Cnidaria, also known as Cnidaria, used to be calledCoelenterata(Coelentera), because its meaning applies toSpinocystandCtenophora, so most of them have been abandoned.
Cnidaria, also known as Cnidaria, used to be calledCoelenterata(Coelentera), because its meaning applies to echinococcus andCtenophora, so most of them have been abandoned.The spinocyst is the most primitive metazoan, which has only two embryonic layers and is radiation or two radiation symmetry.The body wall consists of ectoderm, endoderm and mesoderm.The whole lumen of the body enclosed by the endodermDigestive circulatory cavityOne end of the coelenterate is the mouth, the other end is occluded, and there is no anus.There are prickle cells in the body wall.The bones of coelenterates are mainly exoskeletons with supporting and protective functions.It is mostly composed of chitin, cutin and limestone.In many corals, there are spicules or bone axes, which exist in the mesothelium or protrude from the body surface.Both asexual and sexual reproduction modes often appear in the life history of the same type, that is, the polyp generation uses asexual budding to produce the medusa generation, while the medusa type individual leaves the mother body, grows up and matures, and then produces the polyp type individual by sexual reproduction, and the two generations complete the entire life history alternately.There are about 10000 species, which can be divided into three classes: Hydrozoa, Scleromedusae and Coralzoa. It is generally believed that Hydrozoa is the most primitive.The vast majority of marine products, only a few of which are produced in fresh water, are most abundant in shallow water areas of tropical and subtropical oceans.Smaller species can be used as fish bait, coral skeleton (such asred coral）It can be used as handicrafts, ancient coral and modern coral can form oil reservoirs, and coral reefs at the seaside can be used as natural seawalls;Jellyfish become food after being salted, and some species can be used as medicine;Some planktonic jellyfish can be used as current indicators;However, some large jellyfish (such asJellyfish、Root jellyfish）In case of a large number of fish, the fishing net will be blocked or damaged. Some sea anemones mixed in the catch will cause poisoning and death after eating.Except for a few species that live in fresh water, most of them live in the sea. Most of them live in the shallow sea, and a few are deep-sea species. There are about 11000 species, divided into three classes:Hydra（Hydrozoa）、Medusa（Scyphozoa）、Corallina（Anthozoa）。
In 2023, the international scientific research team found the castle beach biota, a special burial fossil pool of "Burgess shale type" in the Middle Ordovician strata in Wales, England, and found more than 170 kinds of organisms, including many animal phyla such as echinocytes。[2]
morphological character
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Muscle and exercise
Cnidaria
SpinocystVery few can take the initiative to move, and the ability to move is very limited, especially the hydra type.Movement is made up of epidermisMuscle cellinMyofibrilCaused by shrinkage of.For example, the body of hydra can stretch and contract. The body length can reach 15-20mm when stretching and 0.5mm when contracting. This kind of stretching and contraction is explosive. It bursts every 5-10 minutes or so, mainly by the skin muscleCellsIt is caused by the contraction of longitudinal myofibrils.The contraction of myofibrils on one side of the body can cause the body to bend.Sometimes by bending the body andTentacleAnd alternately attach with the base plate to do the turning bucket movement.Mucous cells at the basal disc can secrete a large number of bubbles, which can make hydra float on the water for a short time.Medusae and coralMyofibrilWithEpidermal cellSeparate to form an independent layer of muscle fibers. For example, the myofibrils of jellyfish form a thin layer of muscle ring on the lower umbrella surface and the umbrella edge. Some are separated into sheets by the spoke tube, and their fibers have transverse lines. They contract regularly to make the umbrella surface contract rhythmically.When the umbrella edge contracts, the water inside the umbrella edge is ejected, and the jellyfish body moves upward due to the reaction force. When the umbrella and the umbrella edge muscles relax, the elasticity of the compressed glue layer restores the umbrella to its original shape, and the water enters the umbrella edge again, and the body sinks. However, because the contraction is faster than the expansion, the jellyfish can still move vertically upward.The vertical movement of jellyfish can be seen more clearly in jellyfish with marginal membrane.Once the muscles stop contracting, the jellyfish will sink naturally.Its horizontal movement is mostly passive, mostly driven by water flow and wind.For example, in corals,sea anemoneThe myofibrils of.
