartment
Single opening serving as both mouth and anus
Ring of tentacles on oral surface

9. Passive Predators! All cnidarians are carnivores
Tentacles capture and push food into mouth
Tentacles are armed with stinging cells
Cnidoblasts / cnidocytes
Contain stinging capsules called nematocysts

10. Nutrition
They use tentacles to capture prey and direct it toward the mouth so that it can be digested in the gastrovascular cavity via secretions from gland cells (extracellular digestion)
Some food is phagocytized by special cells and digestion occurs intracellularly
There is no system of internal transport, gas exchange or excretion; all these processes take place via diffusion

11. Cnidocytes
Cnidarians are predators which have tentacles possessing batteries of special cells called Cnidocytes
Cnidocytes contain special stinging capsules called nematocysts.
When triggered, these cells shoot out a discharge thread which can entangle and/or poison it's intended prey

12. How does the stinging cell work??
The tentacle is stimulated
Pressure on “trigger”
Nematocyst is discharged
Thread uncoils
Entangles prey
Some species produce toxins
Injects toxin into prey, paralyzing it!

14. Cnidocytes cover the length of tentacles

15. Caught ya’!! … (now what?!?) Food is forced into the cavity
Extracellular digestion begins
Enzymes secreted into cavity
Intracellular digestion completes process
Partially digested food engulfed by endoderm cells

16. I’m Feelin’ it! (Can Cnidarians feel their prey?)
First true nerve cells in K. Anamalia
Nerve net sends impulses in all directions
Cells of epidermis and gastrodermis arranged into contractile fibers
Do not have a brain to receive information!

17. Reproduction in Cnidarians
Varies among forms
Hydras, anemones and corals
exist only in polyp form
Asexual reproduction (conditions good)
Budding
Sexual reproduction (conditions unfavorable)
Zygotes remain dormant until conditions improve
Budding in Hydra

18. Some colonial polyp forms have a medusa phase in their life cycle
Specialized reproductive polyps produce tiny medusas by asexual budding
Sexual reproduction by medusas produce ciliated larva
planula
Planula settles and develops into new polyp

19. Medusa and polyp stages
What about jellyfish?!?
Medusa and polyp stages
Separate sexes produce gametes
Fertilization occurs in GVC of female
Planula develops and is free-swimming
Planula settles, develops into polyp
Polyp produces new medusas by budding
*Fig. 32-8, PG 690

20. The cnidarians may be a relatively small group comprising the Animal Kingdom, but they play a major role in any marine habitat.

21. 3 Major classes Class Hydrozoa: Hydras Class Scyphozoa: Jellyfish
Class Anthozoa: Anemones and corals

22. Class Hydrozoa: The Hydras
Polyp phase is dominant
Generally microscopic
Generally freshwater

23. Class Hydrozoa
Includes the solitary freshwater hydra; most are colonial and marine
Typical life cycle includes both asexual polyps and sexual medusa stages; however, freshwater hydras and some marine hydroids do not have a medusa stage
Solitary Hydras
Freshwater hydras are found in ponds and streams occurring on the underside of vegetation
Most possess a pedal disc and mouth surrounded by 6-10 tenetacles
Mouth opens to the gastrovascular cavity
The life cycle is simple: eggs and sperm are shed into the water and form fertilized eggs; planula is by passed with eggs hatching into young hydras
Asexual reproduction via budding

24. Possess a skeleton of chiton that is secreted by the epidermis
Class Hydrozoa cont.
Colonial Hydrozoans - e.g., Obelia
Possess a skeleton of chiton that is secreted by the epidermis
All polyps in the colony are usually interconnected
Two different kinds of individuals that comprise the colony: feeding polyps or gastrozooids (C) and reproductive polyps or gonozooids (B)

25. Gonozooids release free swimming medusae
Class Hydrozoa cont.
Life Cycle of Obelia
Gonozooids release free swimming medusae
Zygotes become planula larvae, which eventually settle to become polyp colonies
The medusae of hydroids are smaller than those of jellyfishes (C. Scyphozoa)

26. Single gas-filled float with tentacles
Class Hydrozoa cont.
Other Hydrozoans
Portuguese man-of-war:
Single gas-filled float with tentacles
Tentacles house the polyps and modified medusae of the colony

27. Class Scyphozoa: The Jellies
Generally referred to as jellies
Medusa stage is dominant
Come in a variety of forms
Aggressive predators, and can be dangerous to humans
Jelly
Portugese
Man-o-war

28. Class Scyphozoa
Jellyfish
The medusae are large and contain massive amounts of mesoglea
Possess four gastric pouches lined with nematocysts; these are connected with the mouth an the gastrovascular system

29. Scyphozoan Life Cycle - Aurelia
Gametes develop in gastrodermis of gastric pouches; eggs and sperm are shed through mouth
Fertilized eggs develop into a planula larva; settles on substrate and develops into a polyp - scyphistoma
Scyphistoma produces a series of polyps by budding - strobila
The polyps undergo differentiation and are released from the strobila as free swimming ephyra
Ephyra matures into an adult jellyfish

30. Class Anthozoa: Anemones & Corals
Appear like plants but are animals.
Polyp form is dominant.
Corals build calcium shells to protect themselves
Have symbiotic relationship with algae.
Can build extensive masses which can form land masses.
Anemone
Coral Polyps

31. Class Anthozoa
Exclusively marine; there is no medusa stage
At one or both ends of the mouth is a ciliated groove called the siphonoglyph; generates a water current and brings food to the gastrovascular cavity
Possess a well developed pharynx
The gastrovascular cavity is large and petitioned by septa or mesenteries; increase surface area for digestion or support
Edges of the septa usually have threadlike acontia threads, equipped with nematocysts and gland cells

32. Class Anthozoa cont.
Solitary anthozoans include sea anemones
Most anthozoans are colonial (e.g. corals) and secrete external skeletons composed of calcium carbonate.
Corals obtain much of their energy from microscopic photosynthetic green algae (zooxanthellae) or dinoflagellates that live symbiotically inside the cells of the coral

33. Coral Reefs
Clownfish living in
An anemone