Chordate

Chordates are a type of animals characterized by bilateral symmetry and the presence, at least at certain stages of development of the following symptoms:

• The chord , which is an elastic rod of endodermal origin, is formed by protruding the dorsal side of the intestinal tube. In vertebrates, the chord during embryonic development is completely or partially replaced by cartilage and bone tissue forming the spine .
• Dorsal neural tube . In vertebrates, it develops into the spinal cord and brain .
• Gill slits are paired holes in the pharynx . The lower chordates are involved in filtering water for nutrition. In terrestrial vertebrates, gill slits are laid in early embryogenesis in the form of gill sacs.
• Muscle tail is the postanal part of the body located caudal to the anus displaced to the abdominal side of the anus (the chord and neural tube enter it, but the intestine does not enter).
• Endostyle - groove on the ventral side of the pharynx. In the lower chordate filter, mucus is produced in it, helping to collect food particles and deliver them to the esophagus. Also, iodine accumulates in it and, possibly, it is the precursor of the thyroid gland of vertebrates. As such, endostyle in vertebrates is present only in the assortment .

Attempts to develop evolutionary relations of chordates led to the birth of several hypotheses. At present, a consensus has been reached that chordates are descendants of one common ancestor, who himself is chordate, and the closest relatives of vertebrates ( Latin Vertebrata ) are cephalic chordates ( Latin Cephalochordata ). All discovered fossils of the chordate fossils were found in early Cambrian and include two species of vertebrates classified as fish . Since the chordate fossils are poorly preserved, only the method of molecular phylogenetics suggests a reasonable prospect of studying their occurrence. However, the use of molecular phylogenetics to study evolutionary processes is controversial.

Bilateral animals are divided into two large taxa - primary and secondary . Chordates belong to the secondary. It is very likely that the fossil kimberella , which lived 555 million years ago, belonged to the primary ones ^[1] . Ernietta, who lived 549-543 Ma ago in the Ediacaria, was already clearly a secondary animal ^[2] . Thus, the primary and secondary were to separate before the time of the existence of these animals, that is, before the beginning of the Cambrian period ^[3] .

The first known fossils of two close to chordate groups - echinoderms and hemichordates - are found from the Early and Middle Cambrian, respectively ^{[4] [5]} . In addition, the fossil of the Chengjian biota Yunnanozoon is known , the exact membership of which is not defined as semi-chordate or chordate ^{[6] [7]} . Another fossil belonging to the same biota, Haikouella lanceolata is clearly a chordate and possibly vertebral. He showed signs of the heart, arteries, neural tube and brain, tail, gill lobes, possibly the eyes, but at the same time there are tentacles around the mouth opening ^[7] . Haikouichthys and Myllokunmingia , also from the Shenzheng fauna and pikaya from the Burgess shales, are primitive vertebrates ^{[8] [9]} . On the other hand, the fossils of other chordates are very rare, since they do not have solid parts of the body.

Studies of the kinship relations of chordates began in the 90s of the 19th century. They were based on anatomical, embryological and paleontological data and led to different phylogenetic trees . For a while, the closest relatives of the chordates were considered semi-chordates, but now this hypothesis has been rejected ^[10] . The combination of classical methods data with rRNA gene sequence analysis data led to the hypothesis that shells are living representatives of a group that is basal for other second-rot. Regarding the relationships within the chordates, some scientists believe that the closest relatives of vertebrates are the cephalic chordates, but there are reasons to consider shells as such ^[11] .

The origin of chordates, based on the molecular clock method, was estimated at 896 Ma ^[11] .

Below is a cladogram based on a comparison of mitochondrial DNA sequences ^[12] :

Usually there are three subtypes of chordates (sometimes four). Vertebrates are the highest subtype, to which about 95% of all species of chordates belong. Of the lower chordates, uncranial and tunicans are distinguished.

Below are three commonly recognized subtypes of chordates together with their classes and subclasses ^[13] .

