Chordates (Chordata) is a type of animal that includes vertebrates, larva-chordata, and disembodied ones. Vertebrates such as lampreys, mammals, birds, amphibians, reptiles and fish are the most famous, and are a subtype of animals to which people belong.
Chordates have bilateral symmetry, which indicates the existence of a line of symmetry dividing their body into two halves, which are almost a mirror image of each other.
Bilateral symmetry is found not only in chordates, but also in arthropods and echinoderms (although in the case of echinoderms they are bilaterally symmetrical only during the larval stage of their life cycle, and pentaradial symmetry appears in adults).
All chordates have a chord that is present during some or all of their life cycle. The chord (or dorsal string) is a semi-flexible rod to which the large muscles of the animal are attached. It also plays an important role in signaling and development coordination.
The chorda consists of flat cells enclosed in a fibrous membrane. In vertebrates, the dorsal string is present only during the embryonic stage of development, and then vertebrae develop around it and the spine forms. In the tunicates, the chord remains throughout the entire life cycle of the animal.
Chordates have a single hollow dorsal nerve cord that runs along the back of the animal and in most species forms a brain in the front of the body. They also have gill slits serving as openings between the pharyngeal cavity and the external environment, as well as for filtering water.
Another characteristic of chordates is a structure called endostyle - a flute-like organ that is located near the wall of the pharynx and secretes mucus, as well as catches small particles of food that enter the pharyngeal cavity. Endostyle is present in tunicans and lancelet. In vertebrates, the endostyle is replaced by the thyroid gland - the endocrine gland located in the neck.
Chordates are classified in the following taxonomic hierarchy:
- Domain: Eukaryotes (Eukaryota),
- Kingdom: Animals (Animalia),
- A type: Chordates (Chordata).
Chordates, in turn, are divided into the following subtypes:
- Disembodied (Acrania): now there are about 32 species of skullless. Representatives of this subtype have chords that persist throughout their entire life cycle. Lancelet - this is the only family of inconspicuous, whose members are marine animals with long narrow bodies. The earliest known fossil lancelet, Yunnanozoonlived about 530 million years ago during the Cambrian period. Fossil fossils of these organisms have also been found in the famous Burgess Shales in British Columbia.
- Larva-chordata, or tunicates (Urochordata): At present, there are about 1600 species of shells. This subtype is divided into classes such as ascidia and appendicularia. Shells are marine filtrators, most of which do not move around, but attach to rocks or other hard surfaces on the seabed.
- Vertebrates (Vertebrata) - Today, there are about 57,000 species of vertebrate animals. Among the members of this group are lampreys, mammals, birds, amphibians, reptiles and fish. During development, the vertebral chord is replaced by the spine.
Aromorphoses of chordates
- The emergence of a chord
Chorda is a flexible elastic cord of mesodermal origin, which in representatives of the chordate type can be connective tissue, cartilage or bone. In vertebrates, the chord is replaced by cartilage and bone tissue; it bears the more familiar name for us - the spine.
Tubular type of nervous system
The central nervous system is represented by a narrow tube with a channel inside (neurocele). The neural tube is located on the dorsal side of the body, above the chord. In vertebrates, the neural tube develops in the spinal cord and brain.
Gill slits in the throat
Provide active gas exchange for aquatic animals, which makes life processes more efficient.
Active movement in most animals leads to the appearance of paired limbs: in aquatic - pectoral and abdominal fins, in terrestrial - fore and hind limbs. Paired limbs are absent only in cyclostomes (lampreys and myxinum).
The general structure of chordates is traditionally studied using the example of a lancelet, which is excellent for this.
Lancelet - primitive chordate, belongs to the subtype disembodied. The main signs of chordata persist throughout life. The name is given to this animal due to the fact that the rear end of its body resembles a surgical instrument - a lancet. The length of the lancelet is on average 5-8 cm.
The head of the lancelet is not isolated, the body is flattened laterally. There is bilateral (bilateral) body symmetry. The narrowed dorsal fin gradually turns into the caudal fin, which has a lanceolate shape.
It lives in the seas, prefers a bottom lifestyle. The integument of the body consists of two layers: the upper one is the epidermis (from Greek epi - above and derma - skin) and the deep is the dermis (Greek derma - skin), or cutis. The supporting function is performed by the skeleton - chord. The muscular system is represented by segments - myomers. The lancelet has a secondary and perigastric body cavity.
It digs into the sand, feeds passively, using only those organisms that enter the mouth for food. The mouth funnel is located on the front end of the body and is surrounded by tentacles, with the help of which the lancelet draws water with organic particles and plankton, which it feeds on.
