Cells are unique to cnidarians. Developmental biology of cnidarians (Cnidaria). Structure and lifestyle

Cnidaria include hydra, jellyfish and sea anemones. Largest part such organisms live in the seas and oceans, but hydras are also found in freshwater reservoirs. Corals and sea anemones inhabit mainly warm seas.

The smallest are some hydras, whose size is about 1 mm, while the largest can be called the hairy jellyfish Cyane, the length of its tentacles can reach 40 meters, and the body diameter exceeds 2 meters.

The body of any stinger contains a single large cavity, at one end of which there is a mouth opening surrounded by tentacles. The body cavity is surrounded by a wall consisting of 2 layers of cells, and a gelatinous substance between them. The inner cell layer forms the tissue involved in the digestion of food. Muscle fibers are located in the outer layer of cells, so organisms can react with movement to external stimuli. Nerve cells form a network, the simplest nervous system. In the outer layer of the body and tentacles there are special stinging cells. With a part of such a cage, resembling a harpoon, the animal injects poison into the body of the victim or enemy. All cnidarians are carnivorous and feed by drawing prey to their mouths with tentacles.

Forms of existence

Animals have 2 forms of existence: polyps and jellyfish. The body shape of the polyp (hydra, sea anemones and corals) resembles a vase. The mouth of the polyp opens up, and at the other end it is attached to the bottom of the reservoir or to other surfaces. And the body of a jellyfish resembles an umbrella or an inverted bowl. The mouth of the jellyfish opens down and it drifts freely in the water. Some cnidarians, such as the eared aurelia, are jellyfish at the adult stage, and polyps at the larval stage. This species of aquatic animals leads various Lifestyle.

Lifestyle

Anemones are immobile animals, they live at the bottom of the reservoir, entrenched in one place. They live apart and, thanks to their brightly colored tentacles, resemble a plant rather than an animal. The wide domed body of jellyfish is well suited for swimming in the water. Jellyfish move by swinging on the waves or using the current, alternately contracting and relaxing their body: water pushed out from under the domed body pushes the animal forward. Some of them are extremely poisonous and their touch can be fatal even to humans. Hydroids are fixed on plants, stones, etc. The place is changed extremely slowly: on the surface they are fixed in turn, first with the sole and then with tentacles, i.e. move as if tumbling.

Like some of the animals listed above, corals also live in one place. Most of their species form a hard calcareous skeleton around the lower part of the polyp. Corals live in large colonies in which calcareous skeletons are combined. And those, in turn, fix new corals, so that colonies can reach impressive sizes. Despite the fact that colonies grow only a couple of centimeters per year, over several thousand years they can form coral islands various shapes.

If the water is contaminated or too high temperature algae living in symbiosis with corals die, which leads to the death of corals, as a result of which only a lifeless white skeleton remains from colorful colonies.

Type Intestinal - Coelenterata, or Crackers - - the most ancient and low-organized organisms from real multicellular animals. Cnidarians got their name from the Greek. knide - to burn. Another common name for this type of animal is coelenterata. Radially symmetrical, mostly marine animals armed with tentacles and unique stinging cells (nematocytes) with which they hold and kill prey.

The body wall consists of two layers surrounding the gastrovascular cavity: the outer (epidermis) of ectodermal origin and the inner (gastrodermis) of endodermal origin. These layers are separated by a gelatinous connective tissue called mesoglea. The gastrovascular cavity serves to digest food and circulate water throughout the body.

Cnidarians for the first time had real nerve cells and a diffuse-type nervous system (in the form of a network). Polymorphism is characteristic, i.e. the presence within the same species, sharply differing in appearance forms. One typical form is a sessile polyp attached to the substrate and similar to a cylinder, at the free end of which is a mouth surrounded by tentacles; another form is a free-floating jellyfish, resembling an inverted bowl or umbrella with tentacles hanging down the edges. Polyps form jellyfish by budding. Those, in turn, reproduce sexually: a fertilized egg develops into a larva, giving rise to a polyp. Thus, in the life cycle of many cnidarians, there is an alternation of sexual and asexual generations. Species that do not have a medusoid form reproduce sexually or by budding. They may be dioecious or hermaphroditic.

Their body consists of two layers of cells - the outer one, which forms the ectoderm, and the inner one, which is called the endoderm. Between these layers there is a developed non-cellular layer - mesoglea.

The function of support in the coelenterates is performed by the mesoglea. In polyps, it looks like a thin base plate.

In coelenterates, the most primitive type among multicellular nervous system. In the ectoderm, nerve cells that perceive irritation are relatively evenly distributed. The irritation is transmitted through the contacting processes of the nerve cells to the contractile fibers of the epithelial-muscle cells, and then the response follows - the contraction of the body of the hydra.

