Subtype of Shells. The main features and structure of the ascidian

Type Chordates

Inferior chordates. Subtype Skullless

TYPE CHORD. LOW CHORD

General characteristics of the Chordate type

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

1. Chordates, bilaterally symmetric, deuterostomes, multicellular animals.

2. Chordates have a notochord throughout their life or at one of the developmental phases. Chord Is an elastic rod located on the dorsal side of the body and performing a supporting function.

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

4. Under the chord is located digestive tube... The digestive tube begins mouth and ends anus, or the digestive system opens into the cloaca. The throat is pierced gill slits, which in primary aquatic animals persist throughout their life, and in terrestrial ones they are laid only in the early stages of embryonic development.

5. Under the digestive system lies heart... The circulatory system in chordates closed.

6. Chordates have secondary body cavity.

7. Chordates are segmented animals. Arrangement of organs metameric, i.e. major organ systems are located in each segment. In higher chordates, metamerism manifests itself in the structure of the spinal column, in the muscles of the abdominal wall of the body.

8. The excretory organs in chordates are diverse.

9. Chordate dioecious. Fertilization and development are varied.

10. Chordates evolved through a series of intermediate forms unknown to biology from the earliest coelomic animals.

The Chordate type is divided into three subtypes:

1. Subtype Skullless. These are 30-35 species of small marine Chordates, resembling fish in shape, but without limbs. The chord in the Skulls persists throughout life. The nervous system is in the form of a hollow tube. There are gill slits in the pharynx for breathing. Representatives - Lancelet.

2. Subtype Larva-chordates, or Shells. These are 1500 species of sedentary sedentary animals living 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), on the body there are two siphons - oral and cloacal. Larval chordates are water filtering devices. The body is covered with a thick shell - tunic (hence the name of the subtype - Tunic). In adulthood, the Tunicates lack the notochord and neural tube. However, the larva, which actively swims and serves for dispersal, has a structure typical for Chordates and is similar to Lancelet (hence the second name - Larval Chordates). Representative - Ascidia.

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

The chord is replaced by the vertebral column. The neural tube is differentiated into the spinal cord and the brain. The skull is developed, which protects the brain. The skull carries jaws with teeth for gripping and chopping food. Paired limbs and their belts appear. Cranials have a much higher metabolic rate, a complex population organization, varied behavior and a pronounced individuality of individuals.

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

Subtype Cranial - Acrania

Lancelet

The subtype Cranials includes the only class Cephalochondids, which numbers only about 30-35 species of marine animals living in shallow waters. A typical representative is Lancelet — Branchiostoma lanceolatum(genus Lancelet, class Cephalic, subtype Cranial, type Chordate), the size of which reaches 8 cm. The body of the Lancelet is oval in shape, narrowed towards the tail, laterally compressed. Outwardly, the Lancelet resembles a small fish. On the back of the body is located tail fin in the form of a lancet - an ancient surgical instrument (hence the name Lancelet). Paired fins are absent. There is a small dorsal... On the sides of the body from the abdominal side, two metapleural folds, which grow together on the ventral side and form peribranch, or the atrial cavity communicating with the pharyngeal slits and opening at the posterior end of the body with an opening - atriopore- outward. At the front end of the body, near the mouth, there are perioral tentacles with which the Lancelet grabs food. Lancelet live on sandy soils in the sea at a depth of 50-100 cm in temperate and warm waters. They feed on bottom sediments, marine ciliates and rhizopods, eggs and larvae of small crustaceans, diatoms, burrowing into the sand and exposing the front end of the body to the outside. They are more active at twilight, avoid bright lighting. The disturbed Lancelet swims pretty quickly from place to place.

Veils. Lancelet's body is covered skin consisting of a single layer epidermis and a thin layer dermis.

Musculoskeletal system. A chord stretches along the entire body. Chord Is an elastic rod located on the dorsal side of the body and performing a supporting function. To the anterior and posterior ends of the body, the notochord becomes thinner. The chord protrudes into the anterior part of the body somewhat farther than the neural tube, hence the name of the class - Cephalic. The chord is surrounded by connective tissue, which at the same time forms support elements for the dorsal fin and divides muscle layers into segments using connective tissue

Type Chordate subtype Cranial Lancelet

interlayers. Individual muscle segments are called myomers, and the partitions between them - myosepts... Muscles are formed by striated muscles.

Body cavity at Lancelet secondary, in other words, they are coelomic animals.

