Arthropods, members of the phylum Arthropoda, are a diverse group of animals including insects, crustaceans, spiders, scorpions and centipedes. However, the members of this phylum, despite their incredible diversity and sheer numbers, share a number of important distinguishing characteristics.
Arthropods are invertebrates, which means their bodies do not have internal bones for support. To compensate for this, they produce a hard exoskeleton made of chitin, a mixture of lipids, carbohydrates and protein, which covers and protects their bodies like a suit of armor. As arthropods grow, they must shed or molt their exoskeletons. They first produce new, softer exoskeletons underneath the old ones. Once their hardened, old coverings crack and shed, they sport roomier, albeit soft, exoskeletons. Arthropods are incredibly vulnerable during the molting process, and will often hide until their new exoskeletons harden.
Arthropods have bodies that are internally and externally segmented. The number of segments depends on the individual species; millipedes, for example, have more segments than lobsters.
The name arthropod actually comes from the Greek “arthro,” meaning joint, and “pod,” meaning foot. All arthropods have jointed limbs attached to their hard exoskeletons that allow for flexibility and movement. The joints generally bend in only one direction but allow for sufficient predatory and defensive actions.
An arthropod's body can be divided vertically into two mirror images. This is called bilateral symmetry. An arthropod shares this symmetry with many other animals such as fish, mice and even humans. Other animals such as the jellyfish and sea star exhibit radial symmetry, while coral and sea sponge are asymmetrical -- exhibiting no pattern at all.
Open Circulatory System
An arthropod has an open circulatory system. This means instead of a closed circulatory system of interconnected veins and capillaries, an arthropod’s blood is pumped through open spaces called sinuses in order to reach tissues. An arthropod does, however, have a heart which pumps blood into the hemocoel, the cavity where the organs are located, where it surrounds the organs and tissues.
Christina Stephens is a writer from Portland, Ore. whose main areas of focus are pets and animals, travel and literature. A veterinary assistant, she taught English in South Korea and holds a BA in English with cum laude honors from Portland State University.