They are the earliest of all known arthropods. Various appendages become modified for activities such as eating, sensing the environment, swimming, etc. Arthropods are perhaps the most successful and diverse of all invertebrates, and because their tough exoskeleton offers considerable potential for fossilization their geological record is good. The moulting process is known as ecdysis . When the old skeleton is cast off the arthropod takes up air or water and swells to its next larger size.
Growth is thus rapid and episodic and only possible during moulting.
The animal is now soft-shelled, cannot move much, and is very vulnerable to damage and predation. 80 to 90 percent of arthropod mortality occurs during moulting. The new exoskeleton slowly hardens. Because of ecdysis we have a good fossil record of the moult stages of many fossil arthropods. The three primary divisions of the body are set at right angles to the unique three-lobed longitudinal division from which the group takes its name.