Digestive system. On the front of the body there is mouth opening surrounded by tentacles(up to 20 pairs). The mouth opening leads to 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 endostyle- a groove with a ciliary epithelium, which drives food particles into the intestine. No stomach, but there is hepatic outgrowth homologous to the liver of vertebrates. Middle intestine 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 to 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. There are more than 100 pairs in the throat of the Lancelet gill slits leading to peri-abdominal cavity... The walls of the gill slits are permeated with a dense network of blood vessels, in which gas exchange takes place. With the help of the ciliary epithelium of the pharynx, water is pumped through the gill slits into the peri-occipital cavity and out through the opening (atriopor). In addition, gas-permeable skin also takes part in gas exchange.

Circulatory system. The circulatory system of the Lancelet closed... The blood is colorless and does not contain respiratory pigments. Gases are transported as a result of their dissolution in blood plasma. In the circulatory system one circle blood circulation. The heart is absent, and the blood moves due to the pulsation of the branchial arteries, which pump blood through the vessels in the branchial clefts. Arterial blood enters dorsal aorta from which to carotid arteries blood flows to the anterior part, and through the unpaired dorsal aorta to the posterior part of the body. Then by veins the blood returns to venous sinus and by abdominal aorta heading for the gills. All blood from the digestive system enters the hepatic outgrowth, then into the venous sinus. The hepatic outgrowth, like the liver, neutralizes toxic substances that have entered the bloodstream from the intestines, and, in addition, performs other functions of the liver.

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

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

Type Chordate subtype Cranial Lancelet

hairs, liquid with metabolic products is removed from the nephridium cavity into the peri-occipital cavity, and from there outward.

central nervous system formed neural tube with a cavity inside. The lancelet does not have a pronounced brain. In the walls of the neural tube, along its axis, light-sensitive organs are located - eyes of Hesse... Each of them consists of two cells - photosensitive and pigmented, they are able to perceive the intensity of the light. An organ is attached to the expanded anterior part of the neural tube smell.

Reproduction and development. Lancelet weeds in the Black Sea and Lancelet weeds in the waters of the Atlantic off the coast of Europe start breeding in the spring and spawn until August. Lancelet of warm waters reproduce all year round. Lancelet dioecious, the sex glands (gonads, up to 26 pairs) are located in the body cavity in the pharyngeal region. The reproductive products are excreted into the peri-abdominal cavity through the temporarily formed reproductive ducts. Fertilization outward in water. From the zygote appears 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 goes on to life at the bottom. Lancelet live up to 4 years. Sexual maturity is reached by the age of two.

Significance in nature and for humans. Skulls are an element of biological diversity on Earth. They feed on fish, crustaceans. The Skulls themselves process dead organic matter, being decomposers in the structure of marine ecosystems. Cranials are essentially a living diagram of the structure of Chordates. However, they are not the direct ancestors of vertebrates. In the countries of Southeast Asia, locals collect Lancelet, sifting sand through a special sieve, and eat it.

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

§ nephridial excretory system;

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

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

§ metamerism (repetitive arrangement) of the genitals and nephridia;

§ absence of the heart in the circulatory system;

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

Type Chordate subtype Cranial Lancelet

Rice. The structure of the lancelet.

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

1 - chord; 2. - neural tube; 3 - oral cavity; 4 - gill slits in the pharynx; 5 - periobranular 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. Lancelet Nephridium.

1 - hole as a whole (into the secondary body cavity); 2 - solenocytes; 3 - hole in the peri-abdominal cavity.

Type Chordate subtype Cranial Lancelet

Rice. Lancelet cross section:

A - in the pharynx, B - in the midgut.

1 - neural tube; 2 - musculature; 3 - the roots of the dorsal aorta; 4 - ovary; 5 - endostyle; 6 - abdominal aorta; 7 - metapleural folds; 8 - periabranular (atrial) cavity; 9 - gill slits (due to the oblique position, more than one pair of them can be seen 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.

What are the characteristic features of animals of the Chordate type?

Name the classification of the type into three subtypes.

What is the systematic position of the Lancelet?

Where does the Lancelet live?

What body structure does the Lancelet have?

How does the Lancelet eat and what is the structure of the digestive system of the Lancelet?

How is the release of waste products in the Lancelet?

What is the structure of the nervous system of the Lancelet?

What is the structure of the circulatory system of the Lancelet?

How does the Lancelet reproduce?

What is the significance of the Lancelet in nature?

DRAWINGS TO BE PERFORMED IN THE ALBUM

(3 pictures in total)

Lesson topic:

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Chordates

Chordates (Chordata) are the highest type of deuterostomes. For all species of this type, at least at the stage of embryonic development, the presence of an unsegmented dorsal skeletal axis (chord), dorsal neural tube, and branchial clefts is characteristic.

Type Chordates (Chordata). General characteristics. Structural features

The type is divided into three subtypes: tunicates, cranials, and vertebrates.

