Chord type: features of external and internal structure

Chord type has more than 40 thousandliving animal species. This includes the uncranial (tunicates and lancetniki) and cranial (cyclostomes (lamprey), fish, amphibians, reptiles, birds and mammals). Representatives of this type live around the globe and in all habitats. Most chordates lead an active, moving lifestyle, but there are species that are attached to the substrate - tunicates. The size and weight of the body vary widely in this type and depend on the species and habitat of the animal.

Despite the fact that animals united in the chord type, are very different in appearance, characteristics of the internal structure, lifestyle and habitat,

they have a number of common features. The general characteristics of chordates will help to determine this similarity.

All chordates have:

• The axial skeleton, which is represented by the chord in the uncranial and the spine in the cranial. The skeleton has the shape of a cord, performs a support function and gives elasticity to the body.

• Gill slits in the pharynx.Primordial organisms that live in water all the time and do not leave it, gill slits remain throughout life. And for those with secondary flats who left the aquatic habitat and then returned there again (dolphins, whales, crocodiles), and land animals, gill slits exist only at certain stages of embryonic development, and then disappear. Instead, the lungs are functioning - organs of terrestrial respiration.

• The central nervous system (CNS), which is in the formtube is located on the back side. 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 depart 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 also have similar features with other animals. These similarities are:

• Bilateral symmetry, which is inherent in flatworms, insects and other organisms.

• The whole (otherwise the secondary body cavity), in which the internal organs are located. The secondary cavity appears in annelids.

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

• Metameric arrangement of organs (segmental)clearly expressed at the embryonic stage and in primitive chordates, in adult animals can be traced in the structure of the muscles and axis of the spine. Due to this, the chord type reveals signs of resemblance to annelids and insects.

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

So the chord type combines animals,which are characterized by bilateral symmetry and general, 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 the digestive tube.