The Animal Kingdom represents the most diverse group of organisms on Earth, encompassing millions of species that inhabit a vast range of environments, from the deepest oceans to the highest mountains. The classification of this kingdom into different phyla is based on fundamental features that reflect their evolutionary complexity and body plan. These key criteria include the level of organization, body symmetry, the number of embryonic germ layers, the nature of the body cavity (coelom), patterns of organ systems, segmentation, and the presence or absence of a notochord.

The simplest level of organization is the cellular level, seen in sponges (Phylum Porifera), where cells are arranged as loose aggregates. A higher level is the tissue level, found in coelenterates and ctenophores, where cells performing the same function are organized into tissues. The organ level of organization, exhibited by flatworms (Phylum Platyhelminthes), involves tissues being grouped together to form organs. The most complex is the organ system level, where organs are associated to form functional systems, a characteristic of all higher animal phyla from annelids to chordates.

Body symmetry is another crucial feature. Animals can be asymmetrical (like sponges), radially symmetrical (like coelenterates and echinoderms), where any plane passing through the central axis divides the body into identical halves, or bilaterally symmetrical (like most higher animals), where the body can be divided into identical left and right halves in only one plane. The number of embryonic germ layers also distinguishes animal groups. Diploblastic animals, such as coelenterates, develop from two layers (an external ectoderm and an internal endoderm), while triploblastic animals, from flatworms to chordates, develop a third layer, the mesoderm, between the ectoderm and endoderm.

The presence and nature of a coelom, or body cavity lined by mesoderm, is a key characteristic of triploblastic animals. Acoelomates, like flatworms, lack a body cavity. Pseudocoelomates, such as roundworms, have a body cavity that is not completely lined by mesoderm. Eucoelomates, or true coelomates, from annelids onwards, possess a true coelom.

Based on these fundamental features, the Animal Kingdom is classified into several major phyla.

Phylum Porifera (sponges) are primitive, multicellular, asymmetrical animals with a cellular level of organization. They possess a unique water canal system for feeding, respiration, and excretion.

Phylum Coelenterata (or Cnidaria), including jellyfish and corals, are aquatic, radially symmetrical, and diploblastic animals with a tissue level of organization. They are characterized by the presence of stinging cells called cnidoblasts.

Phylum Platyhelminthes (flatworms) are the first triploblastic and bilaterally symmetrical animals. They are acoelomates with an organ level of organization and often have a flattened body.

Phylum Aschelminthes (roundworms) are bilaterally symmetrical, triploblastic pseudocoelomates. They possess a complete alimentary canal and are often parasitic.

Phylum Annelida (segmented worms), including earthworms and leeches, are the first true coelomates. They exhibit metameric segmentation, where the body is externally and internally divided into segments.

Phylum Arthropoda is the largest phylum in the animal kingdom, including insects, spiders, and crustaceans. They are characterized by a chitinous exoskeleton and jointed appendages.

Phylum Mollusca, the second-largest phylum, includes snails, clams, and octopuses. They are typically unsegmented, with a distinct head, muscular foot, and visceral hump, and are often covered by a calcareous shell.

Phylum Echinodermata, including starfish and sea urchins, are exclusively marine animals with a spiny skin. A unique feature is their water vascular system, which aids in locomotion, feeding, and respiration. Adults are radially symmetrical, but their larvae are bilaterally symmetrical.

Phylum Chordata is characterized by the presence of a notochord (a flexible rod-like structure), a dorsal hollow nerve cord, and paired pharyngeal gill slits at some stage of their life. This phylum includes the most familiar groups of animals. It is divided into three subphyla: Urochordata, Cephalochordata (often called protochordates), and Vertebrata. The subphylum Vertebrata is distinguished by the replacement of the notochord with a cartilaginous or bony vertebral column in the adult. Vertebrata is further divided into several classes, including Cyclostomata (jawless fishes), Chondrichthyes (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia, Reptilia, Aves (birds), and Mammalia. This progression from simpler invertebrates to complex vertebrates illustrates the major evolutionary trends in body organization, complexity, and adaptation within the Animal Kingdom.