What Are the Group Arthropods?

Arthropod Diversity

• Arthropods are extremely numerous and diverse. Over one million species of arthropods have been recorded, but there may be many more in regions, such as rain forests, that have not yet been discovered by humans. The animals that belong to the arthropod phylum are far more numerous than those in any other phyla. Arthropods account for more than 75 percent of all discovered living and fossilized animals. The habitats of this successful group are also varied, with different species being capable of living in almost every type of habitat on Earth. Arthropods also vary greatly in size, weight and appearance.

Common Features

• Although arthropod species can differ greatly in appearance, there are some features that all arthropods have in common. The first of these is the exoskeleton, which is made from a tough compound known as chitin. This provides arthropods with protection and strength as well as limiting water loss. The exoskeleton must be shed at different stages of development to allow for more growth. Another common feature of all arthropods is the segmented body, both externally and internally. In arthropods, the circulatory system is open. This means that they have arteries and a heart but usually do not have blood vessels. Instead, blood is transported through a series of cavities, or sinuses, which provide tissues and organs with a blood supply. Arthropods also generally have jointed legs or similar appendages, depending on whether these have been modified over time.

Subphyla

• Living arthropods can be divided into four main subphyla, the next stage of classification after the phylum, or group, arthropoda. One of these subphyla is hexapoda, consisting mainly of insects, which can be divided into a great number of species. The subphylum chelicerata contains creatures, such as spiders and horseshoe crabs, which have mouthparts that include appendages in front of the mouth, adapted from legs. Crustacea is the name given to another subphylum of arthropods. This group includes crustaceans, which mainly live in water. The final subphylum is myriapoda, consisting of creatures, such as centipedes and millipedes.

Fossils

• Fossils show that arthropods have existed on Earth for hundreds of millions of years. Trilobites were a type of arthropod that became extinct over 240 million years ago. Fossils show that these aquatic creatures shared many features with modern arthropods, such as the segmented body and a line of symmetry that runs from head to tail.