Marine life appeared in the form of fish, which live exclusively in water. Some of these evolved into amphibians which spend portions of their lives in water and portions on land. Other fish evolved into land mammals and subsequently returned to the ocean as seals, dolphins or whales. Plant forms such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton, and particularly phytoplankton, are key primary producers forming the general foundation of the ocean food chain. Marine invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters including breathing tubes (see
Marine life are also present in the anemone. Worms (Old English for "serpent") form a number of phylums. Their body plan typically involves long cylindrical tube-like bodies and no limbs. Marine worms vary in size from microscopic to over in length for some marine polychaete worms (bristle worms) and up to for the marine nemertean worm (bootlace worm). Some marine worms occupy a small variety of parasitic niches, living inside the bodies of other animals, while others live more freely in the marine environment or by burrowing underground. Different groups of marine worms are related only distantly, so they are found in