We saw previously that the bilateral animals diverged into two large groups: protostomes and deuterostomes. Now that we’ve surveyed the important groups of protostomes, it’s time to move closer in the tree of life to our own branch.

Humans and other vertebrates are deuterostomes. Before we examine the phylum to which we belong, let’s look at the other large phylum of deuterostomes: the echinoderms.

Sea stars, also called starfish, are echinoderms.

Brittle stars, basket stars, sea urchins, sand dollars, sea cucumbers, and sea lilies are also members of this group. Do you notice something strange about this group of bilateral animals? What’s most unusual about them, considering their place in the animal family tree? Check the box next to your answer; then click Submit.

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Although all these statements about echinoderms are true, the fact that’s most surprising is that these bilateral animals have radial symmetry, like the radiate animals. Echinoderms only show their bilateral nature clearly when they’re immature, as in the larvae shown on the screen. As we’ll often find in the study of animal relationships, the traits that characterize a group may appear only at particular stages of an animal’s life cycle.

The typical adult echinoderm, such as a sea star, lacks a head, and has a decentralized nervous system without a brain. Echinoderms have two additional important distinctions. They have an internal skeleton of calcified plates, covered by a layer of skin.

They also have a water-vascular system, a system of channels through which seawater can flow inside the body of the animal, including the echinoderm’s hollow tube feet. The water vascular system functions in locomotion, feeding, and gas exchange.

The other large phylum of deuterostomes is the one to which we humans belong: the chordates. All vertebrates are members of this phylum, but there are also some invertebrate chordates, such as lancelets and tunicates, or sea squirts. Chordates are distinguished by some important morphological traits.

The figure on the screen is a generalized diagram of a chordate body, viewed from the side. At some point in their development, all chordates have a notochord, a flexible, supportive rod of tissue running through the back of the animal. In vertebrates, this rod is replaced by the vertebral column later in development. Chordates also have a hollow nerve cord. In vertebrates, this becomes the spinal cord and brain of the adult. Chordates have a distinctive feature associated with their pharynx, the part of the digestive tract that follows the mouth. The pharynx of a chordate has slits that open to the outside of the animal.

The slits develop into structures involved in feeding, breathing, and hearing. Most chordates also have a tail extending beyond the anus at some point in their development.

Vertebrates are a subgroup of chordates distinguished by their vertebral column, or backbone, composed of hard segments called vertebrae. The vertebral column is the main supportive structure for the vertebrate’s body. Vertebrates also have a protective braincase, which together with the vertebral column protects the animal’s central nervous system. These features, along with their well-developed sensory organs, are adaptations that allow for large size and quick movement.

The first vertebrates were aquatic creatures; their fossils trace back to the Cambrian period. By the end of the Silurian period, vertebrates with jaws had appeared. The vertebrate jaw is hinged and works up and down. It was an important adaptation for feeding, and most vertebrates today have jaws. The lamprey, a jawless fish with a sucker mouth, is an exception. The evolution of the jaw allowed vertebrates to dominate the seas, and soon after, vertebrates invaded the land. We’ll look at their descendants, the present-day fishes and land vertebrates, next.

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