Feeding and digestion
The spinocysts are carnivorousplankton, smallCrustacea、PolychaeteEven small fish feed on it.Due to the mechanical and chemical stimulation of food, hydra will stretch their tentacles and release the gill sac to entangle, paralyze and poison the catch, and then send the food into the mouth.The mucus secreted by the gland cells in the oral area is conducive to food swallowing. After the food enters the stomach cavity, the gland cells in the stomach begin to secrete protease, which decomposes and digests the food to form many polypeptides. At the same time, the food can be mixed and pushed in the stomach cavity due to the flagella movement of muscle cells.After this extracellular digestion, the intracellular digestion process begins. The pseudopods that nourish muscle cells engulf food particles, forming a large number of food bubbles in the cells. After the acidic and alkaline chemical processes, nutrients are transported to the whole body by the diffusion of cells.The structure of stomach cavity of medusae and corals is complex.In the stomach cavity of the medusae, there are various radial tubes and circular tubes, and in the stomach sac there are stomach filaments of endoderm origin;The stomach cavity of corals is divided into many chambers by many diaphragms, on which there are diaphragmatic filaments.There are a lot of prickle cells and glandular cells in the stomach silk and diaphragm silk, which are killed and digested after they swallow food into the stomach cavity.Digested nutrients are transported to the whole body through various pipelines, and undigested food residues are still discharged from the mouth.Glycogen and fat are the main storage materials of coelenterates.Many species of spinocysts, especially reef building corals in the sea, have symbiotic algae in their bodies, such as animalsXanthophyta, dinoflagellates, etc.Algae can conduct photosynthesis and produceglycerol, fat, sugar andprolineAnd provide it to echinococcus as its supplementary nutrition.This will be further described in Corals.
Respiration and excretion
SpinocystThere is no special respiratory and excretory organ. Since the body is composed of two layers of cells surrounding the gastric circulation cavity, and the stomach cavity is connected with the outside world through the mouth, in fact, the two layers of cells in the body wall are in contact with the outside environment, so the respiration and excretion can be carried out directly and independently by the cells in the body wall.The water flow at the inlet, outlet and stomach cavity can carry fresh oxygen and take away metabolites.The main nitrogenous wastes of echinococcus animals areammonia。
Nerve conduction and sense
The spiny animals are the first multicellular animals to appear neural structure.The primitive neural structure is composed ofBipolar neuron、Multipolar neuronandnerve fiberCombined into a nerve plexus or nerve network, it is generally located outside the muscle layer at the base of the outer skin cells and is distributed in a network, so it is callednerve net （netnervous system）。Because nerve cells can conduct in all directions, it is also called diffuse nervous system.Hydra has only one neural network, but most coelenterates have a neural network at the base of the stomach layer in addition to the epidermal neural network. The two neural networks are either completely independent or connected by fibers.In 1971, Westfall et al. first proved that hydra and other coelenterates were able toGlandular cellSynaptic transmission also exists between).That is, the conduction of nerve impulse is transmitted from the axon of one neuron to the dendrite of another neuron.There are synaptic vesicles at the ends of axons and no synaptic vesicles at the ends of dendrites.During conduction, synaptic vesicles releaseacetylcholineIt causes the excitation of the latter neuron. After the impulse is transmitted, acetylcholine is exposed tocholinesteraseIt is hydrolyzed to choline and acetic acid and the activation is relieved.Later, it was found that the synaptic conduction of coelenterates can be divided into two types.One type of synapse is symmetrical transmission, that is, there are synaptic vesicles at two or more nerve ends, so impulse can be transmitted to two or more directions at the same time. This kind of transmission is also called non-polar transmission;The other type of synapse is asymmetric, that is, only one end of the nerve cell has synaptic vesicles, and nerve impulses can only be transmitted in one direction, so it is also called polarized transmission.Recent studies have proved that the non-polar conduction of spinocysts is very common.Others believe that one of the two neural networks of echinococcus animals is non-polar conduction formed by multipolar neurons, that is, slow conduction system;The other is the polar conduction formed by bipolar neurons, that is, the fast conduction system.Neurons of the prickly cell animals can have 2, 3 or more long neurites, which are connected with the senses, effectors, other neurons, or the same neuron is connected with