Subphylum larvalochordates or tunicates. Phylum Chordata

Type Chordata

Lower chordates. Subtype Skullless

TYPE CHORDATES. LOWER CHORDATES

general characteristics phylum Chordata

The phylum Chordata unites animals that are diverse in appearance and lifestyle. Chordates are distributed throughout the world and have mastered a variety of habitats. However, all representatives of the type have the following common features of the organization:

1. Chordata are bilaterally symmetrical, deuterostome, multicellular animals.

2. Chordates have a notochord throughout their entire life or at one of the phases of development. Chord- This is an elastic rod located on the dorsal side of the body and performs a supporting function.

3. Located above the chord nervous system in the form of a hollow tube. In higher chordates, the neural tube is differentiated into the spinal cord and brain.

4. Located under the chord digestive tube. The digestive tube begins mouth and ends anus, or the digestive system opens into the cloaca. Throat pierced gill slits, which in proto-aquatic animals persist throughout their lives, but in terrestrial animals are formed only in the early stages of embryonic development.

5. Beneath the digestive system lies heart. Circulatory system in chordates closed.

6. Chordates have secondary body cavity.

7. These are chordates segmented animals. Location of organs metameric, i.e. major organ systems are located in each segment. In higher chordates, metamerism is manifested in the structure of the spinal column and in the muscles of the abdominal wall of the body.

8. The excretory organs of chordates are diverse.

9. Chordates are dioecious. Fertilization and development are varied.

10. Chordata evolved through a series of intermediate forms unknown to biology from the very first coelomic animals.

The phylum Chordata is divided into three subtypes:

1. Subtype Skullless. These are 30-35 species of small marine chordates, shaped like fish, but without limbs. The notochord in the Skullless Ones remains throughout life. The nervous system is in the form of a hollow tube. The pharynx has gill slits for breathing. Representatives – Lancelets.

2. Subphylum Larvalchordates, or Tunicates. These are 1,500 species of sedentary, sedentary marine animals that live in tropical and subtropical regions. Their body is in the form of a bag (the body size of one individual in a colony is no more than 1 mm, and single ones can reach 60 cm); there are two siphons on the body - oral and cloacal. Larval chordates are water filterers. The body is covered with a thick shell - a tunic (hence the name of the subtype - Tunicates). As adults, Tunicates lack a notochord and a neural tube. However, the larva, which actively swims and serves for dispersal, has a typical structure for Chordata and is similar to the Lancelet (hence the second name - Larval Chordates). Representative - Ascidia.

3. Subtype Vertebrates, or Cranial. These are the most highly organized chordates. Vertebrates have active feeding: food is sought and pursued.

The notochord is replaced by the vertebral column. The neural tube is differentiated into the spinal cord and brain. The skull is developed, which protects the brain. The skull bears jaws with teeth for capturing and grinding food. Paired limbs and their belts appear. Cranials have much more high level metabolism, complex population organization, diverse behavior and pronounced individuality of individuals.

The subtypes Cranial and Larval Chordates are called lower Chordates, and the subtype Vertebrates are higher Chordates.

Subtype Skullless - Acrania

Lancelet

The subtype Cephalochordates includes the only class Cephalochordates, which includes only about 30-35 species of marine animals living in shallow waters. A typical representative is Lancelet — Branchiostoma lanceolatum(genus Lancelet, class Cephalochordates, subtype Cranial, type Chordata), whose dimensions reach 8 cm. The body of Lancelet is oval in shape, narrowed towards the tail, compressed laterally. Externally, the Lancelet resembles a small fish. Located on the back of the body caudal fin in the shape of a lancet - an ancient surgical instrument (hence the name Lancelet). There are no paired fins. There is a small dorsal. On the sides of the body from the ventral side hang two metapleural folds, which fuse on the ventral side and form peribranchial, or the atrial cavity, communicating with the pharyngeal slits and opening at the posterior end of the body with an opening - atrioporom- out. At the anterior end of the body near the mouth there are perioral tentacles, with which Lancelet captures food. Lancelets live on sandy soils in the sea at a depth of 50-100 cm in temperate and warm waters Oh. They feed on bottom sediments, marine ciliates and rhizomes, eggs and larvae of small sea crustaceans, diatoms, burying themselves in the sand and exposing the front end of their body. They are more active at dusk and avoid bright lighting. Disturbed Lancelets swim quite quickly from place to place.

Veils. The body of the Lancelet is covered skin, consisting of a single layer epidermis and thin layer dermis.

Musculoskeletal system. A chord stretches along the entire body. Chord- This is an elastic rod located on the dorsal side of the body and performs a supporting function. The chord becomes thinner towards the anterior and posterior ends of the body. The notochord protrudes into the anterior part of the body somewhat further than the neural tube, hence the name of the class - Cephalochordates. The notochord is surrounded by connective tissue, which simultaneously forms supporting elements For dorsal fin and divides muscle layers into segments using connective tissue

Type Chordata subtype Lancelet

interlayers. The individual muscle segments are called myomeres, and the partitions between them are myoseptami. The muscles are formed by striated muscles.

Body cavity at Lanceletnik secondary, in other words, these are coelomic animals.

Digestive system. On the front of the body there is mouth opening, surrounded tentacles(up to 20 pairs). The mouth opening leads into a large throat, which functions as a filtering apparatus. Through the cracks in the pharynx, water enters the atrial cavity, and food particles are directed to the bottom of the pharynx, where the endostyle- a groove with ciliated epithelium that drives food particles into the intestine. There is no stomach, but there is hepatic outgrowth, homologous to the liver of vertebrates. Midgut, without making loops, opens anus at the base of the caudal fin. Digestion of food occurs in the intestines and in the hollow hepatic outgrowth, which is directed towards the head end of the body. Interestingly, Lancelet has preserved intracellular digestion; intestinal cells capture food particles and digest them in their digestive vacuoles. This method of digestion is not found in vertebrates.

