Arthropods and Their Relatives Segmented External Skeletons

In Precambrian times, the cuticle of some wormlike ecdyso-zoan lineages became thickened by the incorporation of layers of protein and a strong, flexible, waterproof polysaccha-ride called chitin. This rigid body covering may originally have had a protective function, but eventually it acquired both support and locomotory functions as well.

A rigid body covering precludes wormlike movement. To move, the animal requires extensions of the body that can be

33.5 Roundworms (a) The body plan of Trichinella spiralis, a roundworm that causes trichinosis. (b) A cyst of Trichinella spiralis in the muscle tissue of a host. (c) This free-living roundworm moves through marine sediments.

Cuticle J*L

Cuticle J*L

manipulated by muscles. Such appendages evolved several times in the late Precambrian, leading to the lineages collectively called the arthropods ("jointed foot"). Divisions among the arthropod lineages are ancient and have been the subject of much research in the past decade. These phylogenetic relationships are being examined daily in the light of a wealth of new information, much of it concerning gene expression. There is currently no consensus on an exact phylogeny, but most researchers agree that these important animal groups are monophyletic, and some taxonomists consider them as members of a single phylum: Arthropoda.

Before presenting one current view of arthropod phy-logeny, let's look at some arthropod relatives that have segmented bodies but unjointed legs, and at an early arthropod lineage that disappeared but left an important fossil record.

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