Cnidaria - Kingdom: Animalia - SS1 Biology Lesson Note

Cnidaria is a phylum of aquatic animals that includes diverse organisms such as jellyfish, corals, sea anemones, and hydroids. They are characterized by the presence of specialized stinging cells called cnidocytes, which they use for defense and capturing prey.

 

Hydroid

Coral

Sea anemone

Jelly Fish

 

(Images from istockphotos.com)

Here are some key features and characteristics of Cnidaria:

1. Radial Symmetry: Cnidarians exhibit radial symmetry, meaning their body parts are arranged around a central axis, allowing them to be divided into similar halves in multiple planes.

2. Tissue Level of Organization: Cnidarians have true tissues, which are groups of specialized cells that perform specific functions. However, they lack complex organs and organ systems found in higher animals.

3. Nematocysts: Cnidarians possess cnidocytes, which are specialized cells that contain stinging capsules called nematocysts. These nematocysts can discharge venomous threads or barbs when triggered, aiding in prey capture or defense.

4. Two Body Forms: Cnidarians typically exhibit two main body forms: the polyp and the medusa. Polyps are cylindrical, sessile (non-motile) organisms attached to a substrate, while medusas are free-swimming, bell-shaped organisms.

5. Gastrovascular Cavity: Cnidarians have a central digestive cavity called the gastrovascular cavity. This cavity has a single opening, serving as both a mouth and an anus. It aids in digestion and the distribution of nutrients throughout the body.

6. Simple Nervous System: Cnidarians possess a decentralized nervous system, consisting of a nerve net that allows them to respond to stimuli. However, they lack a centralized brain or complex sensory organs.

7. Reproduction: Cnidarians can reproduce both sexually and asexually. Sexual reproduction involves the release of eggs and sperm into the water, resulting in the formation of larvae that later develop into polyps or medusas. Asexual reproduction can occur through budding, where new individuals grow from the body of the parent organism.

8. Ecological Importance: Cnidarians play crucial roles in marine ecosystems. Corals, for example, build coral reefs, which are highly diverse and productive habitats supporting numerous other species. Cnidarians also serve as a food source for many marine organisms and contribute to nutrient cycling.