Respiratory system. Lancelet has more than 100 pairs in its throat gill slits, leading to peribranchial cavity. The walls of the gill slits are penetrated by a dense network of blood vessels in which gas exchange occurs. With the help of the ciliated epithelium of the pharynx, water is pumped through the gill slits into the peribranchial cavity and through the opening (atriopore) it is discharged out. In addition, the skin, which is permeable to gases, also takes part in gas exchange.

Circulatory system. Circulatory system of Lancelet closed. Blood is colorless and does not contain respiratory pigments. The transport of gases occurs as a result of their dissolution in the blood plasma. In the circulatory system one circle blood circulation There is no heart, and the blood moves thanks to the pulsation of the gill arteries, which pump blood through the vessels in the gill slits. Arterial blood enters dorsal aorta, from which carotid arteries blood flows to the anterior part, and through the azygos dorsal aorta to the posterior part of the body. Then by veins the blood returns to venous sinus and by abdominal aorta goes to the gills. All the blood from digestive system enters the hepatic process, then into the venous sinus. The hepatic outgrowth, like the liver, neutralizes toxic substances that enter the blood from the intestines, and, in addition, performs other functions of the liver.

This structure of the circulatory system is not fundamentally different from the circulatory system of vertebrates and can be considered as its prototype.

Excretory system. The excretory organs of Lancelet are called nephridia and resemble the excretory organs of flatworms - protonephridia. Numerous nephridia (about one hundred pairs, one for two gill slits), located in the pharynx, are tubes that open with one opening into the coelom cavity, and the other into the peribranchial cavity. On the walls of the nephridium there are club-shaped cells - solenocytes, each of which has a narrow canal with a ciliated hair. Due to the beating of these

Type Chordata subtype Lancelet

hairs, liquid with metabolic products is removed from the nephridium cavity into the peribranchial cavity, and from there out.

central nervous system educated neural tube with a cavity inside. The lancelet does not have a pronounced brain. In the walls of the neural tube, along its axis, there are light-sensitive organs - Hessian eyes. Each of them consists of two cells - photosensitive And pigment, they are able to perceive the intensity of light. The organ is adjacent to the expanded anterior part of the neural tube sense of smell.

Reproduction and development. Lancelets that live in our Black Sea and Lancelets that live in the waters of the Atlantic off the coast of Europe begin breeding in the spring and spawn eggs until August. Warm water lancelets breed throughout the year. Lancelets dioecious, gonads (gonads, up to 26 pairs) are located in the body cavity in the pharynx. Reproductive products are excreted into the peribranchial cavity through temporarily formed reproductive ducts. Fertilization external in water. Emerges from a zygote larva. The larva is small: 3-5 mm. The larva actively moves with the help of cilia covering the entire body and due to the lateral bends of the body. The larva swims in the water column for about three months, then moves on to life at the bottom. Lancelets live up to 4 years. Sexual maturity is reached by two years.

Meaning in nature and for humans. Anesthenes are an element of biological diversity on Earth. Fish and crustaceans feed on them. The Skullless themselves recycle the dead organic matter, being decomposers in the structure of marine ecosystems. The skullless are essentially a living blueprint for the structure of chordates. However, they are not the direct ancestors of vertebrates. In countries South-East Asia local residents collect lancelets by sifting sand through a special sieve and eat them.

Skullless animals have retained a number of features characteristic of their invertebrate ancestors:

§ excretory system nephridial type;

§ absence of differentiated sections in the digestive system and preservation of intracellular digestion;

§ filtering method of feeding with the formation of a circumbranchial cavity to protect the gill slits from clogging;

§ metamerism (repeated arrangement) of the genital organs and nephridia;

§ absence of a heart in the circulatory system;

§ poor development of the epidermis, it is single-layered, like in invertebrate animals.

Type Chordata subtype Lancelet

Rice. The structure of the lancelet.

A - neural tube, chord and digestive system; B - circulatory system.

1 - chord; 2. - neural tube; 3 - oral cavity; 4 - gill slits in the pharynx; 5 - peribranchial cavity (atrial cavity); 6 - atriopor; 7 - hepatic outgrowth; 8 - intestine; 9 - anus; 10 - subintestinal vein; 11 - capillaries of the portal system of the hepatic outgrowth; 12 - abdominal aorta; 13 - pulsating bulbs of arteries pumping blood through the gill slits; 14 - dorsal aorta.

Rice. Nephridium Lancelet.

1 - opening as a whole (into the secondary body cavity); 2 - solenocytes; 3 - opening into the peribranchial cavity.

Type Chordata subtype Lancelet

Rice. Cross section of Lancelet:

A – in the pharynx area, B – in the midgut area.

1 - neural tube; 2 - muscles; 3 - roots of the dorsal aorta; 4 - ovary; 5 - endostyle; 6 - abdominal aorta; 7 - metapleural folds; 8 - peribranchial (atrial) cavity; 9 — gill slits (due to the oblique position, more than one pair of them is visible on one cross section); 10 - nephridia; 11 - whole; 12 - ventral (motor) spinal nerve; 13 - dorsal (mixed) nerve; 14 - chord; 15 - subintestinal vein; 16 - dorsal aorta; 17 - dorsal fin.

Questions for self-control.

Name the characteristic features of animals of the Chordata type.

Name the classification of the type into three subtypes.

Name the systematic position of Lancelet.

Where does the Lancelet live?

What body structure does Lancelet have?

How does the Lancelet feed and what is the structure of the Lancelet's digestive system?

How does Lancelet excrete waste products?

What is the structure of the nervous system of Lancelet?

What is the structure of the circulatory system of Lancelet?

How does Lancelet reproduce?

What is the significance of Lancelet in nature?

DRAWINGS THAT NEED TO BE COMPLETED IN THE ALBUM

(3 pictures in total)

Lesson topic:

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Chordata

Chordata are the highest type of deuterostome animals. All species of this type are characterized, at least at the stage of embryonic development, by the presence of an unsegmented dorsal skeletal axis (notochord), a dorsal neural tube, and gill slits.

Phylum Chordata. General characteristics. Structural features

The phylum is divided into three subphyla: tunicates, tunicates and vertebrates.