the senses and effectors at the same time. This diversity shows the originality of nerve cell conduction.In physiological sense, it represents the initial stage of sensory nerve, motor nerve and central nerve of higher animals.In addition, the non nerve conduction, that is, epithelial conduction, of spiny animals is also very common. For example, spiny cells and epidermal muscle cells can be completely independent of neurons and controlled by themselves.Neurostructural ratio of jellyfish typepolyp Complex, such asHydromedusaeIn addition to the neural network of the fimbria, the epithelial nerve cells of the fimbria margin form two nerve rings at the top and bottom of the fimbria margin.The lower (or outer) nerve ring is more developed.Most of the jellyfish types of medusae do not have this kind of nerve ring, but form 4~8 ganglia in the umbrella edge.Jellyfish type individuals can show a rhythmic contraction movement, which is because the nerve structure of jellyfish can spontaneously generate action potential every certain time and cause the contraction of the whole body through nerve conduction.Research has proved that this spontaneous action potential is caused by neurons at the starting point.The firing point neurons exist in the external nerve ring in the Hydromedusae, and in the sensory ganglion in the Pot medusae.Moreover, each ganglion (generally 4-8) has pacemaker neurons, which are the center of rhythmic pulsation of jellyfish.It seems that a jellyfish body with one starting point is enough to cause rhythmic contraction movement of the body, but experiments have proved that multiple starting points can make the contraction rhythm more regular, more secure and the interval shorter than only one starting point.The coelenterate polyp has no obvious sense organs, and its sensory cells can be distributed throughout the body, but the tentacle and mouth areas are rich.Jellyfish type individuals have rich sensory cells or organs at the umbrella edge.The cell bodies of sensory cells are ciliated.Sensory organs include eye points and balance capsules.Eyespot is a cup shaped object composed of sensory cells, with pigment particles distributed inside. It may have a positive tendency to light, that is, it swims to the water when there is light.Or it has negative tendency to light, that is, it sinks to the bottom when there is light, and floats to the surface when there is no light or weak light.The structure of the balance capsule in the hydra medusa is simple. A small capsule is formed at the base of the marginal membrane or the lower fimbrial facial nerve ring (i.e. the outer nerve ring). The inner wall of the capsule has sensory cells, and the cells also have cilia.There is a calcareous stone at the bottom of the capsule.It is a kind of gravity sensor.When the umbrella edge is tilted, the stone contacts with the cilia of the sensory cells and stimulates the cilia cells to generate action potential, inhibiting the sideMuscle fibreThrough muscle contraction, adjust the body again to restore the balance position.The structure of the balance capsule of medusae is more complex, which will be described in the chapter with joints
Reproduction and life history
Asexual reproductionAnd sexual reproduction are very common in coelenterates.The main forms of asexual reproduction areBudding reproduction, especially in the hydra type.For example, when the hydra sprouts, it protrudes from the base of the body through the body wall and stomach cavity, and then grows tentacles and mouth to form a bud. Later, the bud separates from the mother body to form a new individual.However, if the bud formed in Obelia is not separated from its mother, a colony will be formed.Secondly, asexual reproduction can also be carried out through division, which mainly occurs in the hydra type. For example, sea anemones can divide longitudinally, and the larvae of medusae can reproduce asexually by transverse division.In addition, the Hydra type generally has a strong regenerative ability. For example, the Hydra is cut into several segments, and each segment can generate a new individual when the conditions are appropriate. The polarity of the mouth and the mouth end is unchanged during regeneration, but the regeneration speed of the mouth end is faster than the mouth end.Therefore, the regeneration of hydra is also considered as a way of asexual reproduction.Some reproduction occurs in most hydra types and all medusae types, except for a few species, such as hydraHermaphroditeIn addition, most species are dioecious (or heterogeneous).Germ cells originate from mesenchymal cells and then migrate to a fixed location to form gonads.The gonads of hydrozoa come from the epidermal layer, such as hydra, and the gonads of hydromedusae are located under the radial tube or around the labium pendulum, but they all come from the epidermal layer cells.Germ cells of the medusae originate from the stomach layer and lie in the lower part of the stomach sac.The germ cells of corals are generated from the septum in the stomach cavity, which is also the source of the endoderm.The