The vertebrates comprise a large
group of chordates, and are subdivided into seven classes (3 classes of fish,
amphibians, reptiles, birds, and mammals).
have an internal skeleton of cartilage or bone, with vertebrae surrounding the
dorsal nerve cord.
The subphylum Vertebrata
consists of about 43,700 species of animals with backbones. Vertebrates exhibit
all three of the chordate characteristics at some point during their lives. The
embryonic notochord is replaced by a vertebral column in the adult. The
vertebral column is made of individual hard segments (vertebrae) surrounding the
dorsal hollow nerve cord. The nerve cord is the one chordate feature present in
the adult phase of all vertebrates. The vertebral column, part of a flexible but
strong endoskeleton, is evidence that vertebrates are segmented. The vertebrate
skeleton is living tissue (either cartilage or bone) that grows as the animal
The endoskeleton and muscles
form an organ system that permits
rapid and efficient movement. The pectoral and pelvic fins of fishes evolved
into jointed appendages that allowed vertebrates to move onto land. The skull,
the most anterior component of the main axis of the vertebrate endoskeleton,
encases the brain. The high degree of cephalization in vertebrates is
accompanied by complex sense organs concentrated in the head region. Eyes
developed as outgrowths of the brain. Ears were equilibrium devices in aquatic
vertebrates that function as sound-wave receivers in land vertebrates.
Vertebrates have a complete digestive system and a large coelom. Their
circulatory system is closed, with respiratory pigments contained within blood
vessels. Gas exchange is efficiently accomplished by gills, lungs, and in a few
cases, moist skin. Kidneys are efficient in excretion of nitrogenous waste and
regulation of water. Reproduction is usually sexual with separate sexes.
The first vertebrates were
fishlike. Fishes are aquatic, gill-breathing vertebrates that usually have fins
and skin covered with scales. The larval form of a modern-day lamprey, which
looks like a lancelet, may resemble the first vertebrates: it has the three
chordate characteristics (like the tunicate larva), as well as a two-chambered
heart, a three-part brain, and other internal organs that are like those of
Small, jawless, and finless
ostracoderms were the earliest vertebrates. They were filter feeders, but
probably were also able to move water through their gills by muscular action.
Ostracoderms have been found as fossils from the Cambrian through Devonian
periods, when the group finally went extinct. Although extant jawless fishes
lack protection, many early jawless fishes had large defensive head shields.
These long, eel-like, jawless
fish are free-swimming predators on other fish. Lampreys hatch in freshwater and
many live their lives entirely in freshwater. Some lampreys migrate to the sea,
but must return to freshwater to reproduce. Lampreys have a sucker-like mouth
that lacks a jaw.
Sea lamprey mouth
of the class Myxini have a partial cranium (skull), but no vertebrae. Their
skeleton is made of of cartilage, as is that of sharks. Hagfish lack jaws, and
for this reason used to be classified with the lampreys in a group called the
Agnatha ("no jaws") or the Cyclostomata ("round mouth").
The fish first appeared during
the Cambrian Period. Whether fish first evolved in fresh or salt water is
unclear from the fossil record.
jawless fish are the most primitive group, although they were a very important
group during the Silurian and Devonian periods. Hagfish
and lampreys are the only living members of this class today. They have long, cylindrical
bodies with cartilage skeletons and no paired fins.
The first jawed fish were the
Placoderms, an extinct group of Devonian-aged jawed fishes. Placoderms were
armored with heavy plates and had strong jaws and paired pectoral and pelvic
fins. Paired fins allow fish to balance and to maneuver well in water, which
facilitate both predation and escape.
fossil is a cast of the placoderm, Bothriolepis
The evolution of jaws is an
example of evolutionary modification of existing structures to perform new
functions. Jaws are modified gill arches,
and allowed the exploitation of new roles in the habitats: predators with
powerful jaws. There
are two classes of jawed fish: the cartilaginous fish and the bony fish.