The functions of supporting the body and protecting internal organs in animals can be performed in two ways. In the first case, an external skeleton is formed - the carapace. It consists of chitin, for greater strength, impregnated with calcium, phosphorus or silicon salts. Crustaceans, spiders, ticks, insects - these are animals that have a shell and are called invertebrates. Another group of fauna representatives in the process of embryogenesis forms the internal skeleton, the role of which is played by the chord.
Biology explains its appearance as a result of the process of organogenesis - the formation of a complex of axial organs. It includes the neural tube, intestines and chord. The emergence of an elastic connective tissue strand at the stage of the neurula of the embryo is a hallmark of both humans and animals belonging to the Type of Running. Consider this process in more detail, and also find out the role of the axial skeleton in the body of vertebrates.
What is a chord in biology?
The definition of this part of the embryo is as follows: it is an unsegmented dorsal string consisting of elastic connective tissue. It is an essential component in the embryogenesis of all animals of the Chordata type . The tab of the chord occurs under the neural tube from the primordium of the chordomesoderm, which is a derivative of the ectoderm of the primary intestine. During organogenesis, it is modified into the intestinal tube, located under the chord. The processes described above are quite complex and are studied in the course of general biology of high school. How to explain to elementary school students studying zoology what a chord is? Biology (Grade 7) considers the structure of the formed dorsal axis using the example of a lancelet. Let us dwell on this issue in more detail.
Subtype Characteristic
This systematic group contains one Class - Lancelet. Animals live at the bottom of the Black Sea, spending almost their entire lives buried in the sand and putting to the surface the front end of the body, surrounded by tentacles. A dense tube - a chord - is visible through a translucent body. Biology considers it as an integral part of the musculoskeletal system of the animal. The dorsal string in the lancelet, as in other primitive chordates - lampreys and myxin, belonging to the maxillary vertebrates, is present throughout ontogenesis. It acts as an axial skeleton. In addition to organisms belonging to the cranium and cyclostomes, the chord is preserved in some fish. Consider the features of their structure.
Cartilaginous fish
Sea cat, sea fox, sawfish, katran are the names of deep-sea inhabitants: stingrays and sharks. The structure of their support and movement system includes the cartilaginous skeleton and muscles. Between the vertebral bodies and in the spinal canal itself there are residual fragments of the germinal spinal cord. This feature is a distinctive feature of the internal structure of cartilaginous fish, in which even in adulthood the chord is partially preserved. Biology explains this as follows: during embryogenesis, the dorsal string forms constricts at an equal distance from each other, 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 Sturgeon Series
Together with sharks and stingrays, with which they have many common features, representatives of this group belong to ancient fish. Sturgeon, beluga, stellate sturgeon are characterized by such structural features as the internal cartilaginous skeleton and chord. Biology claims that the ancestors of sturgeon appeared in the Devonian period of the Paleozoic era. From this moment, their anatomy has not changed. Adult forms have a dorsal cord, covered with a shell of dense connective tissue and lasting a lifetime. Specially dried chord of stellate stellate sturgeon, sterlet and other sturgeon fish in Russia has long been used as food as a filling for pies and kulebyak, calling it vyazig.