Molluscs (Latin name: Granulifusus kiranus), also known as shellfish, areMolluscaThe general name of animals is exceptArthropodaThe largest group, about 100000 species.The systems are very different, but they have the same characteristics: the body is soft and not segmented, generally divided into head foot (some heads degenerate or disappear; foot muscle is fleshy) and viscera mantle (from the dorsalVisceral mass、MantleandMantle cavityConsists of) two parts.
Molluscs have a vast habitat, ranging from the tropical continent to the Arctic and Antarctic oceans, from highland ponds above 7000 meters to the deep sea below 800 meters.There are many kinds of molluscs, second only to arthropods - there are more than 100000 kinds of molluscs.Their shapes are very different, but they all have soft bodies.Most of them live in the sea, and only some bivalves and gastropods migrate to brackish water and fresh water[1]。
Body Structure Diagram of Molluscs
The dorsal skin fold extends downward into the mantle, which secretes the calcareous shell (some degenerate into inner shell or without shell) wrapped in the body.No realEndoskeleton。There is aHemocoelom(i.e. a series of expandedvenous sinus)。The blood function of the blood cavity, such as liquid bone, is used to maintain the tension of the body.The blood contains a small amount of stars orAmoebaCells.Blood containsHemocyanin(GastropodaandCephalopoda)。The muscles of the mouth contain myoglobin.realCoelomDegenerate into genital cavity andPericentric cavity。The body surface generally hasciliaOr mucus.IntraoralTooth tongue。Tooth tongue is a unique organ of most mollusks. It is composed of multiple rows of horny tooth plates, similar to a file, and used to help feed.There are often large digestive glands.yesCtenobranchial, with cilia on the surface to excite the water flow (inBivalveHelps to filter food particles in the water).Excretory organIt is kidney.Marine species excrete ammonia or urea,Terrestrial Gastropod platoonuric acid。HermaphroditeOr variant.Cephalopoda and some gastropodaIn vitro fertilization, androgynousAllogamy。There are several pairs of ganglia.The largest molluskKing squidThe wrist span is 12 meters (40 feet), and the smallest one is only 1 cm longsnails 。Molluscs are distributed in various habitats, such as sea water, fresh water, land (especially forest land, even dry areas).Some gastropods are endophytes of other animals, and molluscs have important economic significance.Many aquatic species, especially clamsOysters、Scallop in ShellandMusselThey can be used for food, fishing or breeding.TerrestrialBig snailThe genus Helix is used as a delicacy in Europe.manyshellorPearlIt can be used as decoration,shipwormAnd so on.SomeFreshwater snailsyesParasitic wormThe intermediate host of.
Molluscs（Cephalopod）The ethnic groups of includea squid、octopus、NautilusAnd the extinct ammonite and arrow stone.It has a long mouthTentacleGrab prey and move it for usesiphonApply water spray to advance.Mollusca is the animal kingdom
a squid
Golden snail
The second gate in.Molluscs are trigerm, bilaterally symmetricalCoelomOf animals.The true coelom of mollusks is formed by the split coelom method, that isMesodermThe body cavity formed.The body is soft, generally symmetrical from left to right, and some species show various strange shapes due to twisting and bending.They usually have shells, no joints, fleshy feet or wrists, and degenerated feet.The outer skin folds from the back into a so-called coat, which surrounds the body and secretes calcareous shells for protection.The gills for breathing are born in the cavity between the coat and the body.It is distributed all over the land and water.It includes Polyphylla (such as chitons)Gastropoda(e.gabalone、snail)；Excavatopoda(e.gdentalium ）, Lambranchia (such as clams and oysters)Cephalopoda(e.ga squid、Nautilus)、Anamorpha、monoplacophora (e.gNew dish shellfish）Seven categories.CoelomorphOne door.genusProtostome，CoelomClass,TeloblastDry.Gastropods become secondary left right asymmetry due to the torsion of visceral mass, while the system of other classes is left right symmetry.Body origin（LambranchiaNone), foot and visceral block.The epidermis of visceral mass extends to form an outer coat, which wraps the main part of the body.The space between it and the visceral block is the mantle cavity, in which there are gills. The end of digestive organs and excretory organs open here.There is no somite structure.The irregular spaces in the body are blood cavities.The lumen of the body narrows due to the development of the mesenchyme, forming only the lumen surrounding the heart, the inner lumen of the renal tube, and the inner lumen of the gonad (the genital cavity).The head has eyes and other sensory organs and mouth.As a motor organ, the muscles of the foot aresmooth muscle, which is related to its slow movement.There areTwill muscle。The mantle secretes shells.The number and shape of shells vary in different classes.The digestive tube is long, and there are teeth, tongue andSalivary gland。Midgut glandIt opens in the stomach.Generally availablegill1 pair, correspondinglyheartTwo auricles and one ventricle.But in Cephalopoda, there are 4 species with 2 pairs of gillsauricle。Some gastropods have only one native gill (the heart is one auricle and one ventricle) due to the torsion of visceral mass, and some species have lost their native consciousness and are replaced by mantle gills.In terrestrial gastropoda, some of the mantle of the lung snails has evolved into lungs.OpenVascular system，Respiratory pigmentMost of them are hemocyanins.Clams, snails, etchemoglobin。The excretory apparatus is made of kidney tube（BowyerNahr's organ), but there are also species with Keber's organ (Lamellibranchia).The nervous system is a special type. In addition to a pair of cephalic ganglia on the esophagus, it also hasLateral ganglia、Ganglia pedisAnd visceral ganglia.Each pair of similar ganglia is connected by the transverse nerve, and the cephalic ganglia is connected with other ganglia by the longitudinal nerve.Hermaphrodite orAllogeneic。In the treeshipwormAnd Bao et al., see the conversion of sex.exceptCephalopodsExternal, uniform spiral typecleavage, viaTrochlear larvaPeriod andFlour dish larvaPeriod;However, the cleavage of cephalopods is symmetrical and develops directly.
