11.13–11.15: The phylum Chordata includes vertebrates, animals with a backbone.

Animals in motion: these X-ray images show a tuatara (a New Zealand reptile) walking.
11.13: All vertebrates are members of the phylum Chordata.

Vertebrates are deuterostome animals, having defined tissues and bilateral symmetry. Vertebrates are part of a phylum called Chordata (the chordates) that includes three major groups in total; the two other groups are the tunicates and the lancelets (FIGURE 11-20). For at least a portion of their life cycle, members of all three of the major groups of chordates possess four distinct chordate body structures that are found in no other animal groups (FIGURE 11-21).

Figure 11.20: An overview of chordates.
Figure 11.21: Characteristic chordate body structures.

Although all chordates share these four characteristic structures, the chordates exhibit tremendous physical and ecological diversity.

Tunicates (Urochordata; about 2,000 species) are invertebrate marine animals that have defined tissues, bilateral symmetry, and deuterostome development, in which the gut develops from back to front. The adults are about the size of your thumb and look like balls of brownish green jelly. You can find them attached to docks and the mooring lines of boats. It is the free-swimming larvae of tunicates that reveal their chordate characteristics. Tunicate larvae have a distinct notochord, dorsal hollow nerve cord, and tail, all of which disappear during development. The sessile adult tunicates only vaguely resemble their larvae. Adult tunicates are filter-feeders: cilia draw a current of water through the mouth into the pharynx and out through the pharyngeal slits. Microscopic food items are trapped by a layer of mucus stretched across the slits.

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Figure 11.22: Vertebrate phylogeny.

Lancelets (Cephalochordata; about 20 species), also bilaterally symmetrical deuterostomes, are slender, eel-like invertebrate animals, about the length of your little finger, that live in coastal waters. Unlike tunicates, both embryonic and adult lancelets have all the chordate characteristics. Lancelets are also filter-feeders.

Vertebrates (Vertebrata; about 56,000 species) are the most diverse group of chordates (FIGURE 11-22). Because they are deuterostomes, they are bilaterally symmetrical, with defined tissue and from-back-to-front gut development. Vertebrates differ from the other chordates in two important ways.

  1. They have a backbone, formed when a column made from hollow bones (or cartilage in some organisms), called vertebrae, forms around the notochord. This backbone surrounds and protects the dorsal hollow nerve cord.
  2. They have a head, at the front (anterior) end of the organism, containing a skull, a brain, and sensory organs.

The size range of vertebrates is huge. The smallest is a frog species from New Guinea that is just over a quarter-inch (6.4 mm) long. And some tiny hummingbirds and shrews weigh only a tenth of an ounce (about 3 grams). Blue whales, weighing in at more than 130 tons (120,000 kg), are the largest. Vertebrates move by swimming, burrowing, craphelaning, walking, running, and flying, and they are present in all habitats on earth. But despite all these differences, all vertebrates have the four chordate characteristics at some stage of their lives. In the remaining sections we focus on the vertebrates, which are by far the most diverse and widely dispersed group of chordates.

TAKE-HOME MESSAGE 11.13

All chordates have four characteristic structures: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. The three subphyla of chordates, though superficially very different, are united by possessing these four structures at some stage of their life cycle.

Which four distinct body structures define vertebrates?

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