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The Basic Invertebrate Groups

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About 97 percent of all known animal species are invertebrates. Many of these species are classified in one of eight basic groups (although it should be noted that these basic groups provide a simple overview of invertebrates; there are in fact over 30 phyla of invertebrates). This article explores invertebrate classification in order to better understand the diversity and common characteristics of invertebrates.

Arthropods

This jumping spider is one of millions of species of arthropods.
Photo © Rundstedt B. Rovillos / Getty Images.

Arthropods are a highly diverse group of invertebrates that include centipedes, millipedes, spiders, mites, horseshoe crabs, scorpions, insects and crustaceans. The first arthropods appeared during the early Cambrian Period. They are thought to have evolved from annelids between 500 and 600 million years ago. Perhaps the best known early arthropods were the Triolobites, creatures that lived on the muddy bottoms of shallow marine environments.

Cnidarians

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Photo © Purestock / Getty Images.

Cnidarians are a group of aquatic invertebrates that includes jellyfish, corals, sea anemones and hydras. Cnidarians are radially symmetrical and they have tentacles that encircle their mouth. Other cnidarian characteristics include a gastrovascular cavity has only one opening, a mouth, through which the animal takes in food and releases waste. The earliest known animal fossils are cnidarians that are estimated to have lived about 580 million years ago. Some of these early fossils are tube-like and closely resemble the minieralized remains of corals.

Echinoderms

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Photo © Sami Sarkis / Getty Images.

Echinoderms are a diverse group of marine invertebrates comprising approximately 6000 species. Echinoderms include starfish, sea lilies, feather stars, brittlestars, sea cucumbers, and sea urchins. Early echinoderms first appeared during the Cambrian Period. The ancestors of echinoderms are thought to have been bilaterally symmetrical and although echinoderms are well represented in the fossil record, it is controversial how radial symmetry evolved from bilateral symmetry in the group.

Molluscs

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Molluscs are a group of invertebrates that includes chitons, snails, slugs, limpets, squids, oysters, clams, cuttlefish, octopus and many more. There are about 100,000 species of molluscs alive today. Molluscs are thought to have evolved from a segmented, wormlike animal similar to present-day flatworms. Their closest living relatives are either the annelids (also known as segmented worms) or platyhelminths (non-segmented worms or flatworms).

Segmented Worms

Christmas tree worm - Spirobranchus giganteus
Photo © Nick Hobgood / Wikipedia.

Segmented worms are a group of invertebrates that includes earthworms, ragworms and leeches. There are about 12,000 species of segmented worms alive today. Segmented worms include more than 22,000 species that are categorized into two basic groups, the polychates and the clitellates. The polychaetes include about 12,000 species that are characterized by having multiple hairs on each segment. The clitellates include about 10,000 species that have no nuchal organs or parapoidia. Because segmented worms are soft-bodied animals, fossils of annelids is very rare. A few species secreted minerlized tubes that fossilized. The earliest known segmented worms fossils date back about 505 million years ago to the early Cambrian.

Sponges

Pink lumpy sponge - Monanchora unguifera
Photo © Nick Hobgood / Wikipedia.

Sponges are a group of invertebrates that includes glass sponges, demisponges, and calcareous sponges. There are about 10,000 species of sponges alive today. The earliest fossil evidence of sponges consists of various cells known to belong to sponges—spicules, pinacocytes, porocytes and archaeocytes. These fossil fragments date back to about 580 million years ago during the Ediacaran period. There are three subgroups of sponges, the glass sponges, the demosponges and the calcarious sponges.

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