Molluscs are a phylum with a large number of invertebrates in both quantity and species. There are 120000 modern species and fossil species that have been found, which is the second phylum in the animal kingdom after arthropods.Molluscs are highly adaptable, so they are widely distributed. There are a large number of members in land, fresh water and salt water, such as snailsMussel, Concha squidWe are familiar with the representatives.
Although various molluscs have different shapes and habits, their basic characteristics are very similar. Their bodies are soft and most of them are not segmented. They are generally divided into four parts: head, foot, visceral mass and mantle.The mantle usually secretes a hard calcium shell to protect the outside of the body.Because the shape of the mantle varies from species to species, the hard shell shapes of different kinds of mollusks are also varied.However, except for most adult gastropods, their shells are symmetrical, that is, bilaterally symmetrical.According to these differences in hard shell and soft shell structure, scientists divided molluscs into 10 classes, namely, Monoplate, Polyplate, Anaplate, Gastropoda, ExcavatopodaBivalvia, coracoidea, cephalopodaPhyllostachysAnd Leptoglossa.
The characteristic of the phylum Lamellate mollusks is that their shells are shaped like hats or spoons.They have appeared as early as the Cambrian period, and have been reproducing to the modern times, represented by the modern new bamboo shell.
The body of Polyphylla is symmetrical on both sides and oval in shape. The dorsal shell is composed of 8 bone plates.They appeared in the late Cambrian and continued to modern times.
Anachezia has no bone plates or shells, and its bilaterally symmetrical body looks like a worm.However, their body surface is covered with horny skin with calcareous needle spines, which can also play a protective role.There is no fossil representative of Anachezia, and modern life includes sea hares, etc.
Gastropoda is a kind of mollusk that we are very familiar with. Snails, field snails, sea snails, etc. are all members of this class.Their soft body and shell twist during ontogeny, so the shell is in the form of a single spiral.Gastropods also appeared in the early Cambrian period, and have been breeding to modern times. They are distributed in land, sea and fresh water.
The shell of Digopoda is like a slender tube, with openings at both ends and symmetry on both sides.Geopoda appeared in the Ordovician period and has continued to the modern times. The representative animals are hornshells, etc.
Bivalvia is also a kind of mollusk that we are familiar with. The mussels, sea fans, clams and most of the shells we can pick up at the seaside belong to Bivalvia.They are characterized by having two shell flaps, and a hinged structure between the two shell flaps;Their soft parts lack obvious heads, and their ventral sides have fleshy feet.From Cambrian to modern times, bivalves have always been distributed in the oceans and freshwater on the earth.
Rostroconchia Their shells also look like they have two petals, but there is no hinge structure. Their shells are actually single shells in the form of "false double shells".They are extinct paleontology, living only from Cambrian to Ordovician, representing Hailaerte crust, etc.
Cephalopoda is the most well developed and advanced class in Mollusca, including nautilus, barnstone, ammonite, archery stone, modern octopus, squid, etc.They are all carnivorous animals in the sea and are good at crawling or swimming in the water.Cephalopods are symmetrical on both sides, with an obvious head in front of the body, large developed eyes on both sides, a mouth in the center, and horny jaws in the mouth;Around the mouth is a circle of wrists that can be used to prey on other animals.Some cephalopods have shells, some have inner shells, and some have no shells.Cephalopoda also has a long evolutionary history, from Cambrian to modern times.
