Slide 1 Our representative group of radially symmetrical diploblasts are the Cnidarians.
Slide 2 This group is characterized by radial symmetry and they are diploblastic - they have 2 embryonic tissue layers - endoderm and ectoderm. They do not have mesoderm so they do not have true muscles - this means that they are limited in their mobility and some of them are sessile (non-moving) as adults. This is another completely aquatic group - they live mainly in marine environments but there are some freshwater cnidarians. Phylum Cnidaria consists of jellyfish, corals, and sea anemones.
Slide 3 The Cnidarians exist in either a poly or medusa form. The polyp form is sessile and the medusa form is motile. Most cnidarians have specialized cells known as cnidocytes that they use to sting either potential predators or prey. Within the cnidocytes are small stinging organelles known as nemetocysts.
Slide 4 This image shows the structure of the cnidocytes and nematocysts. The cnidocytes have a trigger - when something brushes against a trigger it causes a nematocyst to discharge and sting the offending organism. If you have ever been stung by a jellyfish - it was because you brushed against these triggers.
Slide 5 The cnidarians are capable of both sexual and asexual reproduction. Asexual reproduction is the result of budding off of polyps from a parent individual.
Slide 6 There are three classes within Phylum Cnidaria. Class Hydrozoa consists of freshwater jellies, freshwater hydras, and the highly toxic Portuguese Man O' War. Organisms in this class can be either in the polyp or medusa stage.
Slide 7 Class Scyphozoa are the true jellyfish. They only occur in the medusa stage.
Slide 8 Class Anthozoa consists of the sea anemones and corals. They occur only as polyps.