Chordates are the most highly organized creatures of all the representatives of the Animal Kingdom. The characteristic features of the structure allowed them to become the pinnacle of evolution.
Signs of chordate
The main features of these animals are the presence of a chord, neural tube and gill slits in the pharynx. Chordates are organisms in which the listed symptoms can significantly change.
So, the skeleton can be external and internal. And the development of chordates in ontogenesis can be characterized by the fact that gill slits overgrow even in the embryonic development of organisms. At the same time, they develop other respiratory organs - air sacs or lungs.
Axial skeleton
The main characteristic of chordates is the presence of a chord. She is an internal axial skeleton, which in the form of a single strand passes through the whole body. Throughout the life of the chord, not many representatives of this type remain. These include different types of lancelet species, representing the class Brachochordate subtype of Invertebrate.
The remaining representatives of the chord develop into a skeleton. Only a few have cartilage tissue. Bony fish, amphibians, reptiles, birds and mammals have a completely ossified skeleton. In the process of evolution, it becomes more complicated. Its components are the skull, spine, chest, belts and directly the upper and lower extremities.
Gill slits in the throat
Chordates are animals in which the organs of the respiratory system are formed as outgrowths of the pharynx. This is their main difference from invertebrates. In this group, they are derivatives of limbs.
Of course, not all chordates have this anatomical sign. Gill slits persist in the lancelet and cartilaginous fish: sharks and rays. In animals that are adapted for respiration by atmospheric oxygen, they are overgrown in the early stages of embryonic development. After the lungs are formed.
Features of the nervous system
The system that ensures the relationship of the organism with the environment in chordates is initially formed as a neural tube. It has an ectodermal origin.
Chordates are highly developed animals largely due to the structural features of the nervous system. So, in mammals, it is represented by the spinal cord located in the spinal canal, as well as the brain. They are parts of the central nervous system. The brain is reliably protected by the bones of the skull, which are connected motionless. It differentiates into departments according to a functional attribute. Anatomically, through the hole formed by the vertebrae, the brain is connected to the spinal cord. The peripheral part of the system is formed by the spinal and cranial nerves. They play the role of a “transportation highway”, combining a complex organism into a single whole and coordinating its work.
The structure of the nervous system determines the complex behavior of chordates, the formation of conditioned reflexes and a clear program of instinctive behavior.
Variety of chordates
This type includes three subtypes: Cranial, Larval-chordate (Envelope), and Cranial (Vertebrate).
The first of them includes only 30 species found in our time. Their representatives are lancelet. These animals look like a surgical instrument called a lancet.
The body of these small animals is almost always half in the sand. So it is more convenient for a lancelet to filter water, swallowing nutrient particles.
The most numerous subtype of chordates are Vertebrates. They have mastered absolutely all habitats, filled food chains and ecological niches.
Water inhabitants are fish. Their streamlined body is covered with scales, they are adapted to gill breathing, move with the help of fins.
Amphibians are the first to land. These are frogs, toads, newts, worms and fish snakes. Their common name is due to the fact that they live on land, breathe with the help of their lungs and skin, but the process of their reproduction occurs in water. Like fish, their females toss caviar into the water, which the males pour with seminal fluid.
Typically terrestrial animals are reptiles. Lizards, snakes, turtles and crocodiles spend only hunting time in the water. They breed eggs, which are laid in special shelters on land. Their skin is dry, covered with dense scales.
The last trait is inherited from reptiles birds. The non-feathered part of their legs is called a forearm. It is she who is covered with small scales. Scientists consider this fact as evidence of origin in the process of evolution. Birds are capable of flying due to many features of the external and internal structure. These are modified forelimbs, feather cover, light skeleton, the presence of a keel - a flat bone to which the muscles that move the wings are attached.
Finally, Beasts, or Mammals, are the crown of evolution. They are viviparous and feed the young with milk.
Chord animals are the most complex, diverse in structure, playing a crucial role in nature and human life.