Tunicates (Tunicata) or larva-chordates (Urochordata) have a sac-like or barrel-shaped body with a length of 0.3 to 50 cm; 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 pierced with gill slits. The hindgut and genital ducts open into the atrial cavity, which connects with the external environment. The nervous system consists of a ganglion located between the mouth and the atriopore, with a nerve trunk extending from it; the sense organs are poorly developed.

Tunicates reproduce sexually; there is also asexual reproduction. All larva 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 sense organs and the nervous system, muscles and chord (in adult forms, it remains only in the appendicularia). It is believed that vertebrates descended from neotenic (which began to reproduce) tunicate larvae. Three classes: tiny primitive appendicularia (Appendicularia), ascidians (Ascidiacea) and pelagic tunicates (Thaliacea), including three subclasses: pyrosomes, salps, and barrels.

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

Cranial (Acrania) or cephalochordata (Cephalochordata) are a subtype of lower chordates.

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

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

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

Vertebrates lose, 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.

Due 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 chord in the embryo, which transforms into a spine in an adult animal, an internal skeleton, a separate head with a developed brain protected by a skull, perfect sense 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) - corymbose and cyclostomes and maxillary (Gnathostomata) - armored, cartilaginous and bony fish, amphibians, reptiles, birds, mammals. Shield and shell fish became extinct in the Paleozoic. Currently, there are about 50,000 known vertebrate species.

General characteristics of the Chordov type

Basic terms and concepts tested in the examination paper: cranial, gill slits, internal skeleton, amphibians, skin, limbs and girdles of limbs, circulatory circles, lancelet, mammals, neural tube, vertebrates, reptiles, birds, reflexes, lifestyle adaptations, fish, bone skeleton, cartilaginous skeleton, chord ...

TO type Chordates include animals that have an internal axial skeleton - notochord or vertebral column.

Chordates have reached in the process of evolution the highest level of organization and flowering in comparison with other types. They live in all areas of the globe and occupy all habitats.

Chordates Are bilaterally symmetrical animals with a secondary body cavity and a secondary mouth.

In chordates, a general plan of the structure and location of internal organs is observed:

- 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 - the heart.

In the Chordate type, two subtypes are distinguished - Cranial and Vertebrate.

It belongs to the uncranial lancelet... All other chordates known today, considered in the school biology course, belong to the subtype Vertebrates.

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

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

The skin is involved in gas exchange and elimination 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 type of chordates can be co-tissue, cartilaginous and bone. Cranials have a connective tissue skeleton. In vertebrates - cartilaginous, osteochondral and bone.

Musculature- is divided into cross-striped and smooth.

The striated muscles are called skeletal muscles. Smooth muscles form the muscular system of the jaw apparatus, intestines, stomach and other internal organs. The skeletal musculature is segmented, although less so than in lower vertebrates. Smooth muscles have 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.

In the process of evolution of chordates, the length of the digestive tract increases, it becomes more differentiated into divisions.

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

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

Lungs during embryonic development are formed from intestinal outgrowths and have an endodermal origin.

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

There is one circulation circle (in fish and amphibian larvae) or two (in all other classes). The heart of fish, amphibian larvae is two-chambered. In adult amphibians and reptiles, the heart is three-chambered. 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.

The 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 cranial and spinal nerves and interconnected nerve nodes that lie along the spinal column.

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

Sense organs well developed. Primary aquatic animals have organs lateral line, perceiving pressure, direction of movement, speed of water flow.

Excretory organs in all vertebrates it is represented by the kidneys. The structure and mechanism of functioning of the kidneys changes in the course of evolution.

Reproductive organs. Vertebrates are dioecious.

The sex glands are paired and develop from the mesoderm. The reproductive ducts are connected with the excretory organs.

Pisces superclass

Fish appeared in Silurian - Devonian from jawless ancestors.

There are about 20,000 species. Modern fish are divided into two classes - Cartilaginous and Bone... 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 Chordata type

Bony fish include animals with bony scales, a bony skeleton, gill slits, covered with a gill cover. The appearance of fish is due to the following aromorphoses :

- the emergence of a cartilaginous or bony spine and skull covering the spinal cord and brain from all sides;

- the appearance of jaws;

- the appearance of paired limbs - pelvic and pectoral fins.

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

The senses are well developed - sight, smell, hearing, taste, lateral line organs, balance. The skin is two-layered, thin, slimy, covered with scales. Muscles are almost undifferentiated, with the exception of the muscles of the jaws and muscles that attach to the gill covers of bony fish.

Digestive system well differentiated into departments.

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

Respiratory organs fishes are gills, and in lungs, gills and lungs. An additional function of respiration 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 gill arteries to the gills, where the blood is saturated with oxygen. Arterial blood flows through the outflowing branchial arteries into the dorsal aorta, which supplies blood to the internal organs.