Tunicates (Tunicata) or larval chordates (Urochordata) have a sac- or barrel-shaped body ranging from 0.3 to 50 cm in length; the size of a colony of pyrosomes can exceed 30 m. The body of tunicates is enclosed in a gelatinous tunic secreted by the outer epithelium.

The pharynx is penetrated by gill slits. The hindgut and ducts of the gonads open into the atrial cavity, which connects to the external environment. The nervous system consists of a ganglion located between the mouth and the atriopore, with a nerve trunk extending from it; sense organs are poorly developed.

Tunicates reproduce sexually; Asexual reproduction also occurs. All larval chordates are marine animals that feed on algae, small animals and detritus.

In contrast to the simplified structure of adult forms leading a sedentary lifestyle, the larvae are active, have developed sensory organs and nervous system, muscles and chord (in adult forms it remains only at the appendiculars). It is believed that vertebrates evolved from neotenic (began to reproduce) tunicate larvae. Three classes: tiny primitive appendicularia (Appendicularia), ascidians (Ascidiacea) and pelagic tunicates (Thaliacea), including three subclasses: pyrosomes, salps and barrel tunicates.

About 3000 species, mainly in the upper layers of seas and oceans.

Cranials (Acrania) or cephalochordates (Cephalochordata) are a subphylum of lower chordates.

The head is not separate, the skull is missing (hence the name). The entire body, including some internal organs, is segmented. Respiratory organs - gills. Blood moves due to the pulsating abdominal vessel. The sense organs are represented only by sensory cells.

The subphylum includes two families (about 20 species), representatives of which live in temperate and warm seas; The most famous is the lancelet.

Vertebrates (Vertebrata) or cranial animals (Craniota) are the most highly organized group of animals.

Vertebrates are inferior, for example, to insects in terms of the number of species, but they are very important for the modern biosphere, since they usually complete all food chains.

Thanks to the presence of a complex nervous system and the ability to live in a wide variety of conditions, vertebrates were divided into sharply different systematic groups and managed to achieve not only high perfection in morphology, physiology and biochemistry, but also the ability to higher forms of behavior and mental activity.

The main features of vertebrates: the presence of a notochord in the embryo, which transforms into a spine in an adult animal, internal skeleton, a separate head with a developed brain, protected by a skull, perfect sensory organs, developed circulatory, digestive, respiratory, excretory and reproductive systems.

Vertebrates reproduce exclusively sexually; most of them are dioecious, but some fish are hermaphrodites.

The first vertebrates appeared in the Cambrian. 8 classes, combined into 2 superclasses: jawless (Agnatha) - shield and cyclostomes and gnathostomes (Gnathostomata) - armored, cartilaginous and bony fish, amphibians, reptiles, birds, mammals. Shield fishes, as well as armored fishes, became extinct in the Paleozoic. Currently, about 50,000 species of vertebrates are known.

General characteristics of the chordate type

Basic terms and concepts tested in the examination paper: skullless, gill slits, internal skeleton, amphibians, skin, limbs and limb girdles, circulatory circles, lancelet, mammals, neural tube, vertebrates, reptiles, birds, reflexes, adaptations to lifestyle, fish, bony skeleton, cartilaginous skeleton, notochord .

TO phylum Chordata These include animals that have an internal axial skeleton—notochord or vertebral column.

In the process of evolution, chordates reached the highest level of organization and flourishing, compared to other types. They live in all areas of the globe and occupy all habitats.

Chordata- These are bilaterally symmetrical animals with a secondary body cavity and a secondary mouth.

In chordates there is a general plan of structure and location internal organs:

– the neural tube is located above the axial skeleton;

– there is a chord under it;

– the digestive tract is located under the notochord;

– under the digestive tract is the heart.

In the phylum Chordata, there are two subtypes - Cranial and Vertebrate.

Refers to the skullless lancelet. All other chordates known today, considered in school course biology, belong to the subphylum Vertebrates.

The subtype Vertebrates includes the following classes of animals: Fish, Amphibians, Reptiles, Birds, Mammals.

General characteristics of chordates. Skin vertebrates protect the body from mechanical damage and other environmental influences.

The skin is involved in gas exchange and removal of decay products.

Derivatives of the skin are hair, claws, nails, feathers, hooves, scales, horns, needles, etc. Sebaceous and sweat glands develop in the epidermis.

Skeleton, representatives of the chordate type can be connective tissue, cartilaginous and bone. The skullless have a connective tissue skeleton. In vertebrates – cartilaginous, osteochondral and bone.

Musculature– divided into striated and smooth.

Striated muscles are called skeletal muscles. Smooth muscle forms muscular system jaw apparatus, intestines, stomach and other internal organs. Skeletal muscles are segmented, although less so than in lower vertebrates. Smooth muscle has no segmentation.

Digestive system represented by the oral cavity, pharynx, always associated with the respiratory organs, esophagus, stomach, small and large intestines, digestive glands - liver and pancreas, which develop from the wall of the anterior intestine.

During the evolution of chordates, the length of the digestive tract increases, and it becomes more differentiated into sections.

Respiratory system formed by gills (in fish, amphibian larvae) or lungs (in terrestrial vertebrates).

For many, the skin serves as an additional respiratory organ. The gill apparatus communicates with the pharynx. In fish and some other animals, it is formed by gill arches on which gill filaments are located.

During embryonic development, the lungs are formed from intestinal outgrowths and are of endodermal origin.

The circulatory system is closed. The heart consists of two, three or four chambers. Blood enters the atria and is sent into the bloodstream by the ventricles.

There is one circulation (in fish and amphibian larvae) or two (in all other classes). The heart of fish and amphibian larvae is two-chambered. Adult amphibians and reptiles have a three-chambered heart. However, in reptiles an incomplete interventricular septum appears. Fish, amphibians and reptiles are cold-blooded animals.

Birds and mammals have a four-chambered heart. These are warm-blooded animals.

Blood vessels are divided into arteries, veins and capillaries.

Nervous system ectodermal origin. It is laid in the form of a hollow tube on the dorsal side of the embryo. The central nervous system is formed by the brain and spinal cord. The peripheral nervous system is formed by the cranial and spinal nerves and interconnected ganglia along the spinal column.

