Anamnia

Anamnia is a term used in zoology to describe a group of vertebrates that lay eggs without a protective amniotic membrane. This group includes various classes of animals such as fish and amphibians. Unlike amniotes, which include reptiles, birds, and mammals, anamniotes do not have the specialized membranes that protect the embryo, such as the amnion, chorion, and allantois.

Characteristics

Anamniotes are characterized by their reproductive strategy, which involves laying eggs in water or moist environments. The lack of an amniotic membrane means that their eggs are more susceptible to desiccation and must be kept in a moist environment to ensure the survival of the embryo. This reproductive strategy is seen in both oviparous and ovoviviparous species within this group.

Evolution

The evolution of anamniotes is a significant topic in the study of vertebrate evolution. Anamniotes are considered to be more primitive compared to amniotes, as they retain many ancestral traits. The transition from anamniotic to amniotic reproduction is a key evolutionary step that allowed vertebrates to colonize a wider range of terrestrial environments.

Classification

Anamniotes are not a monophyletic group, meaning they do not form a single clade that includes all descendants of a common ancestor. Instead, they are a paraphyletic group, as they include some but not all descendants of their most recent common ancestor. The main classes of anamniotes include:

• Agnatha (jawless fish)
• Chondrichthyes (cartilaginous fish)
• Osteichthyes (bony fish)
• Amphibia (amphibians)

Related Pages

• Amniote
• Vertebrate
• Evolution of fish
• Evolution of amphibians
• Reproduction in animals

See Also

• Amnion
• Chorion
• Allantois
• Oviparity
• Ovoviviparity