Arthropods are highly diverse. It is the largest of all animal phylums and accounts for over three quarters of all currently known living and fossil organisms. The exact number of living arthropod species is unknown since many have yet to be identified, especially those inhabiting tropical rainforests.
There are five main characteristics all arthropods share. These include:
- bilateral symmetry - the left and right sides of the arthropod body are mirror images of one another
- segmented body - the arthropod body is made-up of repeating units (pairs of legs, claws, or breathing structures)
- exoskeleton - provides protection, prevents water loss, and provides support
- jointed appendages - enable the arthropods to move their legs, mouthparts, and claws despite the fact that their body is covered by a rigid exoskeleton
- numerous pairs of limbs - arthropods have many pairs of legs, some arthropods have fewer or smaller limbs, others have larger, specialized limbs such as claws
The exoskeleton of an arthropod is a hard external structure made of chitin that protects the arthropod, prevents dessication and provides structural support. Since the exoskeleton is rigid, it cannot grow with the arthropod and must be molted periodically to allow for increases in size. After molting, a new exoskeleton is secreted by the epidermis. Muscles connect to the exoskeleton and enable the animal to control the movement of its joints.
The muscle structure of arthropods is more complex than that of most other invertebrates. Arthropods have longitudinal and circular bands of muscle tissue as well as isolated musles that together enable a wide range of movements. Additionally, arthropods have a well-developed nervous system. In more advanced arthropods, the nervous system consists of a brain and a double nerve cord. More primitive arthropods do not have a brain but instead have ganglia either located in each segment of their body or in a ganglionic mass near the head.