In the most general terms, invertebrates are creatures that lack a backbone and internal skeleton. That being said, there exists within this immense group of animals a mind-boggling number of variations on that general theme.

In contrast to other types of organisms, invertebrates appear to be as numerous in the freezing seas of Antarctica as they are in the warm tropics. Colonies of bizarre sponges, clams, giant tubeworms, and other unique invertebrates have even colonized the hot, mineral laden waters in the proximity of deep sea vents. These animals form the only communities known to exist in the complete absence of sunlight, using chemical reactions as opposed to photosynthesis as the basis of the food chain. The incredible diversity of lifestyles found among invertebrates precludes any attempt at generalizations concerning their senses, reproduction, behavior, or modes of living. Instead, such details are covered during discussions of the phyla, or in the various species' accounts.