On the lower surface of the gill cavity there is a special formation - endostyle. Endostyle is a depression lined with ciliated epithelium and glandular cells that secrete mucus. Food particles are adhered to this mucus during water filtration, which then move into the intestines.
At the same time, water also serves for breathing: entering the pharynx, it washes about 100-120 gill slots located in it. Here, oxygen from the blood enters the blood vessels located on the gill partitions, and carbon dioxide is removed back into the water.
Circulatory system. The function of the vessel-heart is performed by the abdominal vessel, the pulsation of which creates a blood flow. Excretory organs - modified metanephridia, reminiscent of the structure of metanephridia annelids.
The nervous system is tubular, in front of a neurocele (the cavity of the neural tube) expands. The front end of the tube carries out important reflex regulation of the animal. The sense organs are primitive. There are organs of vision - Hessian eyes, photosensitive organs located along the neural tube on the sides of the neurocel. Tactile tentacles are localized around the mouth opening.
Lancelet animals are dioecious animals. Sexual dimorphism is absent, external fertilization occurs in water. From the zygote, a larva develops that swims freely for 3 months. The gills of the larva open outward, the skin fold (dorsal fin) is absent. After some time, the larva sinks to the bottom and buries itself in the sand, after which it turns into an adult animal.
Having studied the structure of the lancelet, it becomes apparent close connection with annelids. Therefore, the lancelet can with confidence be called a form occupying an intermediate evolutionary position between ancient annelids and modern vertebrates.
Shells (larval chordates)
Shells are a subtype of chordates. Sea creatures. The body is bag-shaped, surrounded by a mantle.
By type of nutrition, they are filtrators that trap organic plant residues and plankton suspended in water. The presence of siphons, through which water enters the body and leaves it, is characteristic.
The circulatory system of the open type, an interesting feature is the irregular direction in which the heart pumps blood: this direction is constantly changing.
You and I have already studied two subtypes of the chordates - corpulent and shell. I want to warn a common mistake: "inconspicuous" is a subtype of chordates, and invertebrates are a completely different concept that unites everyone except chordates - enteroacids, worms, mollusks and arthropods.
It's time to move on to the study of the next subtype of chordates - cranial, or - vertebral. The composition of this subtype includes the class of cyclostomes. Famous representatives of cyclostomes: myxines and lampreys.
Cyclostomes differ from fish in many ways:
- Lack of scales
- The absence of paired limbs
- The absence of jaws - that's why they are referred to as jawless
- Chorda persists throughout life
- A round (or oval) mouth in which teeth and a suction device are located
An important common feature with the rest of the vertebral (cranial) is the presence of a cyclostome skull, which has a rather peculiar structure. It surrounds the brain only from the lower side and from the sides (the side parts of the skull are not developed in myxines).
Myxins most often attack sick and weakened fish entangled in the net. They gnaw through the body wall and invade the internal environment, eating organs, tissues, and then muscles.
Lampreys parasitize on fish. They bite with their horn teeth into the victim's body, gradually secreting digestive juices, break down fish tissues and feed on them.
Anamnias and Amniotes
The vertebrate subtype is divided into two large groups: anamnias and amniotes.
Anamnias (from the Greek. Άν (an) - denial and αμνίον (amnion) - the shell of the embryo) - lower primary water-bearing vertebrate animals. This group of animals does not have an embryonic shell - an amnion and a special germinal organ - allantois.
Anamnias are attached to water, in which they spend most of their life or the initial stage of development. Anamnias include jawless, various groups of fish and amphibians.
Amniotes (from the Greek. Αμνιον (amnion) - the shell of the embryo) are higher vertebrates, which in the early stages are characterized by the formation of germinal membranes and a special embryonic organ - allantois.
Amniotes break away from the usual reservoir, gain independence, unlike anamnias. Surprisingly, now this "reservoir" is formed inside the mother's body: the embryo is in the fetal bladder (amnion), filled with amniotic fluid (amniotic fluid), a kind of analogue of the reservoir.
Amniotes (higher vertebrates) include three classes: reptiles, birds, and mammals. Undoubtedly, man also belongs to the group of amniotes.
© Bellevich Yuri Sergeevich
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Origin of the Chordates
To this day, it has not been possible to form a single point of view on the origin of the Chordates. This question was asked by scientists back in the 19th century. It was assumed that their ancestors may be arthropods, but soon this hypothesis was refuted, due to significant differences between them in the embryonic period.
