Cnidarians Flashcards
Origin of Cnidarians?
Production of eggs and larvae by sponges generated a new source of food in the water column Some sponges evolved nematocyst (stinging) to capture eggs, and lost choanocytes and ostia Evolved synapses to link sensory cells and nematocysts Mouth evolved and extracellular digestion Present by early to mid-Cambrian 3 main classes: Endocnidozoa, Medusozoa, and Anthozoa
Structure of a Cnidarian?
Two cell layers: ectoderm (outer) an entoderm (inner) All exhibit both polyp (sessile) and medusae (swimming) phases in their life (dimorphic) Radial symmetry Body cavity: digestive cavity with one opening called the coelenteron No head, centralised nervous system or discrete gas exchange/excretory/circulatory system Mouth is generally surrounded by tentacles
What are nematocysts/cnidae?
Stinging or adhesive structures housed in a cnidocyte One of the largest and most complex single celled structure known When stimulated, the tube inverts piercing the prey’s body Mechanoreceptor or chemoreceptor Combination of hydrostatic pressure caused by rapid influx of water and of squeezing of the capsule by contractile fibres Discharge takes a few milliseconds and occurs at up to 2m/s True nematocysts, spirocysts and ptychocysts
Toxins of Cnidarians?
Penetrant nematocysts inject venom Irukandji jellyfish can cause death with their stings
Alternation of generations in
Polyps are tube like with a sac gut. More diverse due to asexual capacity and colony formation. Medusae are bell shaped. Polyp -> Medusae -> Egg -> Planula -> Polyp
Describe a jellyfish?
Velella Velella. Each colony is male or female. Gonozooids produce numerous tiny medusae by budding. Medusae become mature in about 3 weeks and sink into the deep sea. Eggs develop and rise to the surface with the characteristic sail.
Where do Cnidarians get their name?
From the possession of specialised stinging cells called Cnidae, which are carried on sets of tentacles. Their poisonous secretions are used to subdue or kill prey.
How does the sea anemone secrete its waste?
Through its mouth
Diagram of a sea anemone?
Diagram of a jellyfish?
