1. Phylum Cnidaria

2. General Characteristics
They are radially symmetrical; oral end terminates in a mouth surrounded by tentacles.
They have 2 tissue layers
Outer layer of cells - the epidermis
Inner gastrodermis, which lines the gut cavity or gastrovascular cavity (gastrodermis secretes digestive juices into the gastrovascular cavity)
In between these tissue layers is a noncellular jelly-like material called mesoglea

3. Cnidarian Body Plans Polyp form
Tubular body, with the mouth directed upward.
Around the mouth are a whorl of feeding tentacles.
Only have a small amount of mesoglea
Sessile
Medusa form
Bell-shaped or umbrella shaped body, with the mouth is directed downward.
Small tentacles, directed downward.
Possess a large amount of mesoglea
Motile, move by weak contractions of body

4. Movement
The cnidarian body is capable of some kind of coordinated movement
Both the epidermis and the gastrodermis possess nerve cells arranged in a loose network - nerve net (plexus), which innervate primitively developed muscle fibers that extend from the epidermal and gastrodermal cells
Stimulus in one part will spread across the whole body via the network

5. Nutrition
Cnidarians are carnivores with hydras and corals consuming plankton and some of the sea anenomes consuming small fishes
They use they 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
The gastrovascular cavity exists as 1 opening for food intake and the elimination of waste
There is no system of internal transport, gas exchange or excretion; all these processes take place via diffusion

6. Stinging Organelles
Prey capture is enhanced by use of specialized stinging cells called cnidocytes located in the outer epidermis.
Each cnidocyte has a modified cilium - cnidocil, and is armed with a stinging structure called a nematocyst.
The undischarged nematocyst is composed of a long coiled thread
When triggered to release, either by touch or chemosensation, the nematocyst is released from the cnidocyte and the coiled thread is everted
Some nematocysts function to entangle the prey; others harpoon prey and inject a paralyzing toxin

7. Reproduction
One of the most amazing adaptations is the ability of some cnidarians to regenerate lost parts or even a complete body
Asexual reproduction is common with new individuals being produced by budding
Sea anenomes engage in a form of asexual reproduction called pedal laceration
Cnidariand are dioecious
Fertilization is external, with the zygote becoming a elongated, ciliated, radially symmetrical larva - planula larva
Planula larva

8. Cnidarian Taxonomy

9. 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, mouth, hypostome 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

10. Colonial Hydrozoans - e.g., Obelia
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)

11. 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)
Also, the margin of the bell projects inward forming a shelf-like velum

12. Other Hydrozoans Portuguese man-of-war:
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

13. Class Scyphozoa Jellyfish
The medusae are large and contain massive amounts of mesoglea
The differ from the hydrozoan medusa in that the lack a velum
Possess four gastric pouches lined with nematocysts; these are connected with the mouth an the gastrovascular system

14. 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

15. 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

16. Solitary anthozoans include sea anemones
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