Animals are diverse multicellular organisms that belong to the biological kingdom Animalia. They possess various characteristics that distinguish them from other living things. Animals are eukaryotic, meaning their cells have a nucleus enclosed within a membrane. They are also multicellular, composed of multiple cells that work together to form tissues and organs.
Unlike plants and algae, animals are heterotrophic, which means they obtain their nutrients by consuming organic material. They typically respire aerobically, using oxygen to extract energy from food. Most animals are capable of movement at some stage in their life cycle, although some, like sponges, corals, and barnacles, are sessile (fixed in one place) as adults.
During embryonic development, animals undergo a unique stage called the blastula, which forms a hollow ball of cells. This allows for the differentiation of cells into specialized tissues and organs. Animals have a distinct extracellular matrix, consisting of collagen and elastic glycoproteins, which provides structural support and flexibility.
Animal bodies are composed of cells that are organized into tissues, such as muscles and nerve tissues. Muscles enable animals to move and perform various actions, while nerve tissues transmit signals and coordinate bodily functions. Most animals have an internal digestive chamber, either with one opening (in certain groups like Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
The structure of animal bodies can vary greatly, from simple organisms with few specialized tissues to complex organisms with intricate systems and organs. Some animals develop calcified structures like shells, bones, or spicules. Animal cells also possess unique cell junctions, including tight junctions, gap junctions, and desmosomes, which facilitate cell communication and tissue integrity.
The classification and study of animals is known as zoology. Animals play various roles in ecosystems, and humans interact with them in numerous ways, such as utilizing them for food, materials, pets, sports, art, and cultural and religious significance.
Overall, animals exhibit remarkable diversity in form, function, and behavior, contributing to the complexity and richness of the natural world.