General characteristics of chordates.

More than 40 thousand modern animal species are classified as chordates. These animals are very diverse in external structure, lifestyle and living conditions.

General structural features of chordates:
* the presence of an internal axial skeleton, the basis of which is a dense, elastic and elastic dorsal cord – the chord. It is formed in all chordates at the early stages of the development of their embryos (in the lower chordates it persists throughout their life, in the higher ones it is present only in the embryos, in adults it is replaced by the spine).
* The nervous system looks like a tube located on the dorsal side – above the notochord (formed from the ectoderm layer). In the higher chordates, the anterior neural tube grows and forms the brain.
* All chordates are bilaterally symmetrical animals. Along their body runs a digestive tube – the intestine, which begins with the mouth and ends with the anus.
* All chordates in embryonic development have gill slits – paired transverse openings that penetrate the anterior section of the digestive tube.
* The circulatory system of chordates is closed. The heart is located on the abdominal side of the body under the alimentary canal.
* The Chordate type includes three subtypes: Cranials, Membranes, and Vertebrates (Cranials).

The subtype Cranials is represented by a small group of marine chordates and includes one class – Lancelet, which includes about 30 species of small animals. The name “skullless” means that representatives of this subtype do not have a skull and brain. The structure of the skullless is rather primitive:
* the chord serves them as an internal skeleton throughout their life.
* The functions of the central nervous system are performed by the neural tube.

The tunicate subtype (Larval chordates, or Tunicata) includes about 1500 species of marine chordates. In tunicates, the main characters of the Chordate type are clearly expressed only at the larval age.

At the initial stage of life, tunicates are free-swimming larvae that move with the help of their tail. The tunicate larvae have a complex structure, similar to the structure of the lancelet. As the larva grows into an adult, its structure becomes simpler. In adulthood, most of them lack a notochord and neural tube. The body of an adult tunicate is enclosed in a gelatinous membrane – a tunic – and resembles a bag with two funnels through which water enters and exits. With water, the animal receives oxygen for breathing and food – organic particles. The tunicates are hermaphrodites. Many species reproduce by budding, forming colonies.

The vertebrate subtype unites most of the chordate species. This subtype includes the Classes: Cartilaginous Fish and Bony Fish, Amphibians, Reptiles, Birds and Mammals. In terms of structure and lifestyle, vertebrates are at a higher level of organization than cranials and tunicates.

In contrast to the sedentary and passively feeding cranial ancestors, the ancestors of the vertebrates switched to an active search for food and the associated movement. This led to the development of a powerful internal skeleton and musculature, improvement of the processes of respiration, nutrition, blood circulation, excretion, sensory organs and the central nervous system.

The axial skeleton of most vertebrates is the spinal column (hence the name of the subtype), which performs a supporting function and is a kind of sheath for the spinal cord, thereby protecting it.

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