Marine Invertebrates

Marine invertebrates are multicellular animals that inhabit a marine environment and are invertebrates, lacking a vertebral column. In order to protect themselves, they may have evolved a shell or a hard exoskeleton, but this is not always the case.

As on land and in the air, invertebrates make up a great majority of all macroscopic life in the sea. 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 remarkable 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.