Phylum Cnidaria

They are soft-bodied, carnivorous animals that have stinging tentacles arragend in circles around their mouth. They are the simplest animals to have body symmetry and specialized tissues. Their life cycle typically includes two different- looking stages, a polyp and a medusa.

Cnidarians include jelly fish, sea anemones and corals.

Cnidarian polyps and medusas each have a body wall that surrounds an internal space called a gastrovascular cavity. The gastroderm is the inner lining of the gastrovascular cavity, where digestion takes place. The epidermis is the outer layer of cells. The mesoglea is a layer that lies between these two tissues. It varies from a thin, noncellular membrane to a thick, jellylike material that contains cells

 

 

Reproduction in cnidarians

 

 

Most cnidarians reproduce both sexually and asexually. Polyps can reproduce asexually by budding. The new animal is genetically identical to the parent animal. One type of budding begins with a swelling on the side of an existing polyp. This swelling grows into a new polyp. In another type of budding, polyps produce tiny medusas that separate and become new individuals. In most cnidarians, sexual reproduction takes place with external fertilization in water. External fertilization takes place outside the female’s body. The sexes are usually separate.

 

Respiration in cnidarians

Following digestion, nutrients are usually transported throughout the body by diffusion. Cnidarians respire and eliminate the wastes of cellular metabolism by diffusion through their body walls.