coelenterates only have gonads, and there are no gonadal ducts and accessory gonads. The germ cells are discharged from the mouth after maturity, or released by the rupture of the body wall.Fertilization varies according to species, either in vitro seawater, or on the surface of the vertical tube, or in the gonad in the stomach cavity.The cleavage is complete, forming a hollow blastocyst. The gastrula is formed by migration or invagination, resulting in the formation of two layers of cells, namely, two embryonic layers. The cells clustered inside are the endoderm, which will form the stomach layer of the adult in the future, and the outer layer is the ectoderm, which will form the epidermis of the adult in the future.The solid gastrula is rapidly extended, and cilia appear on the body surface, forming a free swimmingFloating wave larva（planula）。The floating wave larva has no mouth and stomach cavity in its early stage. After swimming for a period of time, it is fixed on water plants, rocks or other objects and develops intopolyp Or through budding reproduction to form a colony.The freshwater hydra has no larval stage, and its fertilized eggs develop directly.[1]
The body is radially symmetric, and some are radially symmetric
Two germ layers and primitive digestive cavity
The cells show original tissue differentiation
Reticular nervous system (diffuse or diffuse nervous system)
Unique prickle cell
Hydra and medusa
Sexual and asexual reproduction, generation alternation, marine species with floating wave larva stage[1]
Radial symmetry and two radial symmetry
The body shape of porous animals is mostly asymmetric.From coelenterates, the body shape has a fixed symmetrical form.The animals in this phylum are generally radially symmetric.That is, most stinging animals can divide their bodies into two equal parts through the central axis (from the oral surface to the anti oral surface).This is a primitive, low-level form of symmetry.This kind of symmetry can only be divided into upper and lower parts, not front and rear and left and right. It is only suitable for life fixed or floating in the water.Use its radially symmetric organs to ingest food or feel stimuli from the surrounding environment.In some species of echinococcus, radiation symmetry has developed into bilateral symmetry, that is, through the central axis of the body, only two sections can divide the body into two equal parts.This is an intermediate form between radial symmetry and bilateral symmetry.
Two germ layers and primitive digestive cavity
Although porous animals have two germ layers, they are different from other metazoans in genesis, so they are generally called two layer cells.The spinocysts are the animals with true two germ layers (endoderm and ectoderm).There is a mesoderm secreted by endodermal and ectodermal cells between the two germ layers.The lumen of the body enclosed by the cells of the endoderm and ectoderm, that is, the gastral lumen during embryonic development.It is different from the central cavity of sponge, and has the function of digestion. It can perform extracellular and intracellular digestion.Therefore, it can be said that this kind of animals began to have digestive cavities.This digestive cavity also has the function of circulation. It can transport the digested nutrients to all parts of the body, so it is also called the digestive cavity.With mouth and no anus, the digested residue is still discharged from the mouth.Its mouth has the function of feeding and discharging.The mouth is the primordial mouth during embryonic development. Compared with higher animals, coelenterates can be said to be in the gastrula stage.The mesoglea is between the epidermis and stomach of the echinococcus. The mesoglea in the body of the hydra is underdeveloped, generally a very thin layer, in which few cells are distributed.The glue layer in jellyfish is very developed, occupying almost the entire thickness of the body, which contains fibers and a small number of cells from the ectoderm.The main component in the middle glue layer is water, which contains a very small amount of protein and polysaccharide. Its concentration is less than 1%, and its osmotic concentration is lower than that of seawater. However, because it contains a small amount of sulfate and other ions (calcium sulfate solution is more concentrated than isotonic calcium chloride solution), its total osmotic concentration is similar to that of seawater.Generally, the rubber layer contains 5% organic matter. Some people also use Metridium to do experiments. They found that the rubber layer contains 8% protein and 1% polysaccharide, and exists in the form of collagen protein. This material has strong elasticity and adhesion.The collagen fibers in the tendons of vertebrates can only extend to 10-20% of their own length, while the collagen fibers in the rubber layer of echinococcus animals can extend three times of their own length (rubber is four times).Therefore, the mesothelium not only allows the echinococcus to stretch and maintain its shape, but also is a bone type of echinococcus in a certain sense.
