Cnidaria - Structure, Classification, Characteristics, Reproduction

Cnidaria: Cnidaria, a phylum of the Animalia kingdom, contains over 11,000 species of aquatic animals that live in both freshwater and marine environments, focusing on the latter. Jellyfish, hydroids, sea anemones, corals, and some of the smallest marine parasites make up this diverse group. Their distinct features include the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella that are mainly used for envenomation and prey capture, as well as a dispersed nervous system throughout a gelatinous body.

Cnidaria Clasification

1. Anthozoa (sea anemones, corals, sea pens): These creatures are primarily sessile and attach themselves to the seafloor or other stable surfaces. Sea anemones have a soft body and stinging tentacles. Corals are colonial marine invertebrates that produce calcium carbonate to form hard skeletons. Sea pens are feather-like cnidarians that live on the seafloor.
2. Scyphozoa (true jellyfish): These free-swimming jellyfish have bell-shaped bodies and stinging tentacles. Jellyfish have a complex life cycle that includes both polyp and medusa stages.
3. Cubozoa (box jellies): Box jellies resemble boxes with four tentacles at each corner and are known for their potent venom, which can be fatal to humans.
4. Hydrozoa (hydras, siphonophores, and colonial jellyfish): This is the most diverse cnidarian group, containing both freshwater and marine species. Hydras are small freshwater polyps. Siphonophores are colonial cnidarians made up of multiple polyps that function as a single entity. The Portuguese man of war is a well-known siphonophore.

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Cnidaria Structure

• Ectoderm: The outer layer, which facilitates movement, sensation, and houses specialized stinging cells known as cnidocytes.
• Endoderm: The inner layer, responsible for digestion and the absorption of nutrients.

• Polyps: Stationary (attached) form characterized by a cylindrical body, a basal disc for attachment, and tentacles encircling a central mouth at the opposite end.
• Medusae: Free-swimming form featuring a bell-shaped or umbrella-shaped body, with tentacles hanging underneath and a mouth on the underside.

Mammary Glands

Cnidaria Body Symmetry

Okazaki Fragments

Cnidaria Characterstics

1. Radial Symmetry: Cnidarians show radial symmetry, where their body parts are arranged around a central axis, similar to the spokes of a wheel. This arrangement enables them to interact with their environment from all directions.
2. Tissue Level of Organization: They have a simple body plan with two layers of cells: an outer layer called the epidermis and an inner layer called the gastrodermis. These layers are separated by a jelly-like substance called mesoglea. This level of organization is known as diploblastic.
3. Specialized Cells: Cnidarians possess specialized stinging cells called cnidocytes, which contain nematocysts. These structures are used for defense and capturing prey. When triggered, the nematocysts fire a barb or venomous filament to immobilize the prey.
4. Gastrovascular Cavity: Cnidarians have a central digestive cavity with a single opening that serves as both the mouth and anus. This cavity is called the gastrovascular cavity and is used for digestion, circulation, and gas exchange.
5. Life Cycle: Cnidarians exhibit two main body forms in their life cycle: a polyp form, which is typically sessile and cylindrical, and a medusa form, which is free-swimming and umbrella-shaped. The life cycle includes both sexual and asexual reproduction.
6. Habitats: They are found in marine environments worldwide, from shallow coastal waters to the deep sea. Some species also inhabit freshwater environments.
7. Examples: Common examples of cnidarians include jellyfish, corals, sea anemones, and hydras.

lymphocytes

Cnidaria Examples

• Jellyfish - These are free-swimming, gelatinous marine animals characterized by their stinging tentacles.
• Sea anemones - Sessile organisms that are attached to a substrate, featuring a cylindrical body and stinging tentacles.
• Corals - Marine animals that form colonies and secrete calcium carbonate to construct reefs.
• Hydras - Freshwater cnidarians known for their simple body structure.
• Portuguese man o' war - A colonial cnidarian with a gas-filled bladder that enables it to float on the water's surface.
• Sea pens - Sessile cnidarians resembling feathers or pens.
• Sea fans - Sessile cnidarians with flattened, fan-shaped bodies.
• Sea whips - Sessile cnidarians with elongated, whip-like bodies.

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Cnidaria  Reproduction

Sexual Reproduction

• Polyps are stationary organisms resembling flower-like structures. They reproduce sexually by releasing sperm and eggs into the surrounding water.
• Medusae are free-swimming organisms resembling jellyfish, which develop from fertilized eggs. They also participate in sexual reproduction by releasing sperm and eggs into the water.

Sexual Reproduction In Flowering Plants

Asexual Reproduction

• Budding: This process involves the development of new individuals as buds on the parent polyp. These buds eventually detach and become independent polyps.
• Fragmentation: In cases where a cnidarian is injured or broken into pieces, each fragment can regenerate into a new individual.
• Strobilation: Certain jellyfish species can undergo asexual reproduction during the polyp stage through a process known as transverse fission. This process involves the horizontal division of the body into multiple segments, each of which develops into a new medusa.