The etymology of the word "arthropod" derives from the Greek words "arthron" (joint) and "podos" (leg). The term refers to the fact that arthropods generally have segmented appendages.
The phylum of arthropods is currently one of the largest phyla (about half of described species belong to this phylum).
Characteristics of this group include:
- segmented appendages (as the etymology implies, with one exception: Onychophora, also called peripatidae);
- an external skeleton (exoskeleton) which is a cuticle formed of chitin (more or less hard) that requires the animal to periodically moult as it grows;
- a body normally divided into a head, thorax and abdomen;
- an open circulatory system.
The arthropod group notably contains insects, spiders, scorpions and centipedes, as well as species belonging to fossil groups, such as trilobites.