The vast majority of animals are invertebrates, and all of them have different life cycles. Some of them hatch from eggs and become adults immediately, while others undergo a larval stage and then emerge as adults. Invertebrates are classified into three major groups: insects, arachnids, and crustaceans. While these three groups have some similarities, the main differences are in their physical features. For instance, invertebrates are characterized by a circular body plan, which means that they lack a backbone. Many of them also have a hard, outer covering that serves multiple functions, including supporting muscles and preventing water from evaporating.