Coelenterates are characterized by radial symmetry and a two-layer body structure.
Most coelenterates have pronounced radial or radial symmetry. In coral polyps, there are deviations towards two-beam or even bilateral (bilateral) symmetry.

Coelenterates are characterized by two life forms: a sessile saccular polyp (coral polyps) and a floating discoid jellyfish. The polyp has the following structure. The part of the body that attaches the body to objects is called the sole. On the upper part of the body there is a mouth surrounded by tentacles. All coelenterates are characterized by the presence of special stinging cells, which are designed to protect against enemies, as well as attacks. This has not been found in other animals.

Stinging cells contain capsules with paralyzing poison. It enters the body of the victim through a special channel located in the stinging thread of these cells. When a sensitive hair is irritated, the stinging thread straightens with force and pierces the victim. After the shot, the stinging cell dies, and a new one is formed from the intermediate cell.

In addition to stinging coelenterates, they also have other specialized cells: skin-muscular, glandular, reproductive, and nervous.

The digestive system of coelenterates is very primitive. The mouth leads into the intestinal or gastric cavity.

Digestion of food at the first stage occurs under the action of enzymes in the gastric cavity. This is extracellular or cavity digestion. Small food particles, into which food breaks down, are captured by endoderm cells, i.e. the inner layer of cells, and are digested intracellularly.

Coelenterates reproduce both asexually and sexually.

Simply arranged cnidarians include hydra, reaching 2.5–3 cm in length and leading a solitary lifestyle. Many form large colonies. Approximately 10,000 species have been described, grouped into three classes.

The type of coelenterates unites about 9000 species - inhabitants of the seas and oceans and about 20 species of inhabitants fresh water. The type of coelenterates includes three classes:
Hydroids (Hydrozoa) Scyphozoa (Scyphozoa) Coral polyps (Anthozoa)

The value of the coelenterates is great. Calcareous skeletons of reef-building coral polyps form reefs and atolls in tropical seas. Coral reefs and the islands are a dangerous obstacle to navigation. Coral polyps play a beneficial role in cleansing sea ​​water from suspended organic particles. Huge strata of limestone were formed from the skeletons of coral polyps that had died off over many millennia. In many tropical coastal countries, it is used in construction. From the skeletons of some types of corals, for example, red coral, various decorations are made.

Jellyfish sensitively pick up sound vibrations that occur when water rubs against air, and long before a storm approaches, they sail away from the coast. Based on this property, bionics scientists have created the Medusa Ear device, which allows you to determine the approach of a storm about 15 hours before its onset.

Some types of jellyfish serve as a refuge for fish fry and hermit crab. Coelenterates have great importance in the food chain of marine biocenoses.

Type Intestinal, or Cnidaria. general characteristics type

Systematic position of the type

Remark 1

Type Intestinal (Coelenterata) belongs to the sub-kingdom of Animals, its representatives are eumetazoans, or true higher multicellular animals.

Representatives of the Supersection Eumetazoi have a number of common features:

• differentiation of tissues, organs;
• the presence of nerve cells;
• clearly manifested integrity and integration of individuals;
• pronounced bilateral (Section Bilateral) or radial (Section Radiant) symmetry.

Type Intestinal are included in the Section Radiant. They, as representatives of this section, are characterized by:

• beam symmetry;
• two-layer structure;
• the presence of a gastric (intestinal) cavity;
• diffuse nervous system.

Type Intestinal includes polyps and jellyfish that have stinging cells, therefore given type also called the Striders.

This type includes three classes:

• Hydroids (Hydrozoa);
• Scyphozoa (Scyphozoa);
• Coral polyps (Anthozoa).

Features of the external and internal structure

Remark 2

The body of the Coelenterates has a central heteropolar axis, around which morphological structures are located in a certain order. This axis penetrates the oral (oral) and aboral poles of the body.

In relation to the heteropolar axis, the body parts of the coelenterates and individual structures are oriented symmetrically:

• radially;
• asymmetrically, or biradially;
• bilaterally.

Through the body of the coelenterates, 2, 4, 6, 8, etc. can be passed. planes of symmetry. Representatives of the type, as a rule, lead a fixed or sedentary lifestyle. In the process of ontogenesis, two germ layer. From the ectoderm (outer leaf) integuments are subsequently formed, and the endoderm (inner leaf) lines the intestinal cavity.

The tissues and organs of the coelenterates are formed by the epidermis and gastrodermis and the mesoglea between them - the intercellular matrix. The epidermis shows a high degree differentiation of cells, tissues, organs.

Type specific features:

• four-beam symmetry - tetramerism;
• development with metamorphosis, the presence of a two-layer planula larva is characteristic;
• stinging cells that perform the functions of attack and defense;
• the main part of the nervous system is the diffuse plexus.