Spinal cord is a long cord lying in the spinal canal. The spinal nerves arise from the spinal cord.

Sense organs well developed. Proto-aquatic animals have organs side line, perceiving pressure, direction of movement, speed of water flow.

Excretory organs in all vertebrates they are represented by the kidneys. The structure and mechanism of functioning of the kidneys changes during the process of evolution.

Reproductive organs. Vertebrates are dioecious.

The gonads are paired and develop from the mesoderm. The reproductive ducts are connected to the excretory organs.

Superclass Pisces

Fish appeared in the Silurian - Devonian from jawless ancestors.

There are about 20,000 species. Modern fish are divided into two classes - Cartilaginous And Bone. TO cartilaginous fish include sharks and rays, characterized by a cartilaginous skeleton, the presence of gill slits, and the absence of a swim bladder.

Characteristics of the type Chordata

Bony fish include animals that have bony scales, a bony skeleton, and gill slits covered by an operculum. The appearance of fish is due to the following aromorphoses :

– the appearance of a cartilaginous or bone spine and skull, covering the spinal cord and brain on all sides;

– appearance of jaws;

– the appearance of paired limbs – ventral and pectoral fins.

All fish live in water and have a streamlined body, divided into a head, body and tail.

Well-developed sense organs - vision, smell, hearing, taste, lateral line organs, balance. The skin is two-layered, thin, mucous, covered with scales. The muscles are almost undifferentiated, with the exception of the muscles of the jaws and the muscles attached to the gill covers of bony fish.

Digestive system well differentiated into departments.

There is a liver with gallbladder and pancreas. Many have developed teeth.

Respiratory organs fish are gills, and lungfish have gills and lungs. Additional function breathing is performed by the swim bladder in bony fish. It also performs a hydrostatic function.

Circulatory system closed. One circle of blood circulation. The heart consists of an atrium and a ventricle.

Venous blood from the heart flows through the afferent gill arteries to the gills, where the blood is saturated with oxygen. Arterial blood flows through the efferent gill arteries into the dorsal aorta, which supplies blood to the internal organs.

Fish have a portal system of the liver and kidneys, which ensures the purification of the blood from harmful substances. Fish are cold-blooded animals.

Excretory system represented by ribbon-shaped primary buds. Urine flows through the ureters into the bladder. In males, the ureter is also the vas deferens.

Females have an independent excretory opening.

Gonads represented by paired testes in males and ovaries in females. Many fish exhibit sexual dimorphism. Males are brighter than females, attracting them with their appearance and mating dances.

In the nervous system The development of the diencephalon and midbrain should be noted.

Most fish have a well-developed cerebellum, which is responsible for coordinating movements and maintaining balance. The forebrain is less developed than in higher classes of animals.

Eyes They have a flat cornea and a spherical lens.

Hearing organs represented by the inner ear - the membranous labyrinth. There are three semicircular canals.

They contain limestone stones. Fish make and hear sounds.

Organs of touch represented by sensory cells scattered throughout the body.

Side line perceives the direction of flow and water pressure, the presence of obstacles, and sound vibrations.

Taste cells are located in the oral cavity.

The importance of fish in nature and human life. Consumers of plant biomass, consumers of the second and third orders; sources food products, fats, vitamins.

EXAMPLES OF TASKS

Part A

Skullless animals include

3) lancelet

4) octopus

A2. The main characteristic of chordates is

1) closed circulatory system

2) internal axial skeleton

3) gill breathing

4) striated muscles

A3. Has a bony skeleton

1) white shark 3) stingray

2) katran 4) piranha

A4. Warm-blooded animals include

1) whale 2) sturgeon 3) crocodile 4) toad

Bony gill covers are present in

1) dolphin 3) tuna

2) sperm whale 4) electric stingray

Has a four-chambered heart

1) turtles 2) pigeons 3) perches 4) toads

1) single-chamber heart and two circles of blood circulation

2) two-chambered heart and one circulation

3) three-chambered heart and one circulation

4) two-chambered heart and two circles of blood circulation

A8. Cold-blooded animals include

1) beaver 3) squid

2) sperm whale 4) otter

Coordination of fish movements is regulated

1) forebrain 3) spinal cord

2) midbrain 4) cerebellum

A10. Doesn't have a swim bladder

1) katran 2) pike 3) perch 4) sturgeon

Part B

IN 1. Choose the correct statements

1) fish have a three-chambered heart

2) the transition of the head to the body in fish is clearly visible

3) the lateral line organs of fish have nerve endings

4) the notochord in some fish remains throughout life

5) fish are not capable of forming conditioned reflexes

6) the nervous system of fish consists of the brain, spinal cord and peripheral nerves

Select signs related to skullless animals

1) the brain is not differentiated into sections

2) the internal skeleton is represented by a chord

3) excretory organs - kidneys

4) the circulatory system is not closed

5) the organs of vision and hearing are well developed

6) the pharynx is penetrated by gill slits

VZ. Establish a correspondence between the characteristics of animals and the type to which these animals belong

Part C

Where can they store oxygen? deep sea fish? Why do they need to do this?

C2. Read the text carefully. Indicate the numbers of sentences in which errors were made. Explain and correct them.

1. The chordate phylum is one of the largest in terms of the number of species in the animal kingdom. 2. The internal axial skeleton of all representatives of this type is the notochord - a bony, dense, elastic cord 3. The type Chordata is divided into two subtypes - Vertebrates and Invertebrates.

4. In the nervous system, the anterior part of the brain receives the greatest development. 5. All chordates have radial symmetry, a secondary body cavity, and a closed circulatory system. 6. An example of primitive chordates is the lancelet.

Subject of zoology. Its position in the system of biological sciences.

Zoology (from ancient Greek ζῷον - animal and λόγος - study) is a biological science, the subject of study of which is representatives of the animal kingdom. Subject of Zoology[edit]

Zoology studies the physiology, anatomy, embryology, ecology, and phylogeny of animals.

