Phylum Cnidaria

Phylum Cnidaria is a group of aquatic animals that includes jellyfish, corals, sea anemones, and hydras. They are characterized by their radial symmetry, tentacles with stinging cells called cnidocytes, and a simple nerve net. Cnidarians can be found in marine environments all over the world, and they play important roles in marine ecosystems. They are also known for their ability to regenerate and reproduce asexually.

Phylum Cnidaria

General Characteristics:

- Multicellular and aquatic: Consist of various cells and live in aquatic environments.
- Radial symmetry: Body parts arranged around a central axis.
- Diploblastic: Have two germ layers (ectoderm and endoderm).
- Gastrovascular cavity: A single opening serves as both mouth and anus, leading to a blind digestive cavity.
- Cnidocytes: Specialized stinging cells used for defense, capturing prey, and locomotion.

Classes:

1. Hydrozoa:

- Small, colonial organisms (e.g., jellyfish, sea anemones, corals)
- Exhibit alternation of generations between polyp and medusa stages

2. Scyphozoa:

- Large, solitary jellyfish with complex life cycles
- Medusa stage is the dominant stage

3. Anthozoa:

- Colonial or solitary animals (e.g., corals, sea anemones)
- Lack a medusa stage and have a single polyp stage

Ecological Importance:

- Primary producers in marine ecosystems (corals through photosynthesis)
- Provide habitats and shelter for other marine organisms
- Important in nutrient cycling and carbon sequestration

Evolutionary Significance:

- Considered the oldest of the animal phyla, with a long evolutionary history
- Precursors to other more complex animal groups
- Their stinging cells are considered a key innovation in animal evolution

Examples:

- Jellyfish
- Sea anemones
- Corals
- Portuguese man o' war
- Bluebottle jellyfish
- Box jellyfish