includes fish, amphibians, reptiles, birds, and mam-
mals; a member of the phylum Chordata.
Three subphyla exist: Urochordata (sea squirts, lar-
vaceans), Cephalochordata (amphioxus, lancelets), and
Craniata (hagfish plus all the vertebrates). Another
smaller phylum Hemichordates, the sawblades and
acorns, are important in the study of vertebrate evolu-
tion. Although they contain only a few hundred
species, they seem to share some of the chordate char-
acteristics: brachial openings, or “gill slits,” into the
pharynx; a rudimentary structure called the stomo-
chord that is similar to a notochord; and a dorsal nerve
cord and a smaller ventral nerve cord. However, DNA
studies are showing that hemichordates may be closer
to echinoderms.
There are approximately 50,000 living species of
vertebrates, with slightly fewer total fish vertebrates
(25,988), the oldest group, than all others combined
(4,500 mammals, 9,100 birds, 7,082 reptiles, 4,880
amphibians). They range in size from 0.1 gram to
100,000 kilograms. The Cephalochordata have the
fewest number of living species with 45, followed by
the Urochordata with 2,000 species, and the Craniata
with 43,000 species.
Vertebrates are chordates with a distinct head and
contain sense organs and a brain. Other characteristics
include a segmented vertebral rod of cartilage/bone, a
closed circulatory system, true coelom, and a bilaterally
symmetrical body.
Vertebrates first appeared on Earth some 500 mil-
lion years ago when continents were fragmented. Then
the continents combined into the supercontinent Pan-
gaea, which included most of the Earth’s crust, some
300 million years ago, fragmenting again 100 million
years ago into Laurasia (the northern continents) and
Gondwanaland (the southern continents). Vertebrates
live in almost every conceivable habitat on earth.
See alsoINVERTEBRATES.vesicle A small, membrane-enclosed sac, cyst, or bub-
ble found in the cytosol, the semifluid inside the cell
membrane of eukaryotic cells. Vesicles are used to trans-
port proteins and lipids to various destinations in the
cell, including the sites of glycolysis (the first stage of
energy production by the cell), and fatty acid synthesis.
Vesicles can import particles in a process called endocy-
tosis or export waste in a process called exocytosis.
In botanical terms, a vesicle is a small bladderlike
body in the substance of a vegetable or on the surface
of a leaf. In medical terms, a vesicle is a small and
more-or-less circular elevation of the cuticle, skin340 vesicle
View of the spine and pelvis of a human skeleton. The spine is a
column of 33 roughly cylindrical bones called vertebrae. Between
each vertebra is a disc-shaped pad of cartilage. At the top of the
image, seven cervical vertebrae support the skull. The 12 thoracic
vertebrae are below this, and each of these supports an attached
rib (not seen). The lumbar spine is five vertebrae that support the
lower back, and these take most of the strain when lifting. At the
bottom, the sacrum consists of 5 fused vertebrae that connect to
the pelvic bone. The pelvic bone protects and supports abdominal
organs.(Courtesy © James Stevenson/Photo Researchers, Inc.)