Animals traditionally included organisms with a certain set of characteristics:

1. Eukaryotic organisms.

2. The presence of an actin-myosin complex in cells (unlike plants and fungi).

3. Nutrition, as a rule, is associated with the absorption of particles of food substrate by the body (unlike fungi).

4. There are no plastids (unlike plants).

5. Capable (as a rule) of active movement.

6. Store glycogen.

7. Chitin as the main component of the exoskeleton of many invertebrates (mainly arthropods; chitin is also formed in the bodies of many other animals - various worms, coelenterates, etc.).

Some simple organisms, in terms of the method of nutrition and the set of subcellular structures, they occupy an intermediate position between animals and plants and therefore can be considered both as objects of zoology and as objects of botany.

Zoology has a number of branches adjacent to other sciences.

Sections of zoology[edit]

Based on research objectives, zoology is divided into a number of main disciplines, and based on objects of research, into a number of auxiliary disciplines.

Basic disciplines[edit]

The main disciplines of zoology, distinguished by research objectives:

· Taxonomy of animals.

· Morphology of animals.

· Animal embryology.

· Physiology of animals.

· Ethology of animals.

· Animal ecology.

· Zoogeography.

Phylum chordata. General characteristics. Position in the animal world and origin.

Chordata

[edit]

Material from Wikipedia - the free encyclopedia

Chordates(lat. Chordata) - a type of deuterostome animals, which are characterized by the presence of a mesodermal axial skeleton in the form of a notochord, which higher forms replaced by the spine. In terms of the structure and function of the nervous system, the chordate phylum occupies the highest place among animals. More than 60,000 species of chordates are known in the world, and 4,300 species in Russia.

Concept chordates unites vertebrates and some invertebrates that have, at least during some period of their life cycle, a notochord, a hollow dorsal nerve cord, gill slits, an endostyle, and a tail located after the anus. The chordate phylum is divided into three subphyla: cephalochordates (lancelets), tunicates, and vertebrates - the only subphylum that has a skull. Previously, hemichordates were considered as the fourth subtype, but they are now included in a separate group.

Structural features[edit]

Scheme of the structure of cephalochordates using the example of Lancelet: 1 - thickening of the neural tube in the front (“brain”); 2 - chord; 3 - dorsal nerve cord (“spinal cord”); 4 - caudal fin; 5 - anus; 6 - digestive canal; 7 - circulatory system; 8 - outlet of the circumbranchial cavity (atriopore); 9 - peribranchial cavity; 10 - pharyngeal (gill) slits; 11 - pharynx; 12 - oral cavity; 13 - perioral tentacles; 14 - mouth opening; 15 - gonads (testes or ovaries); 16 - eyes of Hesse; 17 - nerves; 18 - metapleural fold; 19 - blind hepatic outgrowth. Respiration (gas exchange): the blue arrow indicates the entry of oxygen-rich water, and the red arrow indicates the exit of carbon dioxide-rich water.

Chordata is a type of animal characterized by bilateral symmetry and the presence, at least at certain stages of development, of the following characters:

· Notochord, which is an elastic rod of mesodermal origin. In vertebrates, the notochord during embryonic development is completely or partially replaced by cartilage and bone tissue that forms the spine.

· Neural tube located dorsally. In vertebrates, the spinal cord and brain develop.

· Gill slits are paired openings in the pharynx. In lower chordates, they participate in the filtration of water for nutrition. In terrestrial vertebrates, gill slits are formed in early embryogenesis in the form of gill pouches.

· The muscular tail is the postanal section of the body, located caudal to the anus, which is shifted to the ventral side of the body (the notochord and the neural tube enter it, but the intestine does not enter).

· Endostyle - groove on the ventral side of the pharynx. In lower filter-feeding chordates, it produces mucus, which helps collect food particles and deliver them to the esophagus. It also accumulates iodine and may be a precursor to the vertebrate thyroid gland. As such, only the sandfly has an endostyle in vertebrates.

Subtype Tunicates. The main features and structure of ascidians.

TUNA TYPE (TUNICATA)

Tunicates, or tunicates, which include ascidians, pyrosomes, sebaceous and appendiculars, is one of the most amazing bands sea ​​animals. They got their name because their body is covered on the outside with a special gelatinous membrane, or tunic. The tunica consists of a substance extremely similar in composition to cellulose, which is found only in the plant kingdom and is unknown in any other group of animals. Tunicates are exclusively marine animals, leading a partly attached, partly free-swimming pelagic lifestyle. They can be either solitary or form amazing colonies that arise during alternation of generations as a result of the budding of asexual single individuals. We will specifically talk below about the methods of reproduction of these animals - the most extraordinary among all living creatures on Earth.

Zoologists of ancient times classified larval-chordates, or tunicates (Tunicata), as a type of mollusk. But already in 1816 Lamarck came to the conclusion that it would be more correct to consider these peculiar animals as an independent group of invertebrate animals, only vaguely similar to mollusks. The famous works of A. O. Kovalevsky, devoted to the study of the history of the development of tunicates and lancelets, revealed the known closeness of larval chordates to aracrania and vertebrates. This closeness is indicated by: the pattern of development of the embryonic layers of tunicates, breathing associated with the anterior part of the intestine, the formation of the rudimentary notochord and its position relative to the intestine and the neural tube.

Following short definition may characterize tunicates. These are chordate animals in which the notochord is located exclusively in the caudal region of the body; it usually exists in the larval period of development and disappears at the end of this period. Single layer epithelium The skin secretes a gelatinous membrane (tunic), which covers the entire body of the animal. The pharynx has the appearance of a gill box. Reproduction occurs partly sexually, partly by budding; There is a change of generations. Almost all species are hermaphroditic. Currently, there are up to 1,500 species of tunicates, of which the vast majority live on the bottom; part floats in the water column and is part of the plankton. The size of animals belonging to this subtype ranges from 1/2 millimeter to 400 millimeters, rarely more. Colonial forms sometimes form ribbons several meters long. The subtype contains 3 classes: ascidians(Ascidiae), salpas(Salpae) appendiculars(Appendiculariae).