Differentiation of cell and tissue
Sponge animals mainly have cell differentiation.The spinocysts not only havecell differentiationAnd began to differentiate into simple tissues.Interstitial cells are small undifferentiated embryonic cells, which can differentiate into skin muscle cells, prickle cells, germ cells, etc.Gland cell is a kind ofepithelial cells。The gland cells are particularly developed at the base plate of the hydra and in the epidermis of the tentacles. Its secretions can help the hydra attach and prey.In species with skeletons, such as polyps and corals, the gland cells can secrete a large amount of cutin or calcium to form a chitin sheath or calcium exoskeleton.The stomach layer also contains a large number of gland cells, which contain a large number of secretory particles, which can be converted into digestive enzymes for extracellular digestion of food.Interstitial cell is located between epithelial cells, near the mesothelium. It is some small, round cells, distributed alone or in piles, with large nuclei. It is an undifferentiated cell in the body, and can be transformed into germ cells, prickle cells, glandular cells and other types of cells.The sensory cell body is long and perpendicular to the body surface. It is abundant in the oral area and tentacles. The cell base has many neurites, and the end has sensory hair. It senses various stimuli, and then acts on effectors or cells through the neurites.
Hydra type and medusa type
The spinocysts have two basic shapes, namely, the hydra type living in a fixed camp and the jellyfish type living in a floating camp.In Hydrozoa, the original species, Hydra and Jellyfish, appear alternately in the life history. The asexual reproduction stage is shown as Hydra, and the sexual reproduction stage is shown as Jellyfish. Most Hydrozoa have relatively developed hydra, especially the species that live in groups.In the medusae, the medusae type is developed, while the hydra type is underdeveloped or completely disappeared.Only the polyp type and jellyfish type of corals no longer exist.The hydra type is a single or group, such as Hydra, which is a common single living animal in the pond. The body is cylindrical or tubular, about 5mm long. The body axis from the mouth to the base plate is called the mouth anti mouth axis. The mouth end is free, and there is a mouth at the top. There is a circle of tentacles around the mouth, and the number of tentacles is 5-10,Other species vary from species to species.The opposite end of the hydra forms an adhesive basidium, which is used to attach to aquatic plants or other objects.The species that live in groups, such as Obelia, form stolon at the opposite end.The body wall of the hydra type is composed of two layers of epithelial muscle cells sandwiched with a thin layer of mesoga, which surrounds a gastric circulation cavity in the center.The gastric circulation cavity of polyps is a simple cavity and also tubular, while that of corals is complex and divided into many intervals by many diaphragms extending from the body wall to the center, so as to increase the area of digestion and absorption[1]。
Except for a few species that live in fresh water, most of them live in the sea. Most of them live in shallow sea, and a few are deep-sea species. There are about 11000 species of echinococcus now.
Cell tissue differentiation
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The echinococcus not only differentiated cells, but also began to differentiate into simple tissues.The body wall of echinococcus consists of the ectoderm, the endoderm and the mesoderm between two layers of cells.There are four types of cells in the ectoderm: epithelial muscle cellsGlandular cell, mesenchymal cells, prickle cells.
