Vertebrate anatomy

Vertebrate anatomy

he defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord). However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon[10] and the Latimeria. Jawed vertebrates are typified by paired appendages (fins or legs, which may be secondarily lost), but this is not part of the definition of vertebrates as a whole. Gills Gill arches bearing gills in a pike All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Each gill is supported by a cartilagenous or bony gill arch.[11] The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches, as some of their chordate relatives have more than 50 pairs of gills.[9] In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches.[12] These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibans retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods.[13] While the higher vertebrates do not have gills, the gill arches form during fetal development, and lay the basis

of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and in mammals the malleus and incus.[9] [edit]Central nervous system The vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of the nerve cord is found in the lancelet, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation.[9] The central nervous system is based on a hollow nerve tube running along the length of the animal, from which the peripheral nervous system branches out to enervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups.[14] Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss.[15][16] The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominate in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals.[14] The resulting anatomy of the central nervous system, with a single, hollow nerve cord topped by a series of (often paired) vesicles is unique to vertebrates. All invertebrates with well developed brains, like insects, spiders and squids have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth/gut.