Type chordata has more than 40 thousand living species of animals. These include cranial (shell and lancelet) and cranial (cyclostome (lampreys), fish, amphibians, reptiles, birds and mammals). Representatives of this type live throughout the globe and in all habitats. Most chordates lead an active, mobile lifestyle, but there are species that are attached to the substrate - shells. The size and body weight vary widely in this type and depend on the species and habitat of the animal.
Despite the fact that the animals, united in the type of chordates, are very different in appearance, features of the internal structure, lifestyle and habitat,
they have a number of common features. To determine this similarity will help the general characteristics of the chordates.
All chordates have:
The axial skeleton, which is represented by the chorda in the cranial and the spine in the cranial. The skeleton has the form of a strand, performs a supporting function and gives elasticity to the body.
Gill slits in the throat. In primary organisms, which all the time live in water and do not leave it, gill slits remain throughout life. And in the secondary ones, who left the aquatic environment, and then returned there again (dolphins, whales, crocodiles), and terrestrial animals, gill slits exist only at certain stages of embryonic development, and then disappear. Instead, the lungs function - the organs of ground breathing.
Central nervous system (CNS), which is in the form of a tube located on the back. In primitive chordates, it remains in the form of a hollow tube throughout life, and in highly organized animals it is divided into the brain and spinal cord. And the nerve endings that move away from the central nervous system form the peripheral nervous system.
Circulatory system. The heart, like the neural tube, is located on the abdominal side of the body.
Chordate animals have distinctive features inside the species, which is associated with their lifestyle and habitat, as well as adaptation to it. In addition to signs of difference from other organisms, chordates have similarities with other animals. These similarities are:
Bilateral symmetry, which is inherent in flatworms, insects and other organisms.
The whole (otherwise the secondary cavity of the body) in which the internal organs are located. A secondary cavity appears in annelids.
They have a secondary mouth, which is formed at the gastrula stage by breaking through the wall.
The metameric arrangement of organs (segmented) is clearly expressed at the stage of the embryo and in primitive chordates; in adult animals, it can be seen in the structure of muscles and the axis of the spine. Due to this, the chordate type shows signs of similarity to annelids and insects.
The presence of organ systems - circulatory, respiratory, nervous, digestive, excretory, genital.
Thus, the chordate type unites animals that are characterized by bilateral symmetry and as a whole, the presence of gill slits in the early stages of development and the appearance of the internal skeleton - the chord, above which the neural tube is located. Under the chord is a digestive tube.