The Animal Kingdom: Invertebrates to Vertebrates

Symmetry describes how a body can be divided into matching halves. Sponges are asymmetrical: no cut gives matching halves. Jellyfish and sea anemones have radial symmetry, with body parts arranged around a central axis like slices of a pie, which suits an animal that drifts or sits still and meets food from every direction. Most animals have bilateral symmetry: only one cut, down the midline, gives mirror-image left and right halves. A bilateral animal has a front end that meets the world first, so sense organs and nerve cells cluster there to form a head, a trend called cephalization. That is why bilateral symmetry goes with active, directed movement.

Tissue layers form early in the embryo. Sponges have no true tissues. Cnidarians have two layers (ectoderm outside, endoderm lining the gut). All other phyla are triploblastic, with a third, middle layer, the mesoderm, which builds muscles, the circulatory system and most organs. The mesoderm also determines the body cavity. In acoelomates such as flatworms, solid tissue fills the space between gut and body wall. Pseudocoelomates such as roundworms have a fluid-filled cavity lined by mesoderm on only one side. Coelomates have a true coelom, lined completely by mesoderm, so organs are suspended in fluid and can move and grow independently of the body wall. Finally, segmentation, a body built from repeated units, appears in annelids, arthropods and chordates (look at your own vertebrae and ribs). Segments allow specialization: one segment can carry legs, another wings.

Following the table downward shows features being added. A sponge is a colony-like bag of cells; water drawn through its pores is filtered for food, and there are no organs or nerves. Cnidarians add true tissues, a simple nerve net and a gut cavity, but food enters and wastes leave through the same opening. Flatworms add a third tissue layer and a head with eyespots; being flat puts every cell close to the surface for gas exchange, since they have no lungs or blood. Roundworms add a complete digestive tract, a tube from mouth to anus, so food moves one way and different regions can specialize. Annelids add a true coelom and segments; an earthworm's fluid-filled segments act as a hydrostatic skeleton for burrowing. Molluscs are built on a foot, a mantle and often a shell, and cephalopods such as the octopus have the largest brains of any invertebrates. Arthropods add a jointed exoskeleton, the key to their success on land and in the air. Echinoderms have bilateral larvae, which shows their radial adult form evolved later, and their early embryos develop like those of chordates, so they are our closest relatives outside the chordates.

Every chordate has four features at some point in its life: a notochord (a flexible supporting rod), a dorsal hollow nerve cord, pharyngeal slits (openings in the throat region), and a post-anal tail. In vertebrates the notochord is replaced during development by a backbone of vertebrae that surrounds the nerve cord, now the spinal cord, and the pharyngeal slits become gills in fish or parts of the jaw and ear in land animals. Humans show all four features as embryos. Vertebrates are grouped into classes by how they breathe, reproduce and control body temperature.