Phylum Cnidaria

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

6. Nutrition • 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

7. Stinging Organelles • Prey capture is enhanced by use of specialized stinging cells called cnidocyteslocated in the outer epidermis. • Each cnidocyte has a modified cilium - cnidocil, and is armed with a stinging structure called a nematocyst.

8. Stinging Organelles • 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

9. Reproduction • One of the most amazing adaptations is the ability of some cnidarians to regenerate lost parts or even a complete body • Asexualreproduction is common with new individuals being produced by budding Planula larva

10. Reproduction • 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

11. Cnidarian Taxonomy

12. 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

13. 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

14. Solitary Hydras • 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

15. 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)

16. 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

18. 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

19. 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

20. Class Scyphozoa • Jellyfish • Possess four gastric pouches lined with nematocysts; these are connected with the mouth an the gastrovascular system

21. 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

22. Scyphozoan Life Cycle - Aurelia • 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

23. 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

24. 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

25. Class Anthozoa cont. • Solitary anthozoans include sea anemones • Most anthozoans are colonial (e.g. corals) and secrete external skeletons composed of calcium carbonate.

26. Class Anthozoa cont. • Corals obtain much of their energy from microscopic photosynthetic green algae (zooxanthellae) or dinoflagellates that live symbiotically inside the cells of the coral