Characteristics of Cnidaria: Study Notes

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General Characteristics of Cnidaria: The Stinging-Celled Animals

The phylum Cnidaria is a diverse group of aquatic animals distinguished by the presence of specialized stinging cells known as nematocysts. These organisms display unique anatomical and reproductive characteristics, making them a fascinating study in marine biology.

1. Name Meaning

• The term Cnidaria originates from the Greek word "cnidos," meaning stinging needle. This name is inspired by their defining feature—nematocysts, or stinging cells, which help them defend against predators and capture prey.

2. Former Name

• Cnidarians were formerly classified under the name Coelenterates, a term no longer in use due to advances in classification.

3. Examples

• Cnidarians include a variety of species, such as coral animals, true jellyfish, sea anemones, and sea pens.

4. Habitat

• Cnidarians are exclusively aquatic, with the majority of species found in marine environments. Only a few species inhabit freshwater ecosystems.

5. Number of Species

• Approximately 9,000 species of Cnidaria have been identified, showcasing considerable diversity across marine habitats.

6. Body Cavity

• Cnidarians possess a central Gastrovascular cavity, also known as the enteron, which serves as both the digestive and circulatory chamber. This cavity is not considered a true coelom.

7. Movement

• Cnidarians exhibit two main forms of movement: they can be sedentary (fixed in place) or free-swimming. This flexibility allows them to adapt to various marine environments.

8. Organization Level

• They exhibit a tissue grade of organization, meaning they possess tissues but lack true organs, differentiating them from more complex animals.

9. Symmetry

• Cnidarians display radial symmetry, allowing them to interact with their environment from all directions.

10. Body Wall

• The body wall in Cnidarians is diploblastic, meaning it has two main layers:
  • Ectoderm (epidermis): The outer protective layer.
  • Endoderm (gastrodermis): The inner layer that lines the gastrovascular cavity.
• These layers are separated by a jelly-like substance called the Mesoglea, providing structural support.

11. Reproduction

• Cnidarians have a unique alternation of generation in their life cycle, alternating between two primary body forms:
  • Polyp: A sessile, asexual form attached to a surface.
  • Medusa: A free-swimming, sexual form that produces gametes.

12. Special Features

• Nematocysts: Specialized stinging cells containing poison-tipped barbs, which are used for defense and capturing prey.

The Two Forms of Cnidarians: Polyp and Medusa

Cnidarians exhibit two distinct body forms, each with unique characteristics and roles within the lifecycle.

(i) Polyp

1. Lifestyle and Reproduction: Polyps are sessile and function primarily as asexual zooids. They are attached to a substrate, remaining stationary.
2. Shape and Structure: Cylindrical and elongated along the body’s vertical axis, polyps have a distinctive shape that facilitates their attachment.
3. Attachment: In solitary polyps, the basal end is attached to a surface via a pedal disc, a disc-like holdfast that anchors the polyp.
4. Oral End and Tentacles: The oral end of a polyp contains the mouth, surrounded by a ring of tentacles used for capturing prey and guiding food into the mouth.

(ii) Medusa

1. Lifestyle and Reproduction: Medusae are free-swimming and serve as the sexual zooids of Cnidarians, facilitating reproduction through gamete production.
2. Shape: Medusae vary in shape, often resembling a bell or thin disc. They are generally convex on the upper side and concave on the lower side.
3. Surface Structure: The upper, or ex-umbrellar surface, is the outer side, while the lower, or sub-umbrellar surface, faces inward.
4. Mouth and Velum: The mouth is located on the sub-umbrellar surface and may be partially covered by a membrane called the velum, which aids in controlling water flow.

Additional Unique Features of Cnidaria

• Nematocysts: These stinging cells inject toxins into prey, immobilizing them for easier capture.
• Reproduction: Cnidarians reproduce both asexually (in the polyp form) and sexually (in the medusa form), resulting in a free-swimming, ciliated larval stage called the planula.
• Polymorphism: Many Cnidarians exist in colonial forms, with multiple individuals or zooids serving different functions. This polymorphism allows them to thrive in complex social structures and enhances survival within various ecosystems.

Cnidarians, with their unique nematocysts, radial symmetry, and polymorphic colonies, play vital ecological roles in marine environments, contributing to the biodiversity and health of aquatic ecosystems.

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