1. Phylum Cnidaria

2. Phylum Cnidaria Radial or biradial symmetry
No posterior or anterior ends
Instead they have oral (end with the mouth) and aboral ends
Mostly marine
Over 9,000 species

3. Characteristics of Cnidarians
Tissue level organization
Gelatinous mesoglea
Between epidermis and gastrodermis
Gastrovascular cavity
Nervous system in the form of a nerve net
Specialized cells (cnidocytes) for defense, feeding, and attachment

4. The Cnidarian Body Wall
Two layers of tissues
Ectoderm of the embryo develops into the epidermis (outer layer of the body wall)
Endoderm of the embryo develops into the gastrodermis (inner layer of the body wall)
Mesoglea- jellylike layer between the epidermis and gastrodermis
Usually acellular (lacks cells)

5. Gastrovascular Cavity
Functions in digestion, respiration, metabolic wastes, and discharge of gametes
All particles enter and exit the gastrovascular cavity through the mouth

6. Nervous System Most primitive of all animal nervous systems
Nerve cells located in the epidermis
Interconnect to form a nerve net
React to stimuli

7. Cnidocytes Specialized cells used for attachment, defense, and feeding
Called “stinging cells”
Usually has a modified cilium (cnidocil)
Operculum covers the capsule of the cnidocyte at one end
Cnida- fluid-filled capsule enclosing a hollow tube
Most common type is a nematocyst

8. Structure & Support
Hydrostatic Skeleton- water is confined in a cavity to provide an inflated shape
Epitheliomuscular cells contract and aid in moving water throughout the body for support

9. Feeding Diet consists of small crustaceans and small fish
Nematocysts entangle and paralyze prey
Tentacles shorten and pull the prey to the mouth
Cells in the gastrodermis release enzymes to digest food in the gastrovascular cavity
Food is distributed to the rest of the body via peristalsis (contractile motions)

10. Alternation of Generations
Two body forms
Polyp- asexual and sessile (does not move)
Attaches to a substrate
Mouth surrounded by food-gathering tentacles
Medusa- dioecious and free swimming
Tentacles dangle from it’s margins
Mouth faces downward
Mesoglea more prominent in this form

12. Locomotion Polyps move by somersaulting or “inchworming”
Medusae move by swimming and floating
Use water currents to aid in locomotion
Move vertically when swimming
Jellyfish swimming

13. Reproduction Mostly dioecious Embryo elongates into a planula
Egg and sperm released into the gastrovascular cavity
Embryo elongates into a planula
Ciliated free-swimming larva
Develops into a polyp once it attaches to a substrate
Budding is possible from the body wall (medusas develop this way)
May also form other polyps from budding