In fish, there is 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-like primary kidneys. Urine flows through the ureters into the bladder. In males, the ureter is also the vas deferens.

Females have an independent excretory opening.

Sex glands represented by paired testes in males and ovaries in females. Many fish exhibit sexual dimorphism. Males, brighter than females, attract them with their appearance, 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 that of the higher classes of animals.

Eyes have a flat cornea, a spherical lens.

Organs of hearing represented by the inner ear - a membranous labyrinth. There are three semicircular canals.

They contain limestone stones. Fish make and pick up sounds.

Organs of touch are represented by sensitive cells scattered throughout the body.

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

Taste cells are in the oral cavity.

The value of fish in nature and human life. Plant biomass consumables, second and third order consumers; sources of food products, fats, vitamins.

EXAMPLES OF TASKS

Part A

Skullless animals include

3) lancelet

4) octopus

A2. The main feature of chordates is

1) closed circulatory system

2) internal axial skeleton

3) gill breathing

4) striated musculature

A3. Bone skeleton is in

1) white shark 3) stingray

2) katrana 4) piranhas

A4. Warm-blooded animals include

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

Bony gill covers have

1) dolphin 3) tuna

2) sperm whale 4) electric ray

The four-chambered heart is in

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

1) unicameral heart and two circles of blood circulation

2) two-chambered heart and one circle of blood circulation

3) three-chambered heart and one circle of blood 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

The coordination of fish movements is adjustable

1) forebrain 3) spinal cord

2) midbrain 4) cerebellum

A10. No swim bladder

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

Part B

IN 1. Choose the correct statements

1) fish have a three-chambered heart

2) the transition from the head to the trunk in fish is clearly visible

3) there are nerve endings in the organs of the lateral line of fish

4) the chord in some fish persists throughout their life

5) fish are not capable of the formation of conditioned reflexes

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

Select traits that are relevant to cranial animals

1) the brain is not differentiated into sections

2) the internal skeleton is represented by the chord

3) excretory organs - kidneys

4) the circulatory system is not closed

5) the organs of sight and hearing are well developed

6) the pharynx is pierced with gill slits

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

Part C

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

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

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

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

The Chordate type unites animals that are different in appearance, habitat, and lifestyle. Representatives of this type are found in all main environments of life: in water, on land, in the soil, in the air. They are distributed throughout the Earth. The number of species of modern representatives of chordates is about 40 thousand.

The Chordate type includes skullless, cyclostomes, fish, reptiles, amphibians, mammals and birds. Tunicates can also be attributed to this type - this is a kind of group of organisms that lives on the bottom of the ocean and leads an attached lifestyle. Sometimes chordates are included in the type of intestinal breathing, which have some signs of this type.

Chordates

Despite the wide variety of organisms, they all have a number of common structural and developmental features.

The structure of chordates is as follows: all these animals have an axial skeleton, which first appears in the form of a chord or dorsal string. Notochord is a special non-segmented and elastic cord that develops embryo from the dorsal wall of the embryonic gut. The origin of the notochord is endothermal.

Further, this strand can develop in different ways, depending on the organism. For life, it remains only in the lower chordates. In most higher animals, the notochord is reduced, and a vertebral column forms in its place. That is, in higher organisms, the notochord is an embryonic organ that is displaced by the vertebrae.

Above the axial skeleton is the central nervous system, which is represented by a hollow tube. The cavity of this tube is called a neurocele. Almost all chordates are characterized by a tubular structure of the central nervous system.

In most organisms of the Chordate type, the anterior section of the tube expands to form the brain.

The pharyngeal (anterior) part of the digestive tube extends outward with two opposite ends. The outgoing holes are called visceral slits. In lower organisms such as they have gills.

In addition to the three above-mentioned features of chordates, it can also be noted that these organisms have a secondary mouth, like echinoderms. The body cavity in animals of this type is secondary. Chordata also have bilateral body symmetry.

The type Chordates is divided into subtypes:

• Skullless;
• Tunicates;
• Vertebrates.

Subtype Skullless

This subtype includes only one class - the Cephalic, and one detachment - Lancelet-shaped.

The main difference between this subtype is that these are the most primitive organisms, and all of them are exclusively marine animals. They are distributed in the warm waters of the oceans and seas of temperate and subtropical latitudes. Lancelet and epigonychites live in shallow water, mostly burrowing with the back of the body into the bottom substrate. They prefer sandy soil.

This type of organisms feeds on detritus, diatoms or zooplankton. They breed always in the warm season. Fertilization is external.

The lancelet is a favorite object of study, since all the signs of chordates are preserved in it for life, which makes it possible to understand the principles of the formation of chordates and vertebrates.

Subtype Shells

The subtype includes 3 classes:

• Salps;
• Ascidians;
• Appendicularia.

