Cnidarians: Key Characteristics

Cnidarians are a phylum of animals found mostly in marine habitats. They include familiar organisms like jellyfish, corals, and sea anemones. Understanding their fundamental characteristics is crucial for classifying them within the animal kingdom.

Diploblastic Nature

One of the defining features of Cnidarians is that they are diploblastic. This means their body wall is derived from two embryonic germ layers: the outer ectoderm and the inner endoderm. There is a non-cellular layer called mesoglea between these two layers.

Basic Body Forms: Polyp and Medusa

Cnidarians exhibit two basic body forms, although some species may show only one, and others alternate between them during their life cycle:

• Polyp: This form is sessile and cylindrical, with the mouth and tentacles facing upwards. Examples include sea anemones and hydra.
• Medusa: This form is free-swimming and umbrella-shaped, with the mouth and tentacles facing downwards. Jellyfish are classic examples of the medusa form.

The question correctly points out that medusa is one of the basic body forms found in Cnidarians.

Symmetry and Other Features

Cnidarians typically exhibit radial symmetry, meaning their body parts are arranged around a central axis, and any plane passing through the center can divide the organism into roughly equal halves. They possess tentacles armed with stinging cells called cnidocytes, which contain nematocysts used for defense and capturing prey.

Why Other Options Are Incorrect

• Option 1 suggests triploblastic animals with bilateral symmetry. Cnidarians are diploblastic and generally have radial symmetry, not bilateral.
• Option 3 mentions monoblastic organisms with tube feet. Cnidarians are diploblastic, not monoblastic. Tube feet are characteristic of echinoderms (like starfish), not cnidarians.
• Option 4 describes asymmetric organisms with tentacles containing poison glands. Cnidarians have radial symmetry, not asymmetry. While cnidocytes contain toxins, "poison glands" isn't the precise term used for their stinging cells (nematocysts within cnidocytes).

Based on these fundamental characteristics, the description of Cnidarians as diploblastic animals with medusa as one of the basic body forms accurately reflects their nature.