Chordates are the most highly organized creatures of all representatives of the Animal kingdom. The characteristic features of the structure allowed them to become the pinnacle of evolution.
Signs of chordates
The main features of these animals is the presence of a notochord, neural tube and gill slits in the throat. Chordates are organisms in which the listed characteristics can be significantly modified.
So, the skeleton can be external and internal. And the development of chordates in ontogenesis can be characterized by the fact that gill slits are overgrown 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 notochord. It is an internal axial skeleton, which, in the form of a solid strand, passes through the entire body. Throughout life, the chord remains not in many representatives of this type. These include different types of lancelets, representing the class Cephalochordidae of the subtype Invertebrates.
In other representatives, the notochord develops into a skeleton. Only a few have it made up of cartilage. Bony fish, amphibians, reptiles, birds andmammals have a fully ossified skeleton. In the process of evolution, it becomes more complicated. Its constituent parts are the skull, spine, chest, belts and directly the upper and lower limbs.
Gill slits in 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 the limbs.
Of course, not all chordates have this anatomical feature. Gill slits are preserved in the lancelet and cartilaginous fish: sharks and rays. In animals that are adapted to breathing atmospheric oxygen, they overgrow in the early stages of embryonic development. After the lungs are formed.
Features of the nervous system
The system that provides the relationship of the organism with the environment in chordates is initially formed according to the type of the neural tube. It is of ectodermal origin.
Chordates are highly developed animals largely due to the peculiarities of the structure 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 part of the central nervous system. The brain is reliably protected by the bones of the skull, which are connected motionlessly. It is differentiated into departments on a functional basis. Anatomically, through the opening 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 “transport highway”, uniting the complexorganism 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.
Chordata variety
This phylum includes three subtypes: Non-cranial, Larval-Chordate (Tunicator) and Cranial (Vertebrate).
The first of them includes only 30 species found in our time. Their representatives are lancelets. These animals look like a surgical instrument called a lancet.
The body of these small animals is almost always half in the sand. This makes it easier for the lancelet to filter the water by swallowing nutrient particles.
The most numerous subtype of chordates are Vertebrates. They have mastered absolutely all habitats, filled food chains and ecological niches.
Aquatic inhabitants are fish. Their streamlined body is covered with scales, they are adapted for 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 lungs and skin, but the process of their reproduction takes place in the water. Like fish, their females throw eggs into the water, which the males sprinkle with seminal fluid.
Typically, land animals are reptiles. Lizards, snakes, turtles and crocodiles spend only their hunting time in the water. They reproduce by eggs that they lay in special shelters on land. Their skin is dry and covered with dense scales.
The last trait was inherited from reptile birds. The unfeathered part of their legs is called the tarsus. It is she who is covered with small scales. Scientists consider this fact as proof of the origin in the process of evolution. Birds are capable of flight due to many features of the external and internal structure. These are modified forelimbs, a feather cover, a light skeleton, the presence of a keel - a flat bone to which the muscles that set the wings in motion are attached.
Finally, Beasts, or Mammals, are the pinnacle of evolution. They are viviparous and feed their young with milk.
Chordate animals are the most complexly organized, diverse in structure, playing an important role in nature and human life.
