Spineless Creatures (part 2)
What are Brachiopods?
Like the Bivalves, Brachiopods are a living animal that is enclosed between two shells, however they are not related. Brachiopods have both extinct and extant species, first appearing over 500 million years ago during the Cambrian Period and still surviving today.
The Brachiopod shells consist of two hinged valves, usually one bigger than the other. All brachiopods are filter feeders, catching small particles of food from the water. Most brachiopods live attached to the sea-bed but some can burrow beneath the sand.
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What are Crinoids?
Crinoids are marine animals classified as Echinodermata, the same taxonomic group as sea urchins, starfish, sea cucumbers and brittle stars. They are commonly found in Carboniferous limestones, but have living representatives.
Different species adapted to living at different sea depths. Those adapted to the deep sea are commonly known as sea lilies due to their plant-like appearance. Sea lilies anchor themselves to the sea floor with root-like projections. They have a stem, at the top of which is the head and five or more radiating arms used to capture planktonic food.
The stems comprise of hundreds of segments, called columnals, and each columnal has a central perforation that during life contained soft tissues. Different species have different shaped columnals, ranging from circular to pentagonal forms.
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What are Echinoids?
Echinoids are marine animals closely related to starfish, sea cucumbers and crinoids. Together they comprise the Phylum Echinodermata which means spiny skinned. Fossil echinoids can be found in rocks that date back to 450 million years ago, during the Ordovician Period.
Echinoids have a spiny ball-like appearance. They are made up of numerous plates, known as the test, which form the skeleton. This acts as a protective casing around the internal organs. On the outside of the test are tube feet which help in feeding and movement, as well as articulated spines which act as a defence mechanism. Echinoids either inhabit rocks on the sea-bed or burrow beneath the surface into the sand.
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What are Trilobites?
Trilobites are an extinct group of Arthropods, which died out 250 million years ago. They are closely related to spiders and scorpions, even though they have a woodlice appearance.
The trilobite bodies are divided into three segments; the head, body and tail. This made them quite flexible, and they could role up into a ball for protection. Most lived on the sea-floor but some had the ability to swim, and were active hunters feeding on smaller invertebrates.
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