Key Points

Animals exhibit different levels of organization. These include the cellular level (sponges), tissue level (coelenterates), organ level (platyhelminthes), and organ system level (annelids to chordates).

Symmetry is a basis for classification. Animals can be asymmetrical (sponges), radially symmetrical (coelenterates, ctenophores), or bilaterally symmetrical (platyhelminthes to chordates).

Diploblastic animals have two germ layers, ectoderm and endoderm (e.g., Coelenterata). Triploblastic animals have a third layer, mesoderm, between the ectoderm and endoderm (e.g., Platyhelminthes to Chordata).

The body cavity (coelom) is crucial for classification. Acoelomates lack a coelom (Platyhelminthes), pseudocoelomates have a false coelom (Aschelminthes), and coelomates have a true coelom lined by mesoderm (Annelida to Chordata).

Commonly known as sponges, they have a cellular level of organisation. They possess a water transport or canal system with pores (ostia) and collar cells (choanocytes).

These animals are radially symmetrical, diploblastic, and possess stinging cells called cnidoblasts. They exhibit two body forms: a sessile polyp and a free-swimming medusa.

These are dorso-ventrally flattened, triploblastic, and acoelomate animals with bilateral symmetry. They possess specialized flame cells for osmoregulation and excretion.

They are bilaterally symmetrical, triploblastic, and pseudocoelomate animals. They have an organ-system level of body organisation with a complete alimentary canal.

Annelids are coelomate animals that exhibit metameric segmentation, where the body is divided into distinct segments or metameres. They use nephridia for excretion.

This is the largest phylum, characterized by a chitinous exoskeleton and jointed appendages. They have an open circulatory system and use Malpighian tubules for excretion.

The second largest phylum, molluscs have a soft body covered by a calcareous shell. They possess a muscular foot for movement and a file-like rasping organ called a radula for feeding.

These spiny-bodied marine animals are coelomates with radial symmetry in adults. Their most distinctive feature is the presence of a water vascular system for locomotion and food transport.

Chordates are characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits at some stage of their life.

All vertebrates are chordates, but not all chordates are vertebrates. In vertebrates, the notochord present during the embryonic period is replaced by a cartilaginous or bony vertebral column in the adult.

These are marine fish with a cartilaginous endoskeleton, placoid scales, and a ventral mouth. They lack an air bladder and must swim constantly to avoid sinking.

This class includes marine and freshwater fish with a bony endoskeleton. They have an air bladder for buoyancy and four pairs of gills covered by an operculum.

Amphibians can live in both aquatic and terrestrial habitats. They have moist skin without scales, a three-chambered heart, and undergo indirect development with a larval stage.

Reptiles are characterized by their creeping or crawling locomotion and a body covered by dry, cornified skin or scales. They are poikilotherms (cold-blooded) and fertilization is internal.

The defining features of birds are feathers, forelimbs modified into wings, and hollow pneumatic bones to aid in flight. They are homoiothermous (warm-blooded) and have a four-chambered heart.

The most unique mammalian characteristics are the presence of milk-producing mammary glands and hair on the skin. They are homoiothermous, have a four-chambered heart, and are typically viviparous.