Cnidarians are animals that have a variety of colours and shapes. They range from transparent jellyfishes in the ocean to the vibrant colours of a sea anemone on a intertidal zone. A Cnidarian’s body is soft while their mouths are lined with stinging structures called nematocysts.
Many life cycles have two different and distinct states, a sessile flowerlike polyp, and a motile bell-shaped medusa. Both of these stages have a body wall that surrounds the gastrovascular cavity. A gastrovascular cavity is an internal space inside of the organism where digestion takes place. The body wall has three layers: epidermis, gastroderm, and mesoglea. Epidermis is the layer of cells that is on the outer surface of the body. Gastroderm is the layer of cells that covers the inner surface, lining the gastrovascular cavity. Mesoglea is the layer between the other two, and it ranges from a think noncellular membrane to a thick jellylike material that may contain wandering amebocytes. The mesoglea is thin in polyps and thick in medusae. Most Cnidarians are only a few layers thick because they have not had the need to evolve into more complicated body systems for basic survival.Cnidarians lack a brain and a centralized nervous system. What they have is a simple nervous system that is called a nerve net. Nerve nets are throughout the bodies of Cnidarians. The nerve nets are concentrated where the mouth is to inform the nematocysts when to sting.
Cnidarians lack three organized systems: the circulatory (internal transport) system, the respiratory system, and the excretory system. Instead, they respire, and excrete wastes through their body walls by diffusion.
All Cnidarians have radial symmetry, two germ layers, and specialized cells and tissues.
Most Cnidarians live in marine water, but some live in freshwater. Their habitats range from the middle of the ocean to intertidal zones to lakes. One imporant adaption for Cnidarians is that they live in symbiosis with tiny photosynthetic protists that live inside of them. (more on this later).
Examples of Cnidarians are hydroids, jellyfish, sea anemones (image above), portuguese man of war, sea pansies, sea wasps and corals.
Important vocabulary is in bold.
