Characteristics of Cnidarians 
Cnidocytes (stinging cells) 
Nematocysts (barbs) 
Gastrovascular cavity (digestion) 
Most are radial symmetry, some have asymmetry (corals) 
Cnidarians have two body forms: 
1. Polyp - stationary, vase-shaped 
Examples: hydra, coral, sea anemone 
2. Medusa - swimming, cup-shaped 
Examples: jellyfish, portuguese man of 

1) cnidarians have bilateral symmetry, 2) there are 2 different forms(dimorphism) , a medusa and a polyp, 3) reproduction is asexual or sexual. 4) they have cnidoblasts, special cells that contain nematocysts, the cells that sting, and 5) they have an internal cavity, and a mouth where food comes in and out. digestion is extracellular.

All cnidarians are characterized by radially symmetric body plans, rather than the bilaterally symmetric body plans that are found in most other animal phyla. Although cnidarians are more advanced than sponges (phylum Porifera) in that they possess distinct tissue layers, they lack many of the features of more advanced animal phyla, such as internal organs and central nervous systems. Most cnidarians possess tentacles, and many also have nematocysts (specialized stinging cells). Both are involved in feeding.

Cnidarians are characterized by the presence of three tissue layers, an outer protective epidermis, a middle layer called the mesoglea, and an inner layer called the gastrodermis, whose function is primarily digestive. The mesoglea of cnidarians is not as highly developed as the mesoderm of other animal groups, being primarily gelatinous with only a few fibrous or amoeba-like cells.

Cnidarians possess only one digestive opening, which serves as both the mouth and the anus. This opening is surrounded by tentacles and leads to an internal digestive cavity called the gastrovascular cavity .

Cnidarians feed using tentacles that are embedded with stinging nematocysts. The nematocysts are springing barbs with small hairlike triggers that are activated by contact with prey. Most nematocysts require stimulation in more than one sensory mode before they will fire. For example, a nematocyst may respond only if there is mechanical stimulation from physical contact with the prey as well as chemical stimulation signaling the presence of suitable prey. As nematocysts fire, barbs unfold and become embedded in the tissue of the prey. At the same time, the nematocysts inject the prey with an immobilizing toxin through a long hollow thread within the barb. Once the prey item has been captured and subdued, tentacles are used by the cnidarians to bring the prey item into the gastrovascular cavity. Within the gastrovascular cavity, the food item is broken into small particles by digestive enzymes secreted by gastrodermal cells lining the cavity. The minute particles are then taken in by the gastrodermal cells, and digestion is completed in digestive vacuoles (small cavities) within these cells. The indigestible remnants of the prey are expelled from the mouth of the gastrovascular cavity.

One hypothesis about the origin of nematocysts suggests that they wereprokaryotic endosymbionts which lived within eukaryotic cells as mutualists(mutualisms are symbiotic relationships between individuals of two different species, in which members of both species derive benefits from the relationship), the same way organelles (specialized parts of cells) such asmitochondria and chloroplasts are believed to originate.

Unlike more advanced animal phyla, cnidarians lack a central nervous system. Instead, their nerves are organized in nerve nets that cover the entire body. Impulses spread slowly out from the point of stimulation along the nerve net. Some cnidarians, such as jellyfish, have more complicated arrangements of nerves that allow for more complex responses to stimuli as well as more effective patterns of movement.

Cnidarians also lack certain tissue types found in other animal phyla, such as true muscle cells. However, they do have fibers that can contract and therefore can be used in capturing prey and in moving about.

In short, cnidarians are stinging animals :) hope it helps