Cnidarians contain 2 body shapes: The polyp which is a cylindrical structure with tentacles at the top of the animal. The medusa is an umbrella shaped structure with its tentacles near the bottom of the animal.
They contain tentacles with special stinging cells called nematocysts. These nematocysts come in a variety of structures. They are activated by touch and can only be used once. They function in protecting the animal and helping capture its food. The cnidocytes are the cells that produce the nematocysts.
They contain 2 cell layers: the ectoderm or outer cell layer and the endoderm the inner cell layer.
These 2 layers are separated by a jelly-like layer called the mesoglea.
Within the mesoglea we find a nerve net. These organisms do not contain a brain. .
They contain a central cavity called the Gastrovascular cavity. Digestion takes place here and the contents are absorbed by the endodermal cells. The undigested food is expelled through its mouth. This is considered a two way digestive process.
They contain radial symmetry. Some organisms are sessile while others are free floating.
They reproduce by budding and regeneration. They also reproduce sexually. They contain both male and female reproductive structures in the same body. They are neither male or female, they are called hermaphroditic.
Many of these organisms are marine though some (Hydra) are fresh water.
General Characteristics of flatworms:
These are the first organisms to exhibit three distinct cell layers. The ectoderm, mesoderm , and endoderm.
They are classified as acoleomates meaning they have no true body cavity (coelom ).
They contain bilateral symmetry. These organisms exhibit cephalization. Many of there sense organs are grouped at the anterior end of the organism. This affords them directional movement. Usually the first part of an organism that enters a new environment is the head.
They contain rudimentary organ systems such as: a ladder-like nervous system with a small brain and in some cases photosensitive eye spots. A network of flame cells used to sweep nitrogenous waste out of the body. A primitive digestive system consisting of a mouth, pharynx and branched gastrovascular cavity.
Flatworms exhibit two forms of life styles, free-living and parasitic.
They can reproduce asexually ( regeneration ) or sexually (hermaphroditic).