All animals of the subtype are exclusively marine.

The main difference between these chordates is that almost all organisms in the adult state do not have a chord and a neural tube. In the larval state, all the characteristics of the type in tunicates are pronounced.

Tunicates live in colonies or singly, attached to the bottom. There are much fewer free-swimming species. This subtype of animals lives in the warm waters of the tropics or subtropics. They can live both on the surface of the sea and deep in the ocean.

The body shape of adult tunicates is round, barrel-shaped. The organisms got their name due to the fact that their body is covered with a rough and thick shell - a tunic. The consistency of the tunic is gristly or gelatinous, its main purpose is to protect the animal from predators.

Tunicates are hermaphrodites, they can reproduce both sexually and asexually.

It is known that the ancestors of these organisms were free-swimming, but at the present time only tunicate larvae can move freely in the water.

Subtype Vertebrates

Cranial animals are the highest subtype. Compared to other subtypes, they have a higher level of organization, which is evident from their structure, both external and internal. Among vertebrates, there are no species that lead a completely attached lifestyle - they actively move in space, looking for food and shelter, a pair for reproduction.

Moving, vertebrates provide themselves with the opportunity to change their habitat, depending on changing external conditions.

The above general biological features are directly related to the morphological and physiological organization of vertebrates.

The cranial nervous system is more differentiated than in lower animals of the same type. In vertebrates, the brain is well developed, which contributes to the functioning of higher nervous activity. It is the higher nervous activity that is the basis of adaptive behavior. These animals have well-developed sense organs, which are necessary for communication with the environment.

As a result of the emergence of the senses and the brain, such a protective organ as the skull has developed. And instead of the chord, this subtype of animals has a spinal column, which serves as a support for the whole body and a sheath for the spinal cord.

In all animals of the subtype, a mobile jaw apparatus and an oral slit appear, which develop from the anterior part of the intestinal tube.

The metabolism of this subtype is much more complicated than that of all the animals discussed above. The cranials have a heart that provides fast blood flow. The kidneys are necessary for the elimination of metabolic products from the body.

The subtype Vertebrates appeared only in the Ordovician-Silurian, but in the Jurassic period all the now known types and classes already existed.

The total number of modern species is slightly more than 40 thousand.

Vertebrate classification

The type of Chordates is very diverse. The classes that exist today are not so numerous, but the number of species is enormous.

The cranial subtype can be divided into two groups, these are:

• Primary aquatic organisms.
• Terrestrial organisms.

Primary aquatic organisms

Primary aquatic animals differ in that they either have gills during their entire life, or only in the larval stage, and embryonic membranes are not formed during the development of the egg. This includes representatives of the following groups.

Section Jawless

• Class Roundstomes.

These are the most primitive cranial animals. They actively developed in the Silurian and Devonian; at present, their species diversity is not great.

Section Jawstomes

Pisces superclass:

• Class Bony fish.
• Class Cartilaginous fish.

Superclass Four-footed:

• Class Amphibians.

These are the first animals to have a jaw apparatus. This includes all known fish and amphibians. All of them actively move in water and on land, hunt and seize food by mouth.

Terrestrial organisms

The group of terrestrial animals includes 3 classes:

• Birds.
• Reptiles.
• Mammals.

This group is characterized by the fact that embryonic membranes are formed in animals during the development of the egg. If the species lays eggs on the ground, the embryo membranes protect the embryo from external influences.

All chordates of this group live mainly on land, have internal fertilization, which suggests that these organisms are more evolutionarily developed.

They have no gills at all stages of development.

The origin of chordates

There are several hypotheses for the origin of chordates. One of them suggests that this type of organisms originated from intestinal breathing larvae. Most representatives of this class lead an attached lifestyle, but their larvae are mobile. Examining the structure of the larvae, one can see the rudiments of the notochord, the neural tube, and other features of the chordates.

Another theory is that the Chordate type evolved from the crawling, worm-like ancestors of the intestinal breathing. They had the rudiments of the notochord, and in the pharynx, next to the gill slits, there was an endostyle - an organ that promoted the secretion of mucus and the catch of food from the water column.

The article considered the general characteristics of the type. Chordates are united by many similar features of all organisms, but still each class and each species has individual characteristics.

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. The tunicates got their name due to the fact that their body is dressed on the outside with a gelatinous shell, or tunic. The tunic consists of a special substance - tunicin, which is extremely close in composition to plant cellulose - 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 the alternation of generations as a result of budding of asexual solitary individuals. Ascidians have a tailed larva floating in the water.
All tunicates, with the exception of a few unusual predatory species, feed on suspended organic particles (detritus) and phytoplankton and are active filter feeders. In the vast majority of cases, in adulthood, they have a bag-like or barrel-shaped body with two siphons - inlet and outlet. Siphons are either drawn together on the upper part of the body, or located at its opposite ends.