Type Chordata

• Subtype Cranial (Acraniata) or Cephalochordata

• Lancelet Class (Leptocardii)

• Subtype Shellworms (Tunicata) or Larval-chordate (Urochordata)

• Class Appendicularia (Appendicularia)
• Ascidia Class (Ascidiacea)
• Salpa Class (Thaliacea): Salpids , Firestones and Barrels

• Subtype Vertebrata

• † Conodonta Class

• Overclass Jawless (Agnatha)

• † Class Anaspida , or unaired , or brushless (Anaspida)
• † Class Unparticular or Cephalaspidomorphs (Cephalaspidomorphi)
• † Class Pilar or Pterospidomorphs (Pteraspidomorpha)
• † Thelodonti Class (Thelodonti)
• Myxini Class
• Lamprey Class (Petromyzontida)

• Infratype Maxillary (Gnathostomata)

• † Class Placoderma or Lamellar -skinned fish (Placodermi)
• † Class Acanthoda or Spiny Toothed (Acanthodii)
• Class Cartilaginous Fish (Chondrichthyes)

• Subclass Plate-Gill (Elasmobranchii): sharks and stingrays
• Subclass All- headed (Holocephali)

• Bone Fish Group (Osteichthyes)

• Class Banded Fishes (Actinopterygii)
• Class Lobeyfish (Sarcopterygii)

• Superclass Four- Legged (Tetrapoda)

• Class Amphibians or Amphibians (Amphibia)
• Class Reptiles or Reptiles (Reptilia)

• † Subclass Anapsida
• Subclass Diapsida

• Bird Class (Aves)
• † Synapsida Class
• Class Mammals (Mammalia)

Reptiles, birds, synapsids and mammals are amniotes , the remaining classes of vertebrates are anamnias .

Occasionally, detonator, salp, and barrel units are promoted to classes in the Tunicata subtype. There are also quite a few alternative vertebrate classifications. Their presence is associated, in particular, with the fact that many traditionally distinguished groups of vertebrates are paraphyletic . For example, lapidoptera are paraphyletic (for this group to become monophyletic, terrestrial vertebrates must be included in it) and reptiles (a class of birds should be included in the positions of the cladistic systematics ).

Very often an unsystematic invertebrate group is used, which includes two subtypes of chordates ( cephalic chordates and tunicates ) and all other animal types. Its use emphasizes a dominant position. , which is occupied by vertebrates in the animal kingdom.

The figure shows the chronogram of modern chord classes.

A comparison of the hierarchy of taxonomic groups with the branches of the phylogenetic tree is encountered in the case of chordates with certain difficulties. For example, the class of lapellate fishes contains only 8 species of the actual fish, except for them all terrestrial chordates (four-legged) are included in it, since they are descended from lapellate fishes. Meanwhile, tetrapods are traditionally divided into classes of amphibians, reptiles, birds and mammals, that is, according to the phylogenetic tree, several classes are subordinate to another class.

No lesser problems arise in the superclass of tetrapods. It is impossible to distinguish a separate branch for reptiles, since they are not a monophyletic group (birds descended from reptiles are traditionally allocated in a separate class). As a result, a class of sauropsids was identified, which includes reptiles and birds, and the selection of birds from sauropsids is shown by a thin line. It should be borne in mind that the number of bird species indicated on the tree is among the species of sauropids.

Golovohordovye

Cephalochordata ( lat. Cephalochordata ) or disembodied ( lat. Acrania ) ^[14] - small marine fish-like animals with all the characteristics characteristic of the chordate. Uncranial - a subtype of the lower chordates, unlike other chordates ( tunicates and vertebrates ), which retains the main characteristics of the type ( chord , neural tube and gill slits) throughout life. The brain is absent, the sense organs are primitive ^[15] . They lead a near-bottom lifestyle, by the nature of their nutrition - digging filters. Perhaps they are the ancestors of vertebrates, or are the last living members of the group from which the vertebrates evolved ^{[16] [17]} . Altogether, about 30 species that make up one class — lancelets, belong to the cranky.

Shells

Shells ( lat.Tunicata, Urochordata ) - a subtype of chordate animals . They include 5 classes - ascidium , appendicularia , salp , fire extinguishers, and barrels . According to another classification, the last 3 classes are considered to be units of the Thaliacea class ^[18] . More than 1,000 species are known. Most of them have a chord only at the larval stage. They are spread all over the world and inhabit the seabed.

Three large classes of shells:

• Ascidia - the lower soft-bodied chordate-filter, leading a sedentary lifestyle in adulthood ^[19] ;
• appendiculars retain larval traits, such as tail, throughout life. For this reason, for a long time they were considered as larvae of ascidium and salp ^[20] . Due to the presence of long tails, the larvae of the shells are called lat. urochordata ^[19] ;
• the third group of shells - free-floating salps , feed on plankton . Two generations are known in their life cycle - a single hermaphroditic and budding colonial asexual. The larvae of these animals have all the main signs of chordates, including the chord and tail. They are also equipped with a rudimentary brain and sensors for light and position (roll) ^[19] .

