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 (Dimorphic)
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. Forms of Cnidarians

5. 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

6. Decentralized Nervous System

7. 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

8. 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

10. Everted nematocyst
Nematocyst Firing

11. 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 (male and females)
Fertilization is external, with the zygote becoming a elongated, ciliated, radially symmetrical larva - planula larva
Planula larva

12. Cnidarian Dimorphic Life Cycle
Fig. 13.9
Colonial hydrozoan

13. Cnidarian Taxonomy Phylum Cnidaria Class Hydrozoa (Hydras)
Class Scyphozoa (Jellyfish)
Class Cubozoa (Box Jellyfish)
Class Anthozoa (Anemones and Corals)

14. Phylogeny of Cnidaria
Cladogram based on molecular systematic data

15. 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

16. 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)

19. 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

20. 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

21. Fire Corals NOT TRUE CORAL TRUE CORAL Cnidocytes cause severe pain
Hydrozoan colony
TRUE CORAL
Anthozoan colony
Cnidocytes cause severe pain
Fig

22. Dimorphic Life Cycle and Reproductive Modes
Asexual
Budding
Medusa buds
Polyp buds
Sexual
Gonadal tissue
Gametes
Fertilization, embryogenesis
Planula larvae
Fig. 13.Fig. 13

23. Scyphozoan Characteristics
Jellyfish
Medusa is dominant body form
Go through small polyp stage during life cycle
Stinging tentacles
May live close to shore or the open ocean

24. Parts of a Jellyfish 24 24

25. 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

26. Parts of a Jellyfish 26 26

27. 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

28. Life Cycle of Aurelia

29. Aurelia aurita (Moon jelly)
Fig

30. CNIDARIA Class Scyphozoa
Sea thimble jellyfish (Linuche unguiculata), Honduras
Tiny jellyfish (< 1 inch) that swarm in the spring
Adults and larvae (“sea lice”) may cause a severe skin reaction in humans
Photo Copyright © Diane R. Nelson

31. CNIDARIA Class Scyphozoa
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
CNIDARIA Class Scyphozoa
Jellyfish (Mastigias sp., family Rhizostomeae) from Jellyfish Lake in Palau, western Pacific
Have lost their ability to sting and depend on zooxanthellae for nutrition
Photo Copyright © Diane R. Nelson

32. CNIDARIA Class Scyphozoa
Lion’s Mane Jelly (Cyanea capillata)
Bell – 6 ft
Tentacles – 120 ft

33. CLASS CUBOZA
The medusa form of a box jellyfish has a squarish, box-like bell. From each of the four lower corners of this hangs a short pedalium or stalk which bears one or more long, slender, hollow tentacles

34. underside of the bell is a mobile appendage called the manubrium which somewhat resembles an elephant's trunk
gastrovascular cavity. It is divided by four equidistant septa into a central stomach and four gastric pockets. The eight gonads are located in pairs on either side of the four septa. The margins of the septa bear bundles of small gastric filaments which house nematocysts and digestive glands and help to subdue prey

35. The box jellyfish's nervous system is more developed than that of many other jellyfish. Notably, they possess a nerve ring around the base of the bell that coordinates their pulsing movements
box jellyfish are unique in the possession of true eyes, complete with retinas, corneas and lenses. Their eyes are set in clusters called rhopalia, located in pockets halfway up the outer, flat surfaces of the bell.

37. Class Anthozoa No medusae, polyp only Mouth has a pharynx
Gastrovascular cavity is divided into sections
Mesoglea contains amoeboid cells
Sexual and asexual reproduction

38. Body structure of Anthozoans
Pedal disk
Oral disk
Siphonoglyph – moves water into gastrovascular cavity to maintain hydrostatic pressure.
Acontia – prevents live prey from damaging gastrovascular cavity.

39. Class Anthozoan Characteristics.
Sea Anemones
Brightly colored & Resemble flowers
Solitary polyps
Feed on invertebrates & fish
Corals
Most are colonial
Build limestone case
Live as polyps in their case

40. 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

41. copyright cmassengale
Sea Anemones & Coral
Exist only as Polyps
Clownfish live in anemone tentacles
Corals build a limestone case which build up & form reefs
ANEMONE
copyright cmassengale
CORAL 41

42. 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

43. Distribution of Coral Reefs
copyright cmassengale 43