Representative of the subtype tunicates (Tunicata). Photo: Minette Layne

The position of tunicates in 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 at the beginning of the 19th century it was established that the solitary and colonial forms of some tunicates - salps - represent only different generations of the same species. Before that, they were classified as different types of animals. Solitary and colonial forms differ from each other not only in appearance. It turned out that only colonial forms have genitals, and solitary ones have asexual organs. The phenomenon of alternating 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, attributed the tunicate to the type of molluscs. Colonial forms were attributed to 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, it was established in the middle of the last century that tunicates are close to chordates. A.O. Kovalevsky established that the development of ascidians proceeds according to the same type as the development of the lancelet, which, according to the apt expression of Academician I.I.Shmal'gauzen, “is like a living simplified diagram of a typical chordate animal. The group of chordates is characterized by a number of certain important structural features. First of all, it will be the presence of a dorsal string, or chord, which is the internal axial skeleton of the animal. Tailed ascidian larvae also have a chord, which disappears when they turn into an adult. Larvae and other important structural features are much higher than parental ones. For phylogenetic reasons, that is, for reasons associated with the origin of the group, tunicates attach more importance to the organization of their larvae than to the organization of adult forms. larval stage, with true chordate animals, tunicates are brought together by a number of other pr of signs. 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 grooved longitudinal invagination of the surface integuments of the body of the embryo - the ectoderm, as is the case in all other vertebrates and in humans. In invertebrates, 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, in contrast to what is characteristic of invertebrates. And finally, the anterior part of the intestine, or pharynx, is pierced by numerous openings in the tunicates and has turned not only into a digestive organ that filters food, but also into a respiratory organ. As we saw above, the respiratory organs of invertebrates are very diverse, but the intestines never form gill slits. This is the trait of chordates and is the only trait that persists in the adult tunicates. The tunicates have a secondary body cavity, or the whole, but it is strongly reduced.
According to A.O. Kovalevsky, accepted by many, though not all, modern zoolologists, ascidians descended from free-swimming chordates. The peculiarities of their structure are a secondary simplification, as a result of the second, the chord, the neural tube, and the sensory organs are lost, as well as the presence of a tunic that performs protective and supporting functions, and great specialization is a consequence of adaptation to an attached lifestyle in an adult state. The structure of their complexly organized tailed larva, swimming in water, to some extent reproduces the organization of their ancestors.
The position of tunicates in the animal kingdom remained unresolved for a long time. They were considered either as an independent type, close to chordates, or as a separate subtype of the chordate type. This was due to the poor knowledge, first of all, of the embryonic and ontogenetic development of this group of animals. A recent comparative study of the embryology of the lancelet and ascidians, pyrosomes, salps and appendiculars shows many similarities. And, as you know, the early stages of animal development are extremely important for phylogenetic constructions. It should be considered definitively established that tunicates are a special subtype - Urochordata, or Tuncata, of the type of chordates (Chordata), where they enter together with the subtypes of cranial (Acrania) and vertebrates (Vertebrata). It must be emphasized, however, that even now there are some controversial issues concerning the relationship between these subtypes and within them, as well as the origin of chordates in general.
A well-known specialist in the field of comparative embryology of lower chordates (skullless and tunicates), as well as invertebrates, O.M. Ivanova-Kazas notes that the development of tunicates, despite its extraordinary originality, allows them to be considered 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 the development of the lancelet. In connection with the sedentary lifestyle of ascidians, highly developed and specialized forms of asexual reproduction appeared in tunicates, which is completely not characteristic of other chordates, 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. There are also hermaphrodites among them. The reproduction of tunicates is an amazing example of how extraordinarily complex and fantastic animal life cycles can be found in nature. All tunicates, except for the appendiculars, are characterized by both sexual and asexual reproduction. In the first case, a new organism is formed from a fertilized egg. But in tunicates, at the same time, development to an adult occurs with profound transformations in the structure of the larva towards its significant simplification. With asexual reproduction, new organisms, as it were, bud off from the mother, receiving from her the rudiments of all the main organs.
All genital specimens of tunicates are hermaphrodites, that is, they have both male and female sex glands. 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 genital glands open into the cloacal cavity, and in the appendicularia, spermatozoa enter the water through the ducts that open on the dorsal side of the body, while the eggs can come out only after rupture of the body walls, which leads to the death of the animal. Fertilization in most tunicates occurs in the cloaca, but there is also external fertilization, when the sperm meets the egg in the water and fertilizes it there. In salps 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 the pelagic tunicates led to the loss of their developed free-swimming larva. In complex and in most single ascidians, fertilization of eggs occurs in the cloacal cavity of the mother, where the spermatozoa 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 go outside, that is, a kind of viviparity takes place.
For sedentary organisms, for their successful reproduction, it is necessary that the eggs and spermatozoa of neighboring individuals mature at the same time. This synchronization is achieved by the fact that the sexual products, taken out by the first sexually mature individuals, with the flow of water enter through the inlet siphon to the neighboring animals and in a short time stimulate the beginning of their reproduction in large areas. A special role in this is played by the parasitic 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 the lancelet and tunicates are close to that, for example, in echinoderms or in hemichordates, and this allows us to consider the lower chordates as a kind of connecting link between invertebrates and vertebrates.
However, neither the cranial nor the tunicates seem to be the direct ancestors of vertebrates. The origin of tunicates is currently presented as follows. Some primitive skullless ones switched to a sedentary 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 way of feeding and became filter feeders - stenophages. At the same time, some of the important organs in adult organisms were reduced. They remained only in the active free-swimming larva, which allowed immobile ascidians to spread widely in the ocean. And the amazing ability to asexual reproduction - budding ensured the rapid colonization of new areas. Then tunicates repopulated the aquatic environment and managed to master the reactive method of movement. All this gave them great advantages, but although tunicates are widespread in modern seas and oceans and are a characteristic component of marine fauna, they did not give a progressively developing branch on the evolutionary tree. It is, as it were, an evolutionary dead end, a lateral branch extending from the very base of the phylogenetic trunk of chordates.
Together with other chordates and a small number of invertebrates, tunicates belong to deuterostomes - one of the main trunks of the evolutionary tree in the Animalia kingdom.
In representatives of deuterostomes, or Deuterostomia, in the process of embryonic development, the mouth is not formed in the place of the primary mouth of the embryo, but breaks through again. The primary mouth turns into an anus. In contrast, in protostomes, or Protostomia, the mouth is formed at the site of the mouth of the embryo - the blastopore. Most types of invertebrates belong to them.
The tunicate subtype includes three classes: Ascidiae, Salpae, and Appendiculariae. Ascidians gave rise to the rest of the Tunicata classes.
The subtype includes 1100 marine species. Of these, 1000 species are ascidians. Appendicularia have about 60 species, about 25 types of salps and about 10 types of pyrosomes. The structure of the body of almost all tunicates beyond recognition is very different from the general plan of the structure of the body in the chordate type.



