Chordate, common name for animals of the phylum
Chordata, which includes vertebrates as well as some invertebrates that
possess, at least for some time in their lives, a stiff rod called a notochord
lying above the gut and beneath a single, hollow dorsal nerve cord. About
43,700 living species are known, making the chordates the third largest
animal phylum. Three subphyla exist: Cephalochordata, the fishlike lancelets,
with 25 species; Tunicata, the highly modified tunicates, with about 2000
species; and Vertebrata, animals with backbones made up of vertebrae (including
fishes, amphibians, reptiles, birds, and mammals), with about 41,700 species.
The closest relatives of the chordates, the acorn worms of the phylum
Hemichordata, are sometimes treated as a chordate group but show only
a trace of a dorsal nerve cord and a structure vaguely resembling a notochord.
The best indication of their relationship to chordates is the presence
of holes in the pharynx. In both hemichordates and lower chordates, the
pharynx, with its gill slits, forms a complex structure that strains food
particles from water.
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LANCELETS
These animals, which look like very small
fish, are about 5 cm (about 2 in) long and have a well-developed notochord
that provides support for muscles used in swimming. Lancelets live in
sand and feed with their gill apparatus. Although their bodies have a
much simpler structure than that of fish—no heart or paired fins exist,
and they have only a trace of a brain—the arrangement of parts is similar
in these animals.
TUNICATES
Only the very young tunicates reveal their relationship
to other chordates. The tadpolelike larva has a simple notochord, a globular
body, and a tail used in swimming. When it attaches itself to the seafloor,
however, it loses the tail, notochord, and dorsal nerve cord. The adult
is covered with a protective covering called a tunic and feeds with its
gill apparatus.
VERTEBRATES
Many features found in lancelets and young
tunicates can be detected in modified form in vertebrates, especially
in embryos and in primitive animals such as jawless fishes. The pharyngeal
gill slits, for example, are retained in fishes and in the embryos of
more advanced animals, but in the latter the feeding and then the respiratory
functions of the gill slits become lost. The notochord of a vertebrate
becomes reinforced with bone (or cartilage in the cartilaginous fishes—sharks,
skates, and rays, or chondrichthyes) and is surrounded by the spinal column;
the dorsal nerve cord grows more complex, with a brain and a protective
skull.
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