Fig.1. Tunicates

Top row - ascidians, from left to right: ascidia mentula, colony of Schlosser's botryllus, clavelina, gastric zion. Bottom row, from left to right: Appendicularia oicopleura, Barrel dolioletta, Salp piebald colony, Pyrosoma atlantis

A group of primitive chordates that, in the larval stage of development, have all the characteristics of Chordata type structural features, but upon transition to adult state lose the notochord and experience a profound transformation of the central nervous system, which turns from the neural tube into a compact nerve ganglion (only the appendiculars retain the notochord and neural tube all their lives!. The simplification of the body with the age of animals is associated with the transition from the mobile existence of the larvae to the immobile existence of adults.

Specific structural features: there is a skin-muscular sac (epithelium and layers of longitudinal and circular muscles); the circulatory system is not closed, the heart is tubular, the blood circulation is pendular; the nervous system is represented by a nerve ganglion that does not have an internal cavity, from which nerve cords extend; the excretory system is absent; hermaphrodites, fertilization during external environment. Ascidians and salps also reproduce asexually.

Fig.2. Similarities and differences between larval chordates and skullless

The body of tunicates is never segmented, although in some ascidians it is noticeably divided into 2 or 3 sections. Externally, the body is covered with a gelatinous, leathery or cartilaginous shell-tunic. It is based on a substance extremely close to plant fiber (cellulose).

Musculature. Beneath the outer epithelium lies a layer connective tissue with muscles contained in it; the musculature of ascidians consists of longitudinal and transverse muscle fibers; in salps it forms a series of rings.

Nervous system. The central nervous system in adult tunicates consists of a single node on the dorsal surface with nerves extending from it.

The sense organs are poorly developed: the eye is found in the form of a pigment spot on the nerve ganglion, sometimes with a light-refracting body (in ascidian larvae, in salps, pyrosis), the auditory organ in the form of an unpaired otocyst (in ascidian larvae, in Doliolum), organs of touch in the form outgrowths at the edges of the inlet and outlet holes. Below the ganglion, the wall of the gill sac protrudes, forming an organ that has been compared to the Hypophysis of the vertebrate brain.

Digestive system. Most characteristic feature The intestinal canal is a strong development of the anterior section, which serves as an organ for breathing and eating. In the appendiculars, the wall of this section (gill sac) is pierced by only two openings, which directly open outward; in ascidians, the wall of the gill sac is equipped with numerous openings (gill slits), which open into the so-called peribranchial or perithoracal cavity, which surrounds most of the wall of the gill sac and makes up the anterior section of the cloacal cavity. Blood supply. The heart lies on the ventral side of the body; The appendiculars have no blood vessels; in the remaining tunicates, the anterior and posterior vessels depart from the heart. A remarkable feature of O.'s blood circulation is that the heart contracts for some time in a certain direction, then the contractions stop and then begin again, but in a certain direction. opposite direction; Consequently, the movement of blood does not have a definite direction, and in each vessel and in the heart the blood moves first in one direction, then in the other.

Reproductive system and characteristics of reproduction. All sexual specimens of tunicates are hermaphrodites, i.e. they have both male and female gonads. The maturation of male and female reproductive products always occurs in different time, and therefore self-fertilization is impossible. In ascidians, salps and pyrosomes, the ducts of the gonads open into the cloacal cavity, and in the appendicular, sperm enter the water through ducts that open on the dorsal side of the body, while eggs can come out only after the body walls have ruptured, which leads to the death of the animal. Fertilization in most tunicates occurs in the cloaca, but there is also external fertilization, when a sperm meets an egg in water and fertilizes it there. In salpas and pyrosomes, only one egg is formed, which is fertilized and develops in the mother’s body.

It should be emphasized that the acquisition of mobility by pelagic tunicates led to the loss of their developed free-swimming larvae. In complex and in most solitary ascidians, fertilization of eggs occurs in the mother's cloacal cavity, where the sperm of other individuals penetrate with the flow of water through siphons, and the fertilized eggs are excreted through the anal siphon. Sometimes the embryos develop in the cloaca and only then come out, i.e. a kind of live birth takes place.

For sessile organisms to reproduce successfully, it is necessary for the eggs and sperm of neighboring individuals to mature simultaneously. This synchronization is achieved by the fact that the reproductive products released by the first sexually mature individuals pass with a stream of water through the introductory siphon to neighboring animals and in a short time stimulate the beginning of their reproduction over large areas. A special role in this case is played by the paranervous gland, which communicates with the ripeness of the pharynx and receives the corresponding signal from the water. Through the nervous system, it accelerates the maturation of the gonads.



