anamniote

Anamniotes are a group of vertebrates characterized by the absence of an amnion during their embryonic development. This group primarily includes fishes and amphibians, which lay their eggs in aquatic environments or develop directly within the water. The term "anamniote" is derived from the Greek words "an-" meaning "without" and "amnion," referring to the membrane that surrounds the embryo in amniotes.

Unlike amniotes—such as reptiles, birds, and mammals—that develop within a protective amniotic sac, anamniotes lack this specialized membrane. As a result, their embryos are usually exposed to aquatic or moist environments, which is crucial for their development and survival. The absence of an amnion is a key differentiating feature in the embryonic stages, influencing their reproductive strategies and habitat requirements.

In terms of evolutionary history, anamniotes are considered the more primitive or basal group of vertebrates. They diverged early from the common vertebrate lineage, which later gave rise to amniotes. Today, the group includes diverse species such as cartilaginous fishes (like sharks and rays), bony fishes, and amphibians (like frogs, salamanders, and caecilians).

Anamniotes play a significant ecological role in aquatic ecosystems and are important subjects of study in evolutionary biology and developmental biology. Their reproductive and developmental adaptations offer insight into vertebrate evolution and the transition from aquatic to terrestrial life.