Vertebrates

Vertebrates ( lat. Vertebrata ) - the highest subtype of chordates. A group of animals dominating (along with insects ) on the earth and in the air. They differ from other chordates in the presence of a separate skull and the development of the brain and sensory organs . Chord in most representatives of the higher chordates is replaced by the spine ^[21] , which protects the spinal cord and consists, as a rule, of cartilage and bone tissue . Endostyle , as such, is present only in lamprey larvae ^[22] . Compared to the lower chordates - cranium and shells - they are characterized by a significantly higher level of organization, which is clearly expressed both in their structure and in physiological functions. Among vertebrates, there are no species leading a sedentary (attached) lifestyle. They move widely, actively searching for and capturing food, finding individuals of the other sex to breed, fleeing the pursuit of enemies.

The position of the lampreys is ambiguous. They have an underdeveloped skull and rudimentary vertebrae - therefore, they can be considered as vertebrate and true fish ^[23] . Nevertheless, molecular phylogenetics , which used biochemical reactions to classify organisms, as a result, this group of vertebrates was announced to be the sister family of myxinidae ( lat. Myxinidae ) of the cyclostome class ^[24] . Myxins with a gill skeleton consisting of a small number of cartilaginous plates, rudimentary vertebrae, are not considered true vertebrates ^[25] - they are considered the group from which vertebrates evolved ^[26] .

Semichordate

Semi-chordate ( Latin Hemichordata ) - a type of marine benthic invertebrate from the group of secondary cirrus. Soft-bodied worm-shaped animals. About 100 species are known in the world, 4 species in Russia ^[27] . Initially, the chordates were considered to be a subtype or close relatives of the chordates, with which they are united by the presence of a chord-like organ ( stomochorda ), which is an outgrowth of the intestine that supports the proboscis, gill slits and spinal nerve trunk. However, unlike the chordates, the body of the semi-chordates is clearly divided into 3 departments: the proboscis (in the intestinal tract) or the head shield (in the cirrus ), the collar and the trunk. Other differences from chordates: the structure of the heart, nervous system and other features of the internal organization, for example, in addition to the dorsal, in the chordate there is also an abdominal (ventral) neural cord. Two classes are distinguished ^[27] : intestinal breathing ( lat. Enteropneusta ) - 79 species ^[28] and pinnatobranchic ( lat. Pterobranchia ). The extinct class of graptolites is a fossil ( fossils in structure resemble pinnate gill) ^[29] .

The similarity of semi-chordate larvae ( tornaria ) with echinoderm bipinnaria larvae made it possible to consider hemichordate as an intermediate link between echinoderms and chordata.

Echinoderms

Echinoderm ( Latin Echinodermata ) is a type of exclusively marine bottom animals , mostly free-living, less often sessile, found at any depths of the oceans . There are about 7000 modern species (400 in Russia) ^[30] . Along with the chordates, echinoderms belong to the branch of the Second-Called animals ( Deuterostomia ). Modern representatives of the type are starfish , sea ​​urchins , ophiurs (serpent tail), holothurians (sea cucumbers) and sea ​​lilies . The composition of this type also includes approximately 13,000 extinct species that have flourished in the seas, starting from early Cambrian .

Echinoderms differ from chordates in three ways:

• instead of bilateral ( bilateral ) they have radial symmetry ;
• their bodies are ring-shaped (in the form of a wheel with pipe legs);
• their bodies are supported by a calcite skeleton , a material not found in chordates.

Despite the fact that the bodies of echinoderms are well protected from the environment by a calcified shell (skeleton), they are covered with thin skin on the outside. The legs of the echinoderms are surrounded by muscles and perform the function of respiration and nutrition - they act as pumps, providing water circulation near the blood vessels, exchanging gas and filtering food particles from the water. The modified feather legs also have a defensive function. Most echinoderms, such as sea ​​lilies , live settled (attached to the rocks), and few of them can move very slowly. Other echinoderms are mobile and able to take on various body shapes, such as starfish , sea ​​urchins, and sea ​​cucumbers ^[31] .

• Invertebrates
• Semi-chord

• N. N. Gurtovoi, B. S. Matveev, F. Ya. Dzerzhinsky. Practical zootomy of vertebrates (lower chordates, jawless, fish) . - M .: Higher school, 1976 .-- S. 351.
• Biological Encyclopedic Dictionary / Ch. ed. M.S. Gilyarov. M., "Owls. Encyclopedia ”, 1989. 864 pp., ill. ⟨Electronic version: Biological Encyclopedic Dictionary. - M .: DirectMedia Publishing, 2006 .-- 9000 p. (Series: Encyclopedia Classics)⟩ .