This phylum is occasionally broken down into two phyla, Arthropoda and Insecta.  Whatever their classification, they are everywhere.  With over a million species, the phylum includes nearly one half of all animals.  Beetles, alone account for some 300,000 species.  The phylum is well known to man.  Shrimp, grasshoppers and termites are used as food and in many places represent an important and otherwise unobtainable source of protein.  Insects are the only group of invertebrates that have adapted to flight.  Some (termites, ants, bees, etc.) have social organizations; some have language (bees) and some (e.g. lightning bugs) produce light.  All have exoskeleta (skeleta on the outside of the body) and they all molt periodically to accommodate growth.  Among all this diversification and variation, three groups; the trilobites, crustacea, and insects, have left an outstanding fossil record and are commonly used for zonation.

Class TRILOBITA (the trilobites)

Trilobites are the first successful arthropod experiment and are extensively used to zone the Paleozoic.  As a matter of fact, the first appearance of the trilobite genus Ollenellus marks the classical beginning of the Paleozoic.  They are characterized by a body that is divided in three parts, hence their name.  Some were blind, others had well developed compound eyes.  Many were mud or filter feeders, while still others may have been predators.  Their ultimate demise may well have been due to being outcompeted by fishes. More on trilobites

Class CRUSTACEA (cephalocarids, shrimps, ostracodes, crabs, lobsters, barnacles, pillbugs, etc.)

Barnacles are presently common indicators of the intertidal zone and may ultimately become excellent index fossils.  One other group, the ostracodes, are extremely important index fossils of the Mesozoic and the Cenozoic.  However, they are not included in this discussion because of their generally microscopic size.
 

Class INSECTA (the insects)

The fossil record of insects reaches back into the Carboniferous.  There are some thirty-six orders of insects known at present, 99.9% of which are winged, testifying to the tremendous adaptive advantage that flight gives insects.  Associated with the rise of flowering plants, insects have shown increasing adaptations to feeding on flowers and related structures.  Today some 20% of insects depend on flowers, nectar or pollen for their food source.  This interdependence is even more evident if one considers that some 65% of flowering plants are insect pollinated.  The effect of insects has been pervasive.  To mention but one, as disease carriers, they may even have been responsible for the decimation and extinction of some mammalian groups.

Class MEROSTOMATA

Although not important as fossils, the merostomes are worth mentioning because they include two interesting groups.  Limulus, the horseshoe crab, is often mentioned as a living fossil.  This class also includes the eurypterids, spectacular scorpion-like predators of the Paleozoic waters which reached lengths up to 6 feet.

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