Characteristics: Members of phylum Cnidaria possess radial or biradial symmetry. They have diploblastic and tissue level organization. They contain gelatinous mesoglea between the epidermal and gastrodermal tissue layers. A gastrovascular cavity is present in them. Nervous system in the form of nerve network is present. Specialized cells called cnidocytes are present and they are used for defense, feeding and attachment. The Cnidaria include over nine thousand species and they are mostly marine.
Structure: Cnidarians possess diploblastic and tissue level organization. Cells organize into tissues and the tissues carry out specific functions. All cells are derived from two embryological layers. The ectoderm of embryo form an outer layer of body wall called epidermis. Endoderm forms gastrodermis. Gastrodermis is the inner layer of body wall. A jellylike layer is present between the epidermis and gastrodermis. It is called mesoglea. Cells are present in the middle layer of some cnidarians. But these cells have their origin in the epidermis or gastrodermis. Cells of the epidermis and gastrodermis differentiate into number of cell types. These cells are used for protection, food gathering, coordination, movement, digestion and absorption. Nematocysts: Nematocysts are characteristic of phylum Cnidaria. They are produced by special cells called cnidocytes. Cnidocytes are epidermal or gastrodermal cells. Nematocysts are used for attachment, defense and feeding. Nematocyst contains a fluid filled intracellular capsule. It encloses a coiled hollow tube. A lid like operculum covers the capsule at one end. The cnidocytes has a modified cilium cnidocil. Cnidocil is stimulated and opens the operculum. A coiled tube is discharged from nematocyst. This tube hit the prey and paralyze it. There are 30 kinds of nematocysts. Six or more kinds may be present in one individual. Some of these are: Nematocysts with a long tube armed with spines: These are used for food gathering and defense. They discharge their tube and penetrate the prey. The spines have hollow tips. It injects paralyzing toxins into prey. Nematocysts with unarmed tubes: The tube wrap around prey or a substrate. Nematocysts with sticky secretions: These secretions help the animal to anchor itself with substrate.
Reproduction: Sexual reproduction: Most cnidarians are dioecious. Sperms and eggs are released into gastrovascular cavity or to the outside of the body. In some cases, eggs are retained in parent until fertilization. Blastula is formed early in development. There is migration of surface cells to the interior. The cavity of embryo is filed with these cells. These cells form the gastrodermis. Embryo elongates and forms a ciliated and free swimming larva. This larva is called planula. Planula attaches to substrate. Its interior cells split to form gastrovascular cavity and a young polyp develops.
Asexual reproduction: Medusa is formed by budding from the body wall of a polyp. New polyps are formed from other polyps by budding. Buds may detach from the polyp. Sometimes, They may remain attached to parent and form zooid of the colony.
Alternation of generations: The phenomenon in which asexually reproducing polyp alternates with sexually reproducing medusa is called alternation of generation. Most cnidarians possess two body forms in their life histories. Polyp: The tube like zooid is called polyp. Polyp reproduces asexually. It is sessile and attaches to substrate at the aboral end. It has cylindrical body called column. Its mouth is surrounded by food gathering tentacles. Medusa: The umbrella like zooid is called medusa. It is dioecious and free swimming. Its shape is like inverted bowl. Tentacles hang from its margins. The mouth opening is centrally located at lower side. Medusa swims by gentle pulsation of body wall. Mesoglea is more abundant in a medusa than in polyp. It gives the medusa jellylike appearance.
0 Comments