What is common between a shark, an earth toad, a python,dove, dolphin and man? At first glance, nothing, so striking are the differences of organisms from each other, both externally and in what they eat and where they dwell. How does the science of biology explain their unity?
Chorda is the organ that forms in the processembryogenesis in all the above-mentioned inhabitants of the Earth. What did it originate from? What are the functions of the chord and in which animals does it occur? We will find answers to the questions that have been raised in this article. The examples below illustrate the diversity of chordates and their leading role in natural ecosystems.
All the features of the structure and life activity,inherent in 42 thousand species of living beings belonging to the Chordovye type, studying biology. Chorda is the main anatomical criterion that unites these organisms into a single systematic unit. It is an elastic cord, consisting of cells of connective tissue, which provides support to the entire body. Chorda is located along the body and remains throughout life for some biological species, called lower chordates. These are representatives of the supra-class Bessmouths: lampreys and myxins. In cartilaginous fishes, sharks and rays, the entire skeleton consists of connective tissue and has the same structure as the chorda. It, in turn, is preserved in these organisms throughout life.
What is the value of the chord? Biology considers its appearance in lancelets, and then other animals as the most important aromorphosis. He led to the development of a system of support and movement that appeared in the first land animals. This allowed them to gain maximum advantages in the conquest of the land and to resist, primarily, the negative influence of the force of gravity and other unfavorable environmental factors.
A large group of animals namedvertebral, cartilaginous staple is replaced by an axial structure that consists of bone elements - vertebrae. These include representatives of most classes of terrestrial animals, for example, amphibians, reptiles, birds and mammals. To aquatic inhabitants having a spine, it is possible to carry orders of bone fishes: carp-like, herrings, percids, etc.
In the process of evolution, all the above groupsanimals took the dominant position in the fauna of the planet. This is confirmed by studies conducted by the science of biology. Chorda is the organ that allowed animals to successfully compete with representatives of invertebrates in the struggle for existence.
Many scientists-zoologists wanted to discover the animal,whose structure could be considered as a link between invertebrates and vertebrate organisms. In the middle of the XIX century, the Russian embryologist A. Kovalevsky described the embryonic development of an unremarkable inhabitant of the Black Sea, the lancelet of the common.
A semi-transparent creature up to 8 cm long and similarto fish without a head kept the chord for the rest of his life. Lancet was a transitional form, the existence of which so long sought biology. Chorda is a structure whose appearance best confirms the evolutionary path of the development of nature. Features of the embryonic structure (the presence of three germinal leaves) and ontogenesis of the lancelet have confirmed the assumption that the ancestors of chordates are organisms that do not have an internal skeleton.
Chord formation in different organismsis approximately the same, although the time interval in which this occurs can be different. For example, in the embryo of a human, the connective tissue appears in an earlier period of embryonic development, and its lining proceeds faster than in fish or other vertebrates. In embryos the dorsal axis is formed under the neural tube. From the area of the primary intestine, called the chordomeodermal process, a chord of vertebrates develops.
Biology and medicine will study in detail the processes,occurring in the phase of gastrulation, which allows to identify violations of intrauterine development, which may entail a decrease in viability, the appearance of pathologies and death of the embryo. Failure in the mechanisms of the bookmarking of the axial organs: the neural tube, the chord, the intestine - is the most dangerous, so the normal course of pregnancy in the first three months is very important for the health of the unborn child.
Representatives of this large group of organismsare so well adapted to various environmental factors that all ecological niches that exist on Earth have occupied. In the hydrosphere live more than 20 thousand species of fish, considered the most ancient chordates, the surface and depths of the lithosphere are filled with a huge number of forms of amphibians, reptiles and mammals, and the land and air environment is populated with more than 8 thousand species of birds. To the type of chordates are human populations, whose vital activity radically changed the face of our planet.