Tunicates (larval chordates; Tunicata or Urochordata), a subphylum of chordates, includes three classes (ascidians , Appendicularia and salps), uniting 1100-2000 species. These are widespread, sedentary marine organisms, the body of which is enclosed in a shell secreted by the outer epithelium - the tunic (hence the name). Body length is from 0.3 cm to 30 m. Only larval forms have a notochord. Some lead an attached lifestyle and are solitary forms or branching colonies. Others swim slowly in the water column. The most prominent organ of tunicates is the front part of the U-shaped digestive tract - the pharynx, which occupies most of the body volume. Nutrition is carried out by filtration. Their prey is small unicellular animals and plants and small organic remains. The circulatory system of the tunicates is open, lacunar type, and consists of a heart sac and a developed network of lacunae. Blood moves through large vessels and then pours into the cavities that wash the organs. The nervous system is represented by the cerebral ganglion on the dorsal side of the body and the nerve trunk extending from it. Tunicates are hermaphrodites, many of them are capable of asexual reproduction by budding. Ascidian class ( Ascidiae) . The majority of tunicates belong to this class, represented by sessile forms, both solitary and colonial. Colonial forms sometimes lead a free-swimming lifestyle. Ascidia looks like a two-necked jar. With the base of its body (sole) it is attached to the protrusions of the bottom. On the upper part of the body there is a tube-like outgrowth with an opening leading into a huge pouch-like pharynx. This is an oral siphon. Another hole is located lower on the side - this is the cloacal siphon. The pharynx is pierced by a large number of small openings - gill slits, or stigmas, through which water circulates. At the bottom of the pharynx there is an opening leading into the short esophagus. The esophagus passes into the pouch-shaped stomach. The short intestine opens into the atrial cavity, which communicates with the external environment through an opening - atriopore, located on the cloacal siphon. Passive nutrition. There is an endostyle. Food particles that enter the throat with water are deposited on it. The endostyle begins at the bottom of the pharynx and along its ventral side rises up to the oral opening. Here it bifurcates, forming a peripharyngeal ring, and passes into a dorsal outgrowth stretching along the dorsal side of the pharynx. Food lumps are driven by the ciliated cells of the endostyle upward to the peripharyngeal ring, from where they descend along the dorsal process to the esophagus. There is a stomach, a short intestine opens into the atrial cavity near the cloacal siphon. The circulatory system is open, lacunar. The nervous system consists of a ganglion without an internal cavity, located between the oral and cloacal siphons. There are no sense organs. Reproductive system. Ascidians are hermaphrodites: in the body of one individual there is both an ovary and a testis. During asexual reproduction, a flask-shaped protrusion appears on the ventral side of the mother's body - the kidney stolon. The bud soon separates and turns into a sessile form: in colonial ascidians, the bud remains on the stolon and itself begins to reproduce by budding. All organs of the maternal form are formed in the kidneys. Sexual reproduction ascidians: a free-swimming larva quickly forms from a fertilized egg. Outwardly, it resembles a tadpole: its “head” contains all the organs, and its tail allows it to move quickly. In addition to the muscles and fin fold, the tail contains a notochord and a neural tube. Soon it is attached to the substrate by two outgrowths of the head and undergoes a regressive metamorphosis. The chord disappears. The neural tube, light-sensitive eye and brain vesicle decrease in size and then disappear. Only the posterior thickened part of the vesicle remains, which forms a ganglion. The pharynx grows, the number of gill openings increases sharply. The oral and anal openings move upward. The body takes on the sac-like appearance typical of an adult. A tunic quickly forms on the surface of the body. Tunicates had common ancestors. The ancestors of tunicates were free-swimming animals that moved through the water using a long caudal fin. They had a developed neural tube with an expanded brain vesicle at the anterior end, sensory organs in the form of an auditory vesicle and a pigmented ocellus, and a well-developed notochord. Later, most species switched to a sedentary lifestyle and their body structure became significantly simpler. Adaptations due to a sedentary lifestyle have progressively developed: a thick tunic - reliable protection for internal organs, a complex gill apparatus, endostyle, reproduction not only sexually, but also by budding.

Tunicates, larval chordates, or tunicates, which include ascidians, pyrosomes. Salps and appendiculars are one of the most amazing groups of marine animals. The central place among them belongs to ascidians. Tunicates received their name due to the fact that their body is covered on the outside with a gelatinous membrane, or tunic. Tunica consists of a special substance - tunicin, extremely close in composition to plant fiber - cellulose, which is found only in the plant kingdom and is unknown for any other group of animals. Tunicates are exclusively marine animals. Ascidians lead an attached lifestyle, the rest are free-swimming pelagic. They can be solitary or form colonies that arise during alternation of generations as a result of budding of asexual single individuals. Ascidians have a tailed larva that swims freely in the water.
All tunicates, except for a few unusual ones predatory species, feed on organic particles suspended in water (detritus) and phytoplankton and are active filter feeders. In the vast majority of cases, in the adult state they have a sac- or barrel-shaped body with two siphons - inlet and outlet. The siphons are either close together on the upper part of the body or located at its opposite ends.

