Phylum: Cnidaria

• This phylum is also known as Coelenterata and includes jellyfish, blue bottles, sea anemones and corals
• Aquatic and mostly found in water
• Only a few species occur in fresh water; Hydra
• Simple body plan
• Radially symmetrical and body wall consists of two cell layers: ectoderm on outside and an endoderm on the inside
• A non-cellular, jelly-like layer, the mesoglea, occurs between the two layers, therefore Cnidarians are diploblastic
• No coelom, but does have a central cavity known as the coelenteronm with only one opening to outside, the mouth. Therefore cnidarians are acoelomate animals
• The coelenteron is filled with water that acts as a hydrostatic skeleton and also plays a role in transport and digestion
• A protein-like or calcerous skeleton occurs in corals, which forms coral reeds
• Two basic body forms are distinguished: a polyp and a medusa
• Polyps are cylindrically shaped, wish basal disc attached to a substrate such as a rock or branch
• Sessile and usually occur in colonies
• The free end has a whorl of 6-8 tentacles surrounding the mouth
• Examples of polyps include sea anemones and corals
• Medusa has a flattened, umbrella shape with tentacles on the edges
• The mouth occurs on the underside, between the tentcles
• The medusa is free swimming and include jellyfish and blue bottles
• Cnidaria have a tissue level of organisation with specialised cells that occur in the ectoderm and endoderm
• The ectoderm mainly consists of musculo-epithelial cells
• Sensory cells occur in the ectoderm and endoderm
• An extended nerve net mainly occurs in the mesoglea and plats an important role in locomotion with the musculo-epithelial cells
• Highly specialised stinging cells (nematoblasts) occur on the tentacles
• These cells are adapted to catch, paralyse and hold on to prey
• The stinging cell has a trigger (cnidocil) that causes the cell to respond with stimulated
• An invaginated, sac-like structure, the nematocyst, occurs in the stinging cell and is filled with a toxic fluid
• In the nematocyst there is a long, thread-like filament which discharges when the cell is stimulated
• The filament is forced into the body of the prey and toxin, which paralyses the prey, is released
• Some filaments have barbs to hold onto prey more efficiently
• The tentacles pull the paralysed prey into the mouth
• The food is digested in the coelenteron and surrounding endoderm
• the undigested remains leave the body again via the mouth
• Digestion in Cnidarians is discontinuous, considering the digestive tract is not a through gut- it is an incomplete gut
• A transport system is absent as the body wall only consists of two cell layers
• The ectoderm adapted to react to stimuli, to catch, paralyse and hold onto prey.

  The endoderm is adapted for digestion and absorption of nutrients