• Animals without a backbone (a row of bones called vertebrae)
• make up 97% of all known species of animals
• All major classes have marine species and some classes are exclusively marine

• Simple organisms that are aggregations of specialized cells
• Limited cellular organization - cells that are largely independent of each other. no true tissues or organs
• simplest of multicellular animals
• Sessile, permanently attached to hard substrates
• Share a common body plan
• No symmetry - asymmetrical

• Next step up from sponges - evolution of specialized tissues
• permits coordinated activties: swimming, feeding, responses to external stimuli
• Sea anemones, jellyfish, corals, sighonophores
• Radial symmetry
• Carnivores
• Stinging cells: cnidoblasts
• Look the same from all sides
• oral surface and aboral surface
• 2 basic form: polyp and medusa
• Central mout with tentacles (catching food) and one way gut!

both medusa and polyp stage

Sophonophores

• drifting colonial hydrozoans
• pelagic predators
• powerful toxins

• Large medusae are the dominant life stage
• Large species may reach 3m in diameter
• Swims in rhythmic contractions
• Includes a highly toxic group: Cubomedusae

• deadliest species of jellyfish
• box jellyfish are pale blue and transparent
• box jellyfish is considered the most venomous marine creature
• it can kill more people than stonefish, sharks and crocodiles combined

• Solitary or colonial polyps that lack a medusa stage
• Anemones, gorgonians, corals

• 'soft corals'
• protein matrix
• warm and saltwater