In a last look at protostomes, we surveyed mollusks and arthropods. The soft bodies of mollusks take a variety of shapes, and they’re often protected by a hard shell. Arthropods have jointed legs and a tough external skeleton containing chitin. The insects, a subgroup of arthropods, are tremendously diverse, and they’re the only invertebrates in which flight has evolved.

Echinoderms and chordates are the two major phyla of deuterostomes. Echinoderms include sea stars, sea urchins, and sand dollars. These bilateral animals develop radial symmetry in adulthood, and they have a distinctive water-vascular system.

Chordates are the group to which vertebrates belong. A few groups of invertebrates are also chordates. All chordates have several physical characteristics in common, including the presence of a notochord at some time in their development.

Vertebrates are distinguished by the presence of a supportive vertebral column and a braincase. Early vertebrates gained an additional competitive advantage in the evolution of jaws. The first vertebrates were fishes.

One group of bony fishes gave rise to the tetrapods, the land vertebrates. The first tetrapods were amphibians, which, like modern amphibians, depended on liquid water. The amniotes gained independence from liquid water with the development of waterproof skin and eggshells. Reptiles, the first amniotes, diversified at the end of the Paleozoic era and dominated the Mesozoic, the age of reptiles. An offshoot of the dinosaur lineage evolved into birds. The branch of amniotes leading to mammals diverged early in the Mesozoic era, but it wasn’t until the Cenozoic, after the extinction of the dinosaurs, that mammals achieved their great diversity and dominance. One of the many groups of mammals living today is the primates, which includes our own species and our closest relatives, the apes, monkeys, and lemurs.

This survey of the diversity of animal life was a quick tour of complex terrain. The story of animal relationships is still a work in progress. Biologists are debating many of the details, and new ideas about phylogeny are emerging all the time. We all experience animal diversity every day, but we may not fully appreciate its importance or its beauty. An evolutionary perspective helps us understand the remarkable variety of animals.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education