Chordates Nonvertebrate

The phylum Chordata is traditionally considered to consist of all of the classes of the sub-phylum Vertebrata (that is, fishes, amphibians, reptiles, and birds), plus several other groups that lack a brain case but share with vertebrates three fundamental anatomical features: (1) a so-called notochord (precursor to the vertebral column, or "backbone," of vertebrates), associated with (2) a dorsal hollow nerve cord (the "spinal cord" in vertebrates), and (3) gill slits in the pharynx ("throat"

region) at some point in the life cycle. (Recall that human embryos have gill slits during development).

Of the two or three major groups considered to be nonvertebrate chordates, only one (the Cephalochordata) resembles vertebrates at all; the rest look like either worms or invertebrates with simple, sacklike bodies. Some modern classifications, while recognizing the close evolutionary affinities of these very different-looking groups, prefer to split them into separate phyla: the Hemichordata, the Urochordata, the Cephalochordata, and the Vertebrata (sometimes called the Craniata). All of these groups, along with echinoderms and some other phyla, are deuterostomes (see Evolutionary Biodiversity), considered to be close evolutionary relatives because of features they share in their embryological development.

Hemichordata. The "acorn worms"— enteropneusts and pterobranchs—share with the urochordates and cephalochordates gill slits in the pharynx, but they are not considered to have true hollow nerve cords or noto-chords. Living either as burrowers or secretively in tubes, all species are marine. Hemichor-dates feed either by removing bacteria and organic particles (detritus) from the surface of sand grains, or in some cases by straining suspended plankton directly from seawater ("filter feeding"). Poorly known, the hemichor-dates nonetheless have a long fossil record, dating from the famed Burgess shale some 500 million years old. Paleontologists also consider the graptolites, a diverse group with a rich fossil record in the Lower and Middle Paleozoic, to have been hemichordates.

Urochordata. Urochordates, also known as tunicates, salps, sea squirts, and ascidians, all have the dorsal hollow nerve cord and the notochord and gill slits in their larval stages, but all of them lose the nerve cord and noto-

chord in the adult stage (except for the aptly named "larvaceans," which gain sexual maturity while retaining many of the features of the larval stage, a process known as neoteny).

Most urochordates live as adults attached to the seafloor, rocks, wharves, and the like. They develop a saclike structure that completely envelopes the body—which, internally, consists mostly of the pharynx lined with gills slits, with which they strain food particles. Equipped with a heart, a digestive system (including a liver), gonads for reproduction, and other organs, the deceptively simple-looking urochordates have all the complexities of the advanced animals they are, despite resembling from the outside much more simple invertebrates.

The urochordates are by far the most common of the nonvertebrate chordates. They are exclusively marine and often are found in a variety of bright colors: yellow, green, purple, and red.

Cephalochordates. Sometimes called the "acrania" because they resemble true vertebrates but without a skull, the cephalochor-dates are the "lancelets" of biology dissection labs—picked for study because they are the closest living relatives of true vertebrates. The notochord, dorsal hollow nerve cord, and pharynx with gill slits all remain with the animal throughout its life. Looking like a miniature fish, Branchiostoma (or "amphioxus") uses a circle of small tentacles around the mouth to intercept particles too large for digestion; the water then continues into the pharynx, where food particles are removed as it passes out through the gill slits. Lancelets feed by poking their heads out of the sandy bottom (all are marine, living in sandy, shallow-water environments). But lancelets are not rooted to the seafloor as are their close relatives, the urochordates. Rather, each individual is capable of swimming—mostly to avoid predators if disturbed, or to take up residence in another portion of the seafloor.

—Niles Eldredge See also: Evolutionary Biodiversity Bibliography

Brusca, Richard C., and Gary J. Brusca. 1990. Invertebrates. Sunderland, MA: Sinauer; Margulis, Lynn, and Karlene V. Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3d ed. New York: W. H. Freeman.

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