Chordata type has more than 40 thousand living animal species. This includes non-cranial (tunicates and lancelets) and cranials (cyclostomes (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 - tunicates. The size and weight of the body varies widely in this type and depends on the species and habitat of the animal.
Despite the fact that the animals united in the chordate type are very different in appearance, features of the internal structure, lifestyle and habitat,
they have a number of common features. The general characteristics of chordates will help determine this similarity.
All chordates have:
- Axial skeleton, which is represented by a notochord in non-cranial animals and a spine in cranial ones. The skeleton has the form of a strand, performs a supporting function and gives elasticity to the body.
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Gill slits in the throat. Atprotostomes that live all the time in the water and do not leave it, gill slits remain throughout life. And in deuterostomes that left the aquatic habitat, 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 terrestrial respiration.
- The central nervous system (CNS), which is located in the form of a tube 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 branch off from the CNS form the peripheral nervous system.
- Circulatory closed system. The heart, like the neural tube, is located on the ventral side of the body.
Chordates have distinctive features within a 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 also have similarities with other animals. These similarities are:
- Bilateral symmetry, which is inherent in flatworms, insects and other organisms.
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The whole (otherwise the secondary cavity of the body), in which the internal organs are located. The secondary cavity appears in annelids.
- Have a secondary mouth, which is formed at the gastrula stage by breaking through the wall.
- Metamericthe arrangement of organs (segmental) is clearly expressed at the embryonic stage and in primitive chordates, in adult animals it can be traced in the structure of the muscles and the axis of the spine. Due to this, the chordate type shows signs of similarity with annelids and insects.
- The presence of organ systems - circulatory, respiratory, nervous, digestive, excretory, sexual.
Thus, the type of chordates combines animals that are characterized by bilateral symmetry and the whole, the presence of gill slits in the early stages of development and the appearance of an internal skeleton - a chorda, over which the neural tube is located. Under the notochord is the digestive tube.
