Marine invertebrates are all multicellular animals that inhabit a marine environment apart from the vertebrate members of the chordate phylum; invertebrates, lack a vertebral column. Some have evolved a shell or a hard exoskeleton.

As on land and in the air, invertebrates make up a great majority of all macroscopic life, as the vertebrates makes up a subphylum of one of over 30 known animal phyla, making the term almost meaningless for taxonomic purpose. Invertebrate sea life includes the following groups, some of which are phyla:

  • Acoela, among the most primitive bilateral animals;
  • Annelida, (polychaetes and sea leeches);
  • Brachiopoda, marine animals that have hard "valves" (shells) on the upper and lower surfaces ;
  • Bryozoa, also known as moss animals or sea mats;
  • Chaetognatha, commonly known as arrow worms, are a phylum of predatory marine worms that are a major component of plankton;
  • Cephalochordata represented in the modern oceans by the lancelets (also known as Amphioxus);
  • Cnidaria, such as jellyfish, sea anemones, and corals;
  • Crustacea, including lobsters, crabs, shrimp, crayfish, barnacles, hermit crabs, mantis shrimps, and copepods;
  • Ctenophora, also known as comb jellies, the largest animals that swim by means of cilia;
  • Echinodermata, including sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, crinoids, and sea daisies;
  • Echiura, also known as spoon worms;
  • Gnathostomulids, slender to thread-like worms, with a transparent body that inhabit sand and mud beneath shallow coastal waters;
  • Gastrotricha, often called hairy backs, found mostly interstitially in between sediment particles;
  • Hemichordata, includes acorn worms, solitary worm-shaped organisms;
  • Kamptozoa, goblet-shaped sessile aquatic animals, with relatively long stalks and a "crown" of solid tentacles, also called Entoprocta;
  • Kinorhyncha, segmented, limbless animals, widespread in mud or sand at all depths, also called mud dragons;
  • Loricifera, very small to microscopic marine sediment-dwelling animals only discovered in 1983;
  • Merostomata; also known as horseshoe crabs;
  • Mollusca, including shellfish, squid, octopus, whelks, Nautilus, cuttlefish, nudibranchs, scallops, sea snails, Aplacophora, Caudofoveata, Monoplacophora, Polyplacophora, and Scaphopoda;
  • Myzostomida, a taxonomic group of small marine worms which are parasitic on crinoids or "sea lilies";
  • Nemertinea, also known as "ribbon worms" or "proboscis worms";
  • Orthonectida, a small phylum of poorly known parasites of marine invertebrates that are among the simplest of multi-cellular organisms;
  • Phoronida, a phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies;
  • Placozoa, small, flattened, multicellular animals around 1 millimetre across and the simplest in structure. They have no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened;
  • Porifera (sponges), multicellular organisms that have bodies full of pores and channels allowing water to circulate through them;
  • Priapulida, or penis worms, are a phylum of marine worms that live marine mud. They are named for their extensible spiny proboscis, which, in some species, may have a shape like that of a human penis;
  • Pycnogonida, also called sea spiders, are unrelated to spiders, or even to arachnids which they resemble;
  • Sipunculida, also called peanut worms, is a group containing 144–320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms;
  • Tunicata, also known as sea squirts or sea pork, are filter feeders attached to rocks or similarly suitable surfaces on the ocean floor;
  • Some flatworms of the classes Turbellaria and Monogenea;
  • Xenoturbella, a genus of bilaterian animals that contains only two marine worm-like species;
  • Xiphosura, includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs.

"Good judgement seeks balance and progress. Lack of it eventually finds imbalance and frustration."
President Eisenhower
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