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.

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