Representative of the subphylum Tunicata (Tunicata). Photo: Minette Layne

The position of tunicates in the system of the animal kingdom is very interesting. The nature of these animals remained mysterious and incomprehensible for a long time, although they were known to Aristotle more than two and a half thousand years ago under the name Tethya.
Only in early XIX century, it was established that the solitary and colonial forms of some tunicates - salps - represent only different generations of the same species. Previously they were classified as different types animals. Single and colonial forms differ from each other not only in appearance. It turned out that only colonial forms have sexual organs, and solitary forms are asexual. The phenomenon of alternation of generations in salps was discovered by the poet and naturalist Albert Chamisso during his voyage in 1819 on the Russian warship Rurik under the command of Kotzebue. Old authors, including Carl Linnaeus, classified the tunicate as a type of mollusk. Colonial forms were assigned by him to a completely different group - zoophytes, and some considered them a special class of worms. But in fact, these outwardly very simple animals are not as primitive as they seem. Thanks to the work of the remarkable Russian embryologist A. O. Kovalevsky, in the middle of the last century it was established that tunicates are close to chordates. A. O. Kovalevsky established that the development of ascidians follows the same type as the development of the lancelet, which represents, in the apt expression of Academician I. I. Shmalhausen, “a kind of living simplified diagram of a typical chordate animal. The group of chordate animals is characterized by a number of certain important structural features. First of all, this will be the presence of a dorsal string, or notochord, which is the internal axial skeleton of the animal. The tailed larvae of ascidians also have a notochord, which disappears when they become an adult. The larvae are much higher in other important structural features. parent forms. For phylogenetic reasons, that is, for reasons connected with the origin of the group, tunicates attach greater importance to the organization of their larvae than to the organization of adult forms. Such an anomaly is unknown for any other type of animal. In addition to the presence of a notochord, at least in the larval stage, tunicates are similar to real chordates by a number of other characteristics. It is very important that the nervous system of the tunicates is located on the dorsal side of the body and is a tube with a canal inside. The neural tube of tunicates is formed as a groove-shaped longitudinal invagination of the surface integument of the body of the embryo - the ectoderm, as is the case in all other vertebrates and in humans. In invertebrate animals, the nervous system always lies on the ventral side of the body and is formed in a different way. The main vessels of the tunicate circulatory system, on the contrary, are located on the ventral side, contrary to what is typical for invertebrate animals. And finally, the anterior section of the intestine, or pharynx, is pierced by numerous openings in tunicates and has turned not only into a digestive organ that filters food, but also into a respiratory organ. As we saw above, invertebrate animals have very diverse respiratory organs, but the intestines never form gill slits. This is a characteristic of chordates and is the only characteristic that is retained in adult forms of tunicates. Tunicates have a secondary body cavity, or coelom, but it is greatly reduced.
According to the ideas of A. O. Kovalevsky, accepted by many, although not all modern zoologists, ascidians descended from free-swimming chordates. The peculiarities of their structure are a secondary simplification, as a result of the second, the notochord, the neural tube, and sensory organs are lost, as well as the presence of a tunic that performs protective and support functions, and greater specialization - this is a consequence of adaptation to the attached way of life in adulthood. The structure of their complex, tailed larvae, swimming in water, to some extent reproduces the organization of their ancestors.
The position of tunicates in the system of the animal kingdom for a long time remained unresolved. They were considered either as an independent type, close to chordates, or as a separate subtype of the chordate type. This was due to poor knowledge, first of all, of the embryonic and ontogenetic development of this group of animals. Conducted in Lately A comparative study of the embryology of the lancelet and ascidians, pyrosomes, salps and appendiculars shows a lot common features. And, as is known, the early stages of animal development are extremely great importance for phylogenetic constructions. It should be considered definitively established that tunicates are a special subphylum - Urochordata, or Tuncata, of the phylum Chordata, where they are included along with the subphyla Acrania and Vertebrata. It must be emphasized, however, that even now some questions remain controversial regarding the family relationships between and within these subphyla, as well as the origin of chordates in general.
A well-known specialist in the field of comparative embryology of lower chordates (animals and tunicates), as well as invertebrates O. M. Ivanova-Kazas, notes that the development of tunicates, despite its extreme originality, allows us to consider them even more highly organized animals than the lancelet, which is the most primitive representative of chordates and the type of development of which in the process of evolution led to the ontogeny of vertebrates. The development of tunicates has evolved in a different direction than that of the lancelet. In connection with the sedentary lifestyle of ascidians, highly developed and specialized forms of asexual reproduction, completely unusual for other chordates, arose in tunicates, with complex life cycles, with the emergence of coloniality, polymorphism, etc. From ascidians it was inherited by pyrosomes and salps.
Tunicates reproduce both asexually (budding) and sexually. Among them there are also hermaphrodites. The reproduction of tunicates provides an amazing example of how incredibly complex and fantastic life cycles animals can be in nature. All tunicates, except appendiculars, are characterized by both sexual and asexual methods of reproduction. In the first case, a new organism is formed from a fertilized egg. But in tunicates, development to an adult occurs with profound transformations in the structure of the larva towards its significant simplification. With asexual reproduction, new organisms seem to bud off from the mother, receiving from her the rudiments of all the main organs.
All sexual specimens of tunicates are hermaphrodites, i.e. they have both male and female gonads. The maturation of male and female reproductive products always occurs at different times, and therefore self-fertilization is impossible. In ascidians, salps and pyrosomes, the ducts of the gonads open into the cloacal cavity, and in the appendicular, sperm enter the water through ducts that open on the dorsal side of the body, while eggs can come out only after the body walls have ruptured, which leads to the death of the animal. Fertilization in most tunicates occurs in the cloaca, but there is also external fertilization, when a sperm meets an egg in water and fertilizes it there. In salpas and pyrosomes, only one egg is formed, which is fertilized and develops in the mother’s body. It should be emphasized that the acquisition of mobility by pelagic tunicates led to the loss of their developed free-swimming larvae. In complex and in most solitary ascidians, fertilization of eggs occurs in the mother's cloacal cavity, where the sperm of other individuals penetrate with the flow of water through siphons, and the fertilized eggs are excreted through the anal siphon. Sometimes the embryos develop in the cloaca and only then come out, i.e., a kind of viviparity occurs.
For sessile organisms to reproduce successfully, it is necessary for the eggs and sperm of neighboring individuals to mature simultaneously. This synchronization is achieved by the fact that the reproductive products released by the first sexually mature individuals pass with a stream of water through the introductory siphon to neighboring animals and in a short time stimulate the beginning of their reproduction over large areas. A special role in this case is played by the paranervous gland, which communicates with the pharyngeal cavity and receives the corresponding signal from the water. Through the nervous system, it accelerates the maturation of the gonads.
Many features of the embryonic development of lancelets and tunicates are similar to those, for example, in echinoderms or hemichordates, and this allows us to consider lower chordates as a kind of link between invertebrates and vertebrates.
However, neither the skullless nor the tunicates appear to be the direct ancestors of vertebrates. The origin of the tunicata is currently presented as follows. Some primitive skullless creatures switched to a sessile lifestyle on a hard substrate at the bottom of the sea and turned into ascidians. A powerful tunic protected them well from enemies, and the well-developed filtration apparatus of the pharynx provided a sufficient amount of food for these animals, which switched to a passive method of feeding and became filter feeders - stenophages. Some important organs in adult organisms were reduced. They remained only in the active free-swimming larva, which allowed the immobile ascidians to spread widely in the ocean. And the amazing ability for asexual reproduction - budding - ensured the rapid settlement of new areas. Then the tunicates repopulated the aquatic environment and managed to master the jet mode of movement. All this gave them great advantages, but, although tunicates are widespread in modern seas and oceans and are a characteristic component of the marine fauna, they did not give rise to a progressively developing branch on the evolutionary tree. This is like an evolutionary dead end, a side branch extending from the very base of the phylogenetic trunk of chordates.
Together with other chordates and a small number of invertebrate animals, tunicates belong to deuterostomes - one of the main trunks of the evolutionary tree in the kingdom Animalia.
In representatives of deuterostomes, or Deuterostomia, during embryonic development the mouth is not formed in the place of the primary mouth of the embryo, but breaks through anew. The primary mouth turns into an anus. In contrast, in protostomes, or Protostomia, the mouth is formed in place of the mouth of the embryo - the blastopore. These include most types of invertebrate animals.
The subphylum of tunicates includes three classes: ascidiae (Ascidiae), salps (Salpae) and appendiculariae (Appendiculariae). Ascidians gave rise to the remaining classes of tunicata.
The subphylum includes 1,100 species living in the seas.