Arthropods have an external skeleton which offers protection to the animal, but also limits growth. In order to for an arthropod to increase in size, it must shed its exoskeleton and secrete a new one. Arthropods tend to be divided into well-mark sections with jointed appendages. The distinct separation of the body into divisions, such as head, thorax, abdomen, is usually accompanied by some identity loss of individual segments. Insects comprise the majority of known arthropods and are mostly terrestrial and freshwater organisms; those which are said to be marine are commonly found at the outer reaches of the intertidal zone. (Kozloff, 2000)

References:

Brusca, R.C., and Brusca, G.J. (2003). Invertebrates. Library of Congress Catologing-in-Publication-Data: USA.

Kozloff, Eugene N. (2000). Seashore Life of the Northern Pacific Coast. University of Washington Press: Seattle and London