Tunicates are widespread in the oceans and seas. There are about 1100 species of them, of which about 1000 belong to the class of ascidians, leading an attached lifestyle. Most ascidians are solitary animals, the rest form colonies.

The body is covered with a thick shell - a tunic (which explains one of the names of the subtype), which forms a sac that communicates with the external environment with two wide tubes (siphons). Through one of them water enters the body, through the other it leaves (Fig. 68). The usual size of the body is a few centimeters.

The nervous system is poorly developed. It is represented by a small ganglion lying above the pharynx, and nerves extending from it to various organs. There is a thin musculocutaneous sac.

The digestive system begins with the mouth, communicating with the external environment through the inlet siphon, and consists of the pharynx (there is an endostyle on its dorsal side), the stomach and the horseshoe-shaped intestine, which opens with the anus into the outlet siphon. The pharynx is pierced with small branchial openings that open into the peri-occipital cavity. Receiving food (small organisms and organic pieces) and digesting it occurs like in lancelets.

Rice. 68. Ascidia:

/ - appearance, // - internal structure; 7 - mouth siphon; 2- cloacal siphon; 3 - tunic (shell); 4, 5 - mantle; 6 - pharynx; 7 - pharyngeal cavity; 8 - gill openings; 9 - endostyle; 10, 11 - periobranular cavity; 12 - its wall; 13 - stomach; 14 - hepatic outgrowth; 15 - anus; 16 - testis; 17 - ovary; 18 - ducts of the sex glands; 19 - pericardial sac; 20 - heart; 21 - nerve node

The circulatory system is not closed. The blood is set in motion by the heart, from which vessels depart to various organs, especially those strongly branched in the walls of the gill slits of the pharynx. The latter is very large and performs, like in lancelet plants, the role of a respiratory organ through which water passes, which is removed after gas exchange through an outlet siphon.

Dissimilation products are accumulated by some cells and remain in the body.

All tunicates are hermaphrodites; fertilization, external and internal. Many species also reproduce asexually (by budding).

