Chordovye type: features of external and internal structure

The chord type includes more than 40 thousand living species of animals. This includes the cranial (shells and lancelets) and cranial (cyclostomes (lampreys), fish, amphibians, reptiles, birds and mammals). Representatives of this type live around the globe and in all habitats. Most chordates lead an active, mobile way of life, but there are species that are attached to the substrate - shells. Dimensions and body weight vary widely in this type and depend on the species and habitat of the animal.

Despite the fact that animals, united in a type of chordates, very different in appearance, features of the internal structure, lifestyle and habitat, They have a number of common features. Identify this similarity will help the general characteristics of chordates.

All chordates have:

• Axial skeleton, which is represented by a chord in the non-cranial and the spinal column of the cranial. The skeleton has the form of a strand, performs a supporting function and gives elasticity to the body.

• Gill slits in the pharynx. In primordial organisms, which all the time live in water and do not leave it, the gill slits remain throughout life. And in the second-term ones, which left the aquatic habitat, and then returned there (dolphins, whales, crocodiles), and terrestrial animals, the gill slits exist only at certain stages of embryonic development, and then disappear. Instead, lungs function - the organs of terrestrial breathing.

• The central nervous system (CNS), which in the form of a tube is located on the side of 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 closed system. The heart, like the neural tube, is located on the ventral side of the body.

Chord animals have distinctive features within 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 also have similarities with other animals. These similarities are:

• Two-sided symmetry, which is inherent in flatworms, insects and other organisms.

• Overall (otherwise the secondary body cavity), in which the internal organs are located. Secondary cavity appears in annelid worms.

• They have a secondary mouth, which is formed at the gastrula stage by breaking through the wall.

• The metameric arrangement of organs (segmental), clearly expressed at the embryonic stage and in primitive chordates, is traced in adult animals in the structure of the muscles and the axis of the spine. Due to this the type of chordates reveals signs of similarity with annelids and insects.

• The presence of systems of organs - the circulatory, respiratory, nervous, digestive, excretory, sexual.

Thus, the chordate type unites animals, which are characterized by bilateral symmetry and the whole, the presence of gill slits in the early stages of development and the appearance of the internal skeleton - the chord over which the neural tube is located. Under the chord is the digestive tube.