PHYLUM CNIDARIA

            The phylum Cnidaria includes a diverse group of about 10,000 living species.  The name Cnidaria refers to the specialized cells called cnidocytes (Greek knide = nettle) which are unique to this phylum.  Cnidocytes contain stinging structures, nematocysts that are used to immobilize prey.  The cnidarians are exclusively aquatic animals with the majority of species found in marine habitats.  The phylum includes many brightly coloured and beautiful animals: jellyfish, sea anemones, sea fans, corals and hydras.

Cnidocytes contain stinging structures, nematocysts that are used to immobilize prey.

            Members of the phylum Cnidaria are radially symmetrical organisms characterized by a digestive body cavity  (a gastrovascular cavity) (not a coelom), some muscle fibers and many nematocysts. Cnidarians are composed of only two cellular layers, an outer epidermis (ectoderm) and an inner gastrodermis (endoderm). Thus, they are known as diplobsaltic organisms.  Each of these two layers gives rise to specific structures within the organism.  The epidermis is responsible for the production of much of the sensory system and surface skin while the gastrodermis contributes to the growth of structures such as the lining of the gut.  In cnidarians, these cell layers are thin and the animals are therefore very fragile. Between these two cellular layers is a gelatinous (non-cellular) layer of mesoglea. Contrary to epidermis and gastrodermis, the mesoglea is poorly developed (Figure 1).

Animals above the cnidarians on the evolutionary tree have three layers of cells. Thus, they are known as triplobsaltic organisms (the third layer lies between the ectoderm and endoderm and is called the mesoderm).  This third layer is used to build muscular, excretory, and reproductive structures.  The mesoderm allows animals to become more organized in their structure.  Cnidarians, for example, are organized mainly at the level of tissues, while those with three layers possess organs and elaborate organ systems.

The members of Phylum Cnidaria have one of two forms: a polyp or medusa.  A polyp is a tubular organism closed at one end with a mouth surrounded by tentacles at the other end.  A medusa is an umbrella-shaped, jellylike, free-swimming organism with a mouth at the end of a central projection called a manubrium (Figure 1).  We will examine one representative member for each body form.

Figure 1: Body forms in Cnidaria.

 

Medusa body forms.

Polyp body form

Polyp body form

Other Cnidarians...

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