What is common between a shark, an earthen toad, a python, a pigeon, a dolphin and a man? At first glance, nothing, the striking differences between the organisms from each other, both externally and in what they eat and where they live. How does biology explain their unity?
Chorda is an organ formed during embryogenesis in all of the above-mentioned inhabitants of the Earth. What did he come from? What are the functions of the chord and in which animals does it occur? We will find answers to the questions that have now been raised in this article. The examples below illustrate the diversity of chordates and their leading role in natural ecosystems.
Characteristic of the axial organ of vertebrates
All structural and vital features inherent in 42 thousand species of living creatures of the Chordate type are studied by biology. Chorda is the main anatomical criterion that combines these organisms into a single systematic unit. It is an elastic cord, consisting of connective tissue cells, providing support to the entire body. The chord is located along the body and is maintained throughout life in some species called lower chordates. These are representatives of the superclass Jawless: lampreys and mixins. In cartilaginous fish, sharks and stingrays, the entire skeleton consists of connective tissue and has the same structure as the chord. She, in turn, remains with these organisms all her life.
What led to the appearance of the inner skeleton
What is the meaning of the chord? Biology considers its appearance in lanceolators, and then other animals as the most important aromorphosis. He led to the development of a support and movement system that appeared in the first land animals. This allowed them to get maximum benefits in conquering land and to resist, first of all, the negative influence of gravity and other adverse environmental factors.
In a large group of animals called vertebrates, the cartilage cord is replaced by an axial structure, which consists of bone elements - vertebrae. These include representatives of most classes of terrestrial animals, for example, amphibians, reptiles, birds and mammals. To the aquatic inhabitants having a spine, can be attributed to orders of bone fish: cyprinidae, herring, perch, etc.
In the process of evolution, all the above-mentioned groups of animals occupied a dominant position in the fauna of the planet. This is confirmed by the research that biology conducts. Chorda is an organ that has allowed animals to compete successfully with representatives of invertebrates in the struggle for existence.
First chord animal
Many zoological scientists wanted to find an animal whose structure could be considered the connecting link between invertebrate and vertebrate organisms. In the middle of the XIX century, the Russian embryologist A. Kovalevsky described the embryonic development of the unremarkable inhabitant of the Black Sea - the common lancelet.
A translucent creature up to 8 cm long and resembling a fish without a head kept a chord throughout its life. The lancelet turned out to be a transitional form, the existence of which biology has been looking for so long. Chorda is a structure, the appearance of which best confirms the evolutionary path of development of nature. The features of the embryonic structure (the presence of three germ layers) and the ontogenesis of the lancelet confirmed the assumption that the ancestors of chordates are organisms that do not have an internal skeleton.
Axial Skeleton Tab
The formation of a chord in different organisms is approximately the same, although the time interval in which this occurs can be different. For example, in a human embryo, a connective tissue cord appears in an earlier period of embryonic development, and its laying proceeds faster than in fish or other vertebrates. In embryos, the dorsal axis forms under the neural tube. A vertebral chord develops from a section of the primary intestine called the chordomesoderm process.
Biology and medicine study in detail the processes occurring during the gastrulation phase, which allows to identify intrauterine developmental disorders that may lead to decreased viability, the appearance of pathologies and the death of the embryo. Failure in the mechanisms of laying the axial organs: neural tube, chords, intestines - is the most dangerous, therefore, the normal course of pregnancy in the first three months is very important for the health of the unborn child.
Chord variety and value
Representatives of this large group of organisms are so well adapted to various environmental factors that they occupied all the ecological niches existing on Earth. More than 20 thousand species of fish, considered the most ancient chordates, live in the hydrosphere, the surface and depths of the lithosphere are filled with a huge number of forms of amphibians, reptiles and mammals, and the ground and air environment is inhabited by more than 8 thousand species of birds. Human populations also belong to the type of chordates, whose vital activity radically changed the face of our planet.