Trilobites
Trilobites (try'-low-bites) have been extinct for about 250 million years. They commonly are preserved in great detail and are prized as fossils.
Two grooves extending down the back o the animal divide it into three lobes; hence, the name “trilobite.”
Trilobites had a head with eyes and a mouth, a jointed body, and a tail. The animals were cousins of crabs and lobsters and lived in the sea.
They were covered with a horny armor, jointed so that the animal could move. Trilobites shed their armor much as snakes shed their skins, so each animal could have provided several fossils.
Trilobites were abundant in Cambrian, Ordovician, and Silurian times and were among the most important animals then on Earth. They became extinct during Permian time.
When trilobites are found, only the exoskeleton is preserved (often in an incomplete state) in all but a handful of locations. A few locations (Lagerstätten) preserve identifiable soft body parts (legs, gills, musculature & digestive tract) and enigmatic traces of other structures (e.g. fine details of eye structure) as well as the exoskeleton. all line drawings ©1999 - 2009 by S. M. Gon III
Trilobites range in length from 1 mm to 72 cm (1/25 inch to 28 inches), with a typical size range of 3 to 10 cm (1 to 4 inches).
The exoskeleton is composed of calcite and calcium phosphate minerals in a protein lattice of chitin that covers the upper surface (dorsal) of the trilobite and curled round the lower edge to produce a small fringe called the doublure. Three distinctive sections are present: cephalon (head); thorax (body) and pygidium (tail).
Like many animals, Trilobites molted. During molting, the exoskeleton generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other. Similar to lobsters & crabs, trilobites would have physically "grown" between the molt stage and the hardening of the new exoskeleton.
One of the unique features of Trilobites was its eyes. Even the earliest trilobites had complex, compound eyes with lenses made of calcite, a characteristic of all trilobite eyes.
Trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied: some trilobites had only one, while some had thousands of lenses in a single eye. In compound eyes, the lenses were typically arranged hexagonally. [14] The fossil record of trilobite eyes is complete enough that their evolution can be studied through time
For an outstanding website on Trilobites, go here:
http://www.trilobites.info/index.htm
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