How can an arthropod grow if it is enclosed in an exoskeleton?

Crabs, lobsters, and shrimps shed their outer covering once or more each year in a process called molting. To molt, the crustacean secretes a new exoskeleton inside the old one, which it splits. The animal then pushes its body out through a gap between the thorax and the abdomen. After molting, the crustacean has a soft new exoskeleton that gradually hardens.

It's cell theory. easy name i would think.