The position of tunicates in the animal system remained unclear for a long time, until A.O. Kovalevsky thoroughly studied the development of ascidians, showing that it is very similar to the development of lancelets and ends with the formation of a planktonic larva, similar in body shape to tadpoles and moving with the help of a tail. The larvae have a well-developed neural tube and notochord. After a short period of planktonic life, the larvae attach to a solid substrate and their organization undergoes radical restructuring, mainly regressive: the tail, together with the neural tube (except for its anterior end, which turns into a ganglion) and chorda, are reduced (as redundant in a sedentary lifestyle), others the organs necessary for adult animals develop. The tunicates, thanks to their well-developed filtration apparatus, have become a large group, foraging for themselves in all parts of the oceans and seas. The subtype is divided into 3 classes: ascidians, salps and appendiculars.

Zoologists of ancient times attributed larval-chordates, or tunicates (Tunicata), to the type of molluscs. But already Lamarck in 1816 came to the conclusion that it would be more correct to consider these peculiar animals as an independent group of invertebrates, only vaguely similar to molluscs. The famous works of A.O. Kovalevsky, devoted to the study of the history of the development of tunicates and lancelet, elucidated the known proximity of larval-chordates to cranials and vertebrates. Such closeness is indicated by: the picture of the development of embryonic layers of tunicates, respiration associated with the anterior intestine, the formation of a rudimentary chord and its position relative to the intestine and neural tube.

The following brief definition can characterize tunicates. These are chordates, in which the chord is located exclusively in the tail section of the body; it usually exists in the larval period of development and disappears at the end of this period. The monolayer epithelium of the skin secretes a gelatinous membrane (tunic) that covers the entire body of the animal. The pharynx looks like a branchial box. Reproduction occurs partly sexually, partly by budding; there is a change of generations. Almost all species are hermaphrodite. Currently, there are up to 1500 species of tunicates, of which the vast majority live on the bottom; some of them float in the water column and are 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), salpy(Salpae), appendiculars(Appendiculariae).

Fig. 1. Tuners

Top row - ascidians, from left to right: mentula ascidian, colony of Botrillus Schlosser, clavelin, gastric zion. Bottom row, from left to right: appendicular oikopleura, barrell dolioletta, colony of piebald salps, Atlantic pyrosome

A group of primitive chordates, which in the larval stage of development have all the structural features of the Chordata type, but upon transition to an adult state, they lose the chord and undergo a profound transformation of the central nervous system, which turns from a neural tube into a compact nerve ganglion (only the appendiculars retain the chord and the neural tube all life! The simplification of the organism with the age of animals is associated with the transition from the mobile existence of the larva 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 open, the heart is tubular, the blood circulation is pendulum; the nervous system is represented by a nervous ganglion, which does not have an internal cavity, from which nerve cords extend; the excretory system is absent; hermaphrodites, fertilization in the external environment. Ascidians and salps reproduce asexually.

Fig. 2. Similarities and differences between larval-chordates and cranials

The body of tunicates is never segmented, although in some ascidians it is noticeably subdivided into 2 or 3 sections. Outside, the body is clad in a gelatinous, leathery or cartilaginous tunic. It is based on a substance that is extremely close to plant cellulose (cellulose).

Musculature. Under the outer epithelium lies a layer of connective tissue with muscles enclosed in it; the musculature in 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 one node on the dorsal surface with nerves extending from it.

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

Digestive system. The most characteristic feature of the intestinal canal is the strong development of the anterior region, which serves as the respiratory and eating organ. In the appendicular, the wall of this section (gill sac) is penetrated by only two openings that open directly outward; in ascidians, the wall of the branchial sac is provided with numerous openings (gill slits), which open into the so-called peribranchial or perithoracic cavity, which surrounds most of the wall of the branchial sac and constitutes the anterior part of the cloacal cavity. Blood s-ma. The heart lies on the ventral side of the body; the appendicularia have no blood vessels; in the rest of the 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 the opposite direction; Therefore, 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 another.

Reproductive system and reproductive characteristics. All genital specimens of tunicates are hermaphrodites, that is, they have both male and female sex glands. 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 genital glands open into the cloacal cavity, and in the appendicularia, spermatozoa enter the water through the ducts that open on the dorsal side of the body, while the eggs can come out only after rupture of the body walls, which leads to the death of the animal. Fertilization in most tunicates occurs in the cloaca, but there is also external fertilization, when the sperm meets the egg in the water and fertilizes it there. In salps 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 the pelagic tunicates led to the loss of their developed free-swimming larva. In complex and in most single ascidians, fertilization of eggs occurs in the cloacal cavity of the mother, where the spermatozoa 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 go outside, i.e. a kind of viviparity takes place.

For sedentary organisms, for their successful reproduction, it is necessary that the eggs and spermatozoa of neighboring individuals mature at the same time. This synchronization is achieved by the fact that the sexual products, taken out by the first sexually mature individuals, with the flow of water enter through the inlet siphon to the neighboring animals and in a short time stimulate the beginning of their reproduction in large areas. A special role in this is played by the parasitic 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.