The following hypothesis: chordate origin from intestinal (due to the presence of gill slits, stomochorda). Over time, it became clear that they have more differences than similarities. Modern biologists still can not solve this issue.
Allocate a group of secondary ones, which have a number of similar characteristics with the Chordates:
- Digestive system
- nervous system,
- circulatory system.
It is believed that the predecessors of the Chordates were free-swimming animals, which were divided into sedentary individuals Lancelet and Shell, and the other branch gave Vertebrates, which began to develop actively. The sides of the vertebral body made a revolution: the transition from the abdominal to the dorsal side. It is believed that this event became the key in their further evolution.
Type Chordata is divided into three subtypes.
Subtype Shelling (Larval chordata) totals about 1000 species having a chord only in the initial stages of development.
Ascidia Class leads a sedentary lifestyle, they cling to stones at the bottom of the sea, or to the base of the vessels, forming continuous layers of small individuals. Ascidia has openings: oral serves as a filter for trapped water, filtered food enters the intestines and then digestion products are removed through the anus.
Salps and Appendiculars able to move freely in water open spaces.
Hermaphrodite Shells (both testes and ovaries are present), larvae emerge from the eggs, which are little similar to an adult, have signs of an internal skeleton, which indicates a relationship with vertebrates.
Subtype Cranial - marine inhabitants of the near-bottom territory, retained the characteristics of typical Chordates (neural tube, gills, chord). The brain is not developed, they lead a primitive way of life, reaching a maximum value of 7 cm, representatives of dioecious individuals, spawn, live in sand at the bottom of the sea depths and oceans, leaving only a part of the body with an oral cavity for nutrition. Single out Lancelet Class.
Subtype Vertebrates - highly organized individuals, with the presence of developed hemispheres of the brain and cranium, spinal column, which serves as a protective membrane for the spinal cord. In the process of evolution, they got a complex morphological structure, improved sense organs, switched from a sedentary lifestyle, and began to easily move through the expanses of the earth, air and water. Vertebrates include amphibians, fish, birds, reptiles, mammals and cyclostomes.
Chord Classification - Scheme
Features of the structure of chordates
What structural features were formed in chordates during evolution?
Skin integument In Scalpless, they consist of a single-layer epithelium, in vertebrates it is fluff, feathers, scales, and also derivatives of the epidermis - claws, horns, hooves. Glands that secrete sweat, sebum, and odorous substances appear. For example, in cows, goats, sheep, mammary glands are formed to feed offspring.
Phylogenesis circulatory system chordates comes from the mesoderm, a blood circulation circle (closed) is formed, the heart in the abdominal region, the dorsal and abdominal arteries, which are connected by anastomoses. The walls of blood vessels were sent by endothelial cells, which helps maintain homeostasis, the amount of blood glucose and proteins (the beginnings of immunity) increased.
Nervous system in the embryonic period, it looks like a layer of thickened ectoderm, its ends close and form the neural tube. Phylogenesis of the chordal brain occurs from the anterior end of the neural tube, the spinal cord from the posterior. The formation of the brain (kefalization) - has become a prerequisite for the evolutionary development of the Chordates. They began to distinguish external stimuli, perceive the world around them with the help of their senses. Higher chordates have a brain with five sections, which are formed from three brain bubbles.
The development of the respiratory system and digestive system is closely related, there is a common bookmark under the guise of a straight tube, which in the higher chordates differentiates into two independent systems.
The lower chordates digestive tract it has no departments, in the lancelet, the mouth opening with many tentacles communicates with the pharynx, where the filter apparatus is located, the food then goes through the posterior groove into the intestine and is excreted by the anus, the glands are not developed. In vertebrates, the digestive system already consists of separate structures: pharynx, esophagus, gastrointestinal tract, secreting glands appear: the liver and pancreas.
Breath carried out by the gills and lungs (terrestrial vertebrates). After exiting the water, amphibians formed lungs, but their functionality is insufficient, so additional gas exchange is carried out through the skin. Thus, for the Class of Amphibians, skin-pulmonary respiration is characteristic.
Birds are characterized by the presence of double breathing. The respiratory apparatus consists of the larynx, trachea, bronchi, lungs with air sacs. During the flight, birds use air sacs for gas exchange, the inhaled air passes through the lungs twice - both on inhalation and on exhalation - this is the double breathing mechanism.
Breeding system in chordates, it is represented by testes and vas deferens in males, ovaries and oviducts in females. Chordates, with the exception of larval chordates, dioecious.