Epithelial muscle cells are characterized byepithelial cellsIt contains myofibrils.This kind of cell has the function of epithelium and muscle, so it is called epithelio muscular cell, which is called dermatomyocyte for short.At the same time, the epithelium of echinococcus animals also has the same conduction function as nerves, which is due to the application of electrophysiological technology andelectron microscopeThis is a discovery for studying the nerves of coelenterates.Non nerve conduction or neuroid conduction was first confirmed in coelenterates.Epithelial muscle cells belong to both epithelium and muscle.This shows that the epithelium is not separated from the muscle, which is a primitive phenomenon.Generally, one or several slender projections extend from the base of the epithelial muscle cells, including myofibrils, and some epithelial components are underdeveloped and become myocells. Some epithelial components are developed, with flat cells and unidirectional myofibrils, or two rows of myofibrils are vertically arranged, and some epithelial components are developed and cylindrical,There are a series of smooth muscle rings around.Muscle fibers are also divided into striated muscle, twill muscle and smooth muscle.Each myofibril is composed of a bunch of filaments, which can be divided into thick filaments and thin filaments. These filaments are similar to the thick filaments (myosin) and thin filaments of higher animals（Actin）Silk is similar, and its contraction mechanism is also similar to that of higher animals.Little is known about the innervation of muscle. Some experiments have proved that the ultrastructure of neuromuscular synapses and neuromuscular junction, the contact part between nerve and muscle in cell stimulating animals, are also similar to those in higher animals.[1]
Gland cell is a kind of epithelial cell with secretory capacity.The gland cells are particularly developed at the base plate of the hydra and in the epidermis of the tentacles. Its secretions can help the hydra attach and prey.In species with skeletons, such as polyps and corals, the gland cells can secrete a large amount of cutin or calcium to form a chitin sheath or calcium exoskeleton.The stomach layer also contains a large number of gland cells, which contain a large number of secretory particles, which can be converted into digestive enzymes for extracellular digestion of food.[1]
Mesenchymal cell(Interstitial cell) is located between epithelial cells, near the mesothelium. It is some small, round cells, distributed alone or in piles, with large nuclei. It is an undifferentiated cell in the body, which can be transformed into germ cells, prickle cells, glandular cells and other types of cells.[1]
Prickle cell(cnidoblast) is an aggressive and defensive cell peculiar to echinococcus.Hydra is distributed in the epidermis, especially in the mouth area, tentacles and other parts. In addition to the body surface and tentacles, in the medusae and corals, there is also a large number of distribution on the stomach and diaphragm filaments in the digestive cavity to help prey.The prickle cell is a kind of specialized epithelial muscle cell. The nucleus is located at the base, and the top of the cell has a needle (cnidocil), which protrudes from the body surface. Its ultrastructure is similar to that of flagella;The base of the thorn also has grana.There is a nematocyst in the prickle cell. The top of the cyst is a lid, and the inside of the cyst is a slender coiled prickle.When the prickles or prickle cells are stimulated, the prickle sacs are discharged from the prickle cells, and the prickles also turn out from the prickle sacs to form prickles of different lengths for predation and defense.The experiment proved that the discharge mechanism of the gill sac was caused by the combined effect of mechanical and chemical stimuli, and the single use of any of these stimuli did not cause emissions.External stimulation acts on the prickle cells, causing the prickle sac to absorb water from the surrounding cytoplasm, changing the permeability of the cyst wall, and the prickle cells shrink with it, increasing the pressure in the prickle sac. The prickle breaks through the cover plate and turns out, causing emissions.Therefore, its emission is due to the direct effect of external stimulation on the gill sac, rather than through nerve cells.Nerve conduction may only play a regulatory role in the discharge of a large number of prickle cells.The tip of the discharged gill sac continuously exudes liquid, which has anesthetic and toxic effects on the arrested objects.Someone used Aiptasia, a kind of sea anemone, to collect a large amount of prickly sac venom from the gastric filaments in its digestive tract for analysis, and found that there were four protein components.Then the four protein components were used for the toxicity test on Uca and Procambarus. The results showed that some of the protein components had toxic and killing effects, some had the ability to destroy the cell membrane and nerve cord to transmit action potential, and some protein components caused strong convulsions in experimental animals, which indicated that it was caused by the role of the nervous system.The experimental results show that these proteins collected from gastric filaments haveneurotoxin、Muscle toxinHemolytic and necrotic characteristics.Some echinococcus, such as jellyfish and chardonnay jellyfish, have paralytic effects even on people due to the toxin of their gill sac.Each prickly cell can only be discharged once, but it can be continuously supplemented and renewed by mesenchymal cells. According to the shape of the prickly sacs and prickly sacs discharged from it, there are more than 30 prickly sacs in coelenterates, but each animal generally has 1 to 7 prickly sacs.For example, there are four kinds of hydra, one is the penetrant, which is used to puncture and release venom;One is the winding of the vent, which does not release venom but can wind the catch;There are also two kinds of adhesive gill sac (glutinant), and the gills discharged by them have adhesive and predatory functions.The first two kinds of gill sac are sensitive to chemical stimulation, especially food stimulation, while the latter two kinds are sensitive to mechanical stimulation.It has been observed that the hydra can discharge 25% of the gill sacs on its tentacles during a single feeding and renew within 48 hours.[1]
