Sea anemone is a completely non-parasitic form among the given options.

Explanation:
Sea anemones are marine animals that belong to the phylum Cnidaria. They are closely related to corals and jellyfish. Sea anemones are sessile organisms, meaning they are attached to a surface and do not move around actively. They are found in various marine habitats, including rocky shores, coral reefs, and sandy bottoms.

Non-Parasitic Nature:
Sea anemones are entirely non-parasitic, which means they do not rely on other organisms for their survival. They obtain their nutrients through a process called mutualistic symbiosis. They have a mutually beneficial relationship with certain types of photosynthetic algae known as zooxanthellae. These algae live within the tissues of the sea anemone and provide them with food through photosynthesis. In return, the sea anemone offers protection and a stable environment for the algae to grow.

Feeding Mechanism:
Sea anemones are predatory organisms that feed on a variety of small marine animals, including fish, shrimp, and plankton. They have a specialized structure called a "cnidocyte" that contains venomous cells called nematocysts. When prey comes into contact with the tentacles of the sea anemone, these nematocysts are triggered, releasing venomous threads that immobilize the prey. The tentacles then bring the immobilized prey towards the mouth of the sea anemone for digestion.

Importance in Ecosystem:
Sea anemones play an essential role in marine ecosystems. They provide habitat and shelter for many other organisms, including small fish, crabs, and shrimp. Some fish species even have a symbiotic relationship with sea anemones, using them for protection from predators. Additionally, sea anemones contribute to nutrient cycling in the ecosystem through their feeding activities and the release of waste products.

Conclusion:
Among the options provided, sea anemone is the only completely non-parasitic organism. Its mutualistic relationship with zooxanthellae and predatory feeding habits make it an important component of marine ecosystems.