Vertebrados

Vertebrados
Classificação científica
Domínio: Eukariota Reino: Animal Filo: Chordata Sub-filo: Craniata Clade: Vertebrata
Classes
Petromyzontida (lampreias) Placodermi - extincta Chondrichthyes (peixes cartilaginosos) Acanthodii - extincta Actinopterygii (peixes com raios nas barbatanas) Actinistia (celacanto) Dipnoi (peixes pulmonados) Amphibia (anfíbios - rãs, salamandras) Reptilia (répteis) Aves Mammalia (mamíferos)

Vertebrata é o subgrupo dos Craniata, composto por todos os animais com coluna vertebral ou "espinha dorsal" formada por vértebras que protegem a medula espinal.

A maioria dos animais com maior grau de organização a que estamos habituados: peixes (com excepção das mixinas), anfíbios, répteis, aves e mamíferos - incluíndo o Homem - pertencem a este grupo.

Outras características adicionais são a presença dum sistema muscular geralmente simétrico - a simetria bilateral é também uma característica dos vertebrados - e dum sistema nervoso central, formado pelo cérebro e pela medula espinal localizados dentro da parte central do esqueleto (crânio e coluna vertebral).

Sistema esquelético

O esqueleto interno que define os vertebrados é formado por cartilagem, osso ou, na maior parte dos casos, por estes dois tecidos e consiste no crânio, na coluna vertebral e em dois pares de membros, embora em alguns grupos, como as cobras e as baleias, os membros estejam ausentes ou apenas na forma vestigial. O esqueleto dá suporte ao organismo durante o crescimento e, por essa razão, a maioria dos vertebrados são de maiores dimensões que os invertebrados.

A presença dum crânio também possibilitou o desenvolvimento do cérebro, pelo que os vertebrados têm maior capacidade de se adaptar ao meio ambiente e até de o modificar (ver por exemplo, o caso dos castores que constroem verdadeiras represas!)

Metamerically arranged endoskeletal elements flanking the spinal cord. There are primitively two pairs of such elements in each metamere and on each side: the interdorsals and basidorsals. In the gnathostomes, there are two additional pairs ventrally to the notochord: the interventrals and basiventrals. These elements are called arcualia and can fuse to a notochordal calcification, the centrum. The ensemble of the arcualia + centrum is the vertebra, and the ensemble of the vertebrae is the vertebral column.

Both the vertebral column and the limbs support the body of the vertebrate overall. This support facilitates movement. Movement is normally achieved with muscles that are attached directly to the bones or cartilages. The contour of the body of a vertebrate is formed by the muscles. A skin covers the inner parts of a vertebrate's body. The skin sometimes acts as a structure for protective features, such as horny scales or fur. Feathers are also attached to the skin.

The trunc of a vertebrate is hollow and houses the internal organs. The heart and the repiratory organs are protected in the trunc. The heart is located behind the gills, or where there are lungs, in between the lungs.

Sistema nervoso

The central nervous system of a vertebrate consists of the brain and the spinal cord. Both of these are characterizes by being hollow. In lower vertebrates the brain mostly controls the functioning of the sense organs. In higher vertebrates the size of the brain relative to the size of the body is larger. This larger brain enables more intensive exchange of information between the different parts of the brain. The nerves from the spinal cord, which lies behind the brain, extend to the skin, the inner organs and the muscles. Some nerves are directly connected to the brain, linking the brain with the hear and lungs.

The acoustic organs also comprise a special component, the lateral sensory system, which is lost in most terrestrial craniates (Amniota). It consists of lateralis nerve fibres derived from the acoustic nerve and superficial mechanoreceptors, the neuromasts, which are housed in grooves or canals on the surface of the head. These extend onto the body in the Vertebrata. True neuromasts, however, seem to be unique to the Vertebrata, and have never been observed in hagfishes.

Sistema circulatório

Craniates have a circulatory system of arteries, capillaries and veins, and a chambered, muscular main heart located ventrally and anteriorly in the trunk. In the Vertebrata, the circulatory system is entirely closed. The two heart chambers, the atrium and ventricle are well apart. There are additional accessory venous hearts in the head and tail, which help in venous blood circulation, but these are lost in the Vertebrata. In gill-breathing craniates, the heart pumps venous blood anteriorly into arteries and capillaries in the gills for gas (oxygen and carbon dioxide) exchange with water. Oxygenated blood then collects dorsal to the gills and flows anteriorly to the head and posteriorly to the organs and muscles, and back to the heart. In some Vertebrata (Osteichthyes) diverticles of the digestive tract (lungs or air bladder) supplements or replaces gills as the repiratory organ.

Aparelhodigestivo

The digestive tract of craniates is longitudinally differentiated into mouth and oral cavity, pharynx, esophagus, intestine, rectum and anus. A stomach is developed in the Gnathostomata and some fossil jawless Vertebrates. All craniates have a pancreas that produces digestive enzymes and hormones (insulin and glucagon) that regulate blood sugar level. The pancreas was ancestrally disseminated along the anterior part of the gut, but becomes condensed into a well-defined organ in the Vertebrata.

All craniates and the related cephalochordates have a liver or hepatic organ that serves many functions including food storage and production of fat emulsifiers (bile).

Rins

The kidneys are the chief excretory organs of vertebrates and these organs play an important role in water and salt balance. Although kidneys vary greatly in size, shape and position among species, all contain nephrons as the basic functional units. Each nephron is a nearly microscopic tubule that receives a filtrate of blood (lacking blood cells and very large molecules). The filtrate is processed by selective secretion and reabsorption of materials to produce an excretory product (generally called urine) that contains nitrogenous waste and other materials. Long and complex kidney tubules occur only in the vertebrates.

Vertebrates have been traced back to the Silurian period (444 million to 409 million years ago).

Reprodução

The reproductive biology of craniates is highly diverse. The majority of species are bisexual with distinct male and female individuals. Of course, male and female sexes are always distinct in the type of gonads they possess (testes or ovaries), and sex cells they produce (gametes: sperm or ova). Sperm is shed directly into the coelom and then toward the exterior through a pore. In the Gnathostomata, however, testes are linked with the kidneys and sperm pass through the excretory ducts. External sex dimorphism may be non-existent to dramatically pronounced. There are some fishes that are naturally hermaphroditic. In certain hermaphroditic species individuals are "protogynous," i.e. first functioning as females that may later transform into functional males. In other species the opposite sequence of sex change obtains -- "protandrous." There are a few "all-female" species of fishes, amphibians and lizards in which mothers produce only female offspring. In many of these mating with males of related species is necessary to trigger egg development, but fathers do not contribute to the genetic perpetuation of the "all-female" lineages. Among the more typical bisexual craniates there is a very broad spectrum of reproductive mode. The majority of fish and amphibian species are oviparous (egg-laying) with external fertilization of the eggs by the male's sperm. Other fishes, amphibians, many reptiles, all birds and the monotreme mammals (platypus and spiny anteaters of the Australian region) are also oviparous but fertilization is internal. Opposite these are the "live-bearers" or viviparous species in which fertilization is necessarily internal and young develop within the mother's reproductive tract. Here the mother must provide some form nutriment to the embryo (either yolk in the egg or via the blood across permeable placental membranes). Also, the viviparous species have mechanisms for embryonic gas exchange and waste removal. Viviparity has evolved many times in the craniates - among cartilaginous and bony fishes, a handful of amphibians, several snakes and lizards, and most mammals.

External links

• Tree of Life