Anamniotes

Anamniotes is a group of vertebrates that lack an amnion during embryonic development. The amnion is a membrane that surrounds the embryo, creating a fluid-filled sac that protects it from desiccation and provides a cushioning environment. Anamniotes rely on an aquatic environment for reproduction and development, as their eggs do not possess the membranes necessary to prevent desiccation in terrestrial conditions.

The anamniotes include the jawless fishes (Agnatha), cartilaginous fishes (Chondrichthyes), bony fishes (Osteichthyes), and amphibians (Amphibia). These animals typically lay their eggs in water, and the larval stages often undergo metamorphosis before reaching adulthood. The absence of the amnion is a key characteristic differentiating them from amniotes, which include reptiles, birds, and mammals.

While anamniotes are primarily aquatic or require moist environments for reproduction, some amphibians have developed adaptations to survive in drier conditions, such as burrowing underground during dry periods. However, their dependence on water for reproduction remains a defining feature.

The term "Anamniota" is often used in a historical context in zoological classification, but modern cladistics emphasizes evolutionary relationships rather than simply the absence of a particular characteristic. Therefore, while the term remains useful for describing a group of animals with similar developmental strategies, it does not represent a monophyletic group in the strict cladistic sense.