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Phylum Animals that exhibit radial or biradial symmetry and are diploblastic, that is, they form from two embryonic layers, are classified as Cnidaria. About 99 percent of cnidarian species are marine-based.

Cnidocytes, or "stinging cells," are specialised cells found in cnidarians that possess nematocysts as their organelles (stingers). These cells, which are found surrounding the mouth and tentacles, contain toxins that can immobilize animals. Nematocysts have coiling threads that could be studded with barbs.

• The phylum Porifera contains sponges, most of which are aquatic and rely on their physical makeup to carry out all vital tasks. They lack cnidocytes.
• In addition to having no cnidocyte-like features, the phylum Nemertea includes probosci's worms and ribbon worms.
• Roundworms are members of the phylum Nematoda, which lacks cnidocyte-like features.