The axial skeleton is a chord. Biology about its functions and meaning

The functions of body support and protection of internal organsAnimals can be performed in two ways. In the first case, an outer skeleton is formed - a shell. It consists of chitin, for greater strength impregnated with salts of calcium, phosphorus or silicon. Crustaceans, spiders, mites, insects are animals that have a shell and are called invertebrates. Another group of fauna in the process of embryogenesis forms an internal skeleton, whose role is played by the chorda.

Biology explains its appearance as a resultprocess of organogenesis - the formation of a complex of axial organs. To it carry a nervous tube, an intestine and a chord. The emergence of an elastic connective tissue strand at the neurula stage of the embryo is a distinctive feature of both man and animals belonging to the Chodovy type. Let's consider this process in more detail, as well as clarify the role of the axial skeleton in the vertebrate body.

What is a chord in biology?

The definition of this part of the fetus soundsas follows: it is an unsegmented spinal cord, consisting of an elastic connective tissue. It is an obligatory component in embryogenesis of all Chordovye type animals. The chord laying occurs under the neural tube from the rudiment of the chordomesoderm, which is the derivative of the ectoderm of the primary intestine. During organogenesis, it is modified into an intestinal tube located under the chord. The processes described above are rather complicated and are studied in the course of the general biology of high school. How to explain to younger school students studying zoology what is a chord? Biology (Grade 7) considers the structure of the formed dorsal axis by the example of a lancelet. Let us dwell on this issue in more detail.

Characteristics of the Subtype Cranial

This systematic group contains one Class -Lancets. Animals live on the bottom of the Black Sea, spending almost their entire life, buried in the sand and exposing the surface of the front end of the body, surrounded by tentacles. Through a translucent body, a dense tube, a chord, appears through. Biology considers it as an integral part of the animal's locomotor system. The spinal cord in the lancelet, as in other primitive chordates - lamprey and myxin, belonging to the incomplete vertebrates, is present throughout the ontogenesis. It acts as an axial skeleton. In addition to organisms belonging to the non-cranial and cyclostomes, the chord remains in some fish. Let us consider the features of their structure.

Cartilaginous fish

Sea cat, sea fox, fish-saw, katran -these are the names of deep-sea inhabitants: rays and sharks. The structure of their system of support and movement includes the cartilaginous skeleton and muscles. Between the bodies of the vertebrae and in the vertebral canal are residual fragments of the embryonic spinal cord. This feature is a distinctive feature of the internal structure of cartilaginous fishes, in which the chord is partially retained even in the adult state. Biology explains this in the following way: during embryogenesis, the spinal string forms at equal distances from each other the constrictions in which the process of deposition of calcareous structures is initiated. They gradually divide the chord into the bodies of the cartilaginous vertebrae, and its connective tissue parts remain inside the intervertebral discs.

Features of the structure of fish of the Series Sturgeon

Together with sharks and rays, with which they havemany common features, the representatives of this group belong to the ancient fish. Sturgeon, beluga, stellate sturgeon are characterized by such features of the structure as the inner cartilaginous skeleton and chorda. Biology argues that the ancestors of sturgeon fish appeared in the Devonian period of the Paleozoic era. From that moment their anatomy practically did not change. Adult forms have a dorsal cord, covered with a sheath of dense connective tissue and lasting a lifetime. The specially dried chord of stellate sturgeon, sterlet and other sturgeons in Russia has long been used as food for pies and kulebyak, calling it a tie.