1) The axial skeleton is represented by a chord – a cartilaginous elastic rod located along the dorsal side of the animal’s body. In most adult higher chordates, it is present only at the embryonic stage of development, and in adults it is replaced by the spine.
2) The central nervous system has the form of a tube, the cavity of which is filled with cerebrospinal fluid. In vertebrates, the front end of this tube is transformed into the brain, in the trunk and tail sections it is represented by the spinal cord.
3) The anterior section of the digestive tube – the pharynx – is penetrated by gill slits, through which it communicates with the external environment. In terrestrial animals, gaps exist only in the early period of embryonic development, while in aquatic chordates they persist throughout life.
4) The circulatory system is closed, the heart is located on the ventral side, under the digestive tube.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.