Direct development is rare. The body of all coelenterates is a bag consisting of two layers with a gastric cavity. The cavity of the sac is lined with endoderm, where food is digested. The function of the mouth is performed by the opening of the “bag”, undigested food residues are also removed through it. The simplest representatives of the coelenterates in structure can be compared with a typical gastrula. This group of animals has a high ability to regenerate.

Morpho-ecological forms of coelenterates

There are two morpho-ecological forms of Coelenterates:

• polyp (benthic attached form);
• jellyfish (planktonic form).

Type Coelenterates is characterized by the presence of floating forms of animals with tentacles. Colonies are sometimes formed from medusoid and polypoid individuals. Often you can find a symbiosis of Cnidaria and unicellular algae. For most representatives of the type, it is characteristic life cycle with alternation of sexual and asexual reproduction, the so-called metagenesis between a jellyfish and a polyp. As a rule, a jellyfish is formed from a polyp as a result of:

• formation of special transverse constrictions;
• metamorphosis;
• strobilation (terminal department);
• lateral budding.

The formation of a polyp occurs as a result of sexual reproduction of the jellyfish through the stage of planula formation.

(Greek cnidos - thread)

The type of cnidarians, or cnidarians, includes numerous diverse animals, among which the most famous are hydras, jellyfish and corals. They lead a planktonic or benthic immobile, mostly attached way of life,

settling in colonies or alone. These are exclusively aquatic, more often marine, less often brackish or freshwater organisms. Benthic forms inhabit at all depths, up to the abyssal. The body shape is different. The embryo develops two layers of cells: ectoderm and endoderm. Due to the ectoderm, an epidermal layer arises in an adult, consisting of muscle, nerve, stinging, skeletal and other cells. Due to the endoderm, an internal gastric layer is formed, consisting mainly of a variety of digestive cells. In an adult animal, an unstructured gelatinous layer is formed between the epidermal and gastric layers - mesoglea, which is formed due to cellular secretions and the introduction of various cells of ecto- and endodermal origin. In the adult state, cnidarians are represented by two life forms: polyps and jellyfish. Jellyfish have the form of an umbrella, bell or mushroom, and single polyps are most often bag-shaped (Fig. 107). Colonies of polyps of various shapes; they are, as a rule, polymorphic, as they consist of individuals of various structures that perform various functions. Polyps are benthic organisms, mostly attached, in rare cases they can lead a planktonic lifestyle, such as siphonophores, or move along the bottom, such as hydra and

Rice. 107. Longitudinal and cross sections of a hydroid polyp (a, b), a scyphoid jellyfish (c, d) and coral polyp(e, f) d - pharynx, gs - gastric (inner) layer, m - mesoglea, n - digestive cavity, p - mouth, s - skeleton, u - tentacles, es - epidermal (outer) layer

anemones. Many polyps have a skeleton: mineral (calcareous) or organic (chitin and protein), rarely agglutinated. Jellyfish are planktonic organisms; as an exception, sessile bottom forms are found.

Cnidarians have five functional systems: digestive, muscular, nervous, reproductive, skeletal. Such systems as excretory, circulatory and others are absent. Actually the digestive cavity in the evolution of the animal kingdom first appears in cnidarians, therefore for a long time this type was called Coelenterata (Greek koilos - full; enteron - gut, entrails) - coelenterates. The digestive cavity is saccular, folded or non-folded. A single mouth opening leads into it, which performs the function of both oral and anal. mouth opening polyps and jellyfish are surrounded by tentacles, the number of which can reach 100; they form one or more cycles. Tentacles - with a large number of stinging capsules, each of them has inside a spirally coiled thread with a point at the end. When defending and attacking, the thread unfolds with lightning speed and, penetrating, like a harpoon, into the body of the victim, paralyzes it. With the help of tentacles, food is transferred to the mouth.

The type of cnidarians is divided into three classes: Hydrozoa, Scyp-hozoa and Anthozoa, which differ from each other in many ways, but primarily in the structure of the digestive system and reproduction features.

Cnidarians reproduce both sexually and asexually. In the first case, after the formation of reproductive products and fertilization, the process of crushing the egg begins and a two-layer planktonic larva, planula, appears. Then the planula settles to the bottom and the polyp begins to grow. Asexual reproduction occurs in two main ways: division and budding. When dividing, regeneration (restoration) of the missing parts occurs, as a result of which new individuals appear. When budding, outgrowths appear in various parts of the animal - buds, the further growth of which leads to the formation of a new individual. The result of asexual reproduction is the formation of colonies of polyps. WITH asexual reproduction the occurrence of jellyfish is also associated.