Arthropoda is a class of ancient molluscs with radially symmetric, single conical shells, distributed from Ordovician to Devonian, and represented by Arthropoda, etc.
The mollusca is also a kind of ancient molluscs with a single conical shell, but they are bilaterally symmetrical, distributed from Cambrian to Ordovician, representing the mollusca, etc.
In 2023, the international scientific research team found the "Burgess shale type" special buried fossil pool – Castle Beach Biota in the Middle Ordovician strata of Wales, England, and found more than 170 kinds of organisms, includingMolluscs[4]。
Main features
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Molluscs
The main characteristics are: the soft body surface hasMantle, mostly withshell；The motor organ isfoot。
Morphological structure of molluscsvariationLarger, but the basic structure is the same.The body is soft and has a hard shell. The body is hidden in the shell for protection. Since the hard shell will hinder the movement, they all move very slowly.It is not segmented and can be divided into three parts: head, foot and visceral mass. It is covered with membrane outside the body and often secretes shells.The foot is shaped like an axe and has two shells, such as a tree.
Marine mollusks
It is speculated that protomolluscs appeared beforeCambrian, living in the shallow sea, the body is oval, the body length is not more than 1cm, both sides are symmetrical, the head is located in the front, with a pair of antennae, and the base of the antennae has eyes.The belly of the body is flat and muscular, forming feet suitable for crawling.The back of the body is covered with a shield shaped convex shell, which protects the whole body.Shells may initially have beenKeratinIt is formed, called conchiolin, and then calcium carbonate is deposited on the conchiolin to increase its hardness.Under the shell isbody wallDouble layer formed by extending to the webCellular structureThe membrane of the shell, called mantle (or pallium), has a strong secretory capacity, and the shell is formed by the mantle.Under the coat filmVisceral sac。There is a cavity between the back end of the body, the upper part of the foot and the visceral sac, namely the mantle cavity, which is connected with the outside world.There are many gills for respiration in the mantle cavity, as well as the openings of the posterior kidney, anus, and genital pores.
Molluscs
The structure of protomollusk gills may be similar to the existing gastropod gills. It is composed of a long gill with triangular gill filaments extending alternately on both sides of the axial direction. This kind of gill is called a ctenidium.The gill axis extends outward from the mantle or body wall, which contains blood vessels, muscles and nerves. The front edge of the gill filament (i.e. the ventral edge) haschitinThe bone rod is supported to increase the hardness of the gill.The gills are fixed by the back and peritoneum on both sides of the mantle cavity, so the gills divide the mantle cavity into upper and lower chambers.Water flows into the mantle cavity from the lower chamber at the rear end of the mantle cavity, and flows out of the mantle cavity through the surface of the gill filament and the upper chamber. The front edge and surface of the gill filament are covered with cilia, and the swing of the cilia causes the flow of water in the mantle cavity.There are two blood vessels on the gill axisBlood vessel entering the gill(subordinate blood vessel), the ventral edge is the efferent blood vessel, and the blood flows from the entering branchial vessel to the exiting branchial vessel, and also from the microvessels on the surface of the gill filament directly from the dorsal edge to the ventral edge, so the direction of blood flow is just opposite to the direction of water flow on the surface of the gill, which can more effectively conduct gas exchange.
Nudibranchs
Like many existing species, protomolluscs are not only covered with cilia on the surface of gills, but also covered with cilia on their mantle and skin (including foot skin). The oscillation of these cilia causes continuous flow of water, which is conducive to gas exchange and predation. Therefore, skin respiration in protomolluscs is very important.FootCiliary movementIt also combines with the contraction of muscles to form the movement of the body.Protomolluscs may be herbivorous, feeding on algae growing on shallow sea rocks.It has the same feeding structure as the existing molluscs.The mouth is located at the front of the head, and behind the mouth is the buccal cavity. At the back of the mouth is a pocket shaped lingual sac (radulasac).At the bottom of the odontoglossal sac is a membrane band that can move back and forth. There are rows and orderly rows of chitin fine teeth distributed on the membrane band. The tooth tip is backward, and the membrane band and teeth form the radula. At the bottom of the odontoglossal sac are teethcartilage(odontophora), the tongue and cartilage are attached with extensor muscle and retractor muscle, which can stretch out of the mouth by muscle expansion, cartilage and membrane band to scrape food. Due to the abrasion of feeding, the fine teeth at the front end gradually age and lose, and the membrane band can continuously secrete new teeth at the back end. The replacement speed is several rows a day in many mollusks.This tooth tongue structure exists in most living mollusks.There is a pair on the back of the mouthsalivary glandsIts secretion can smooth the tooth tongue and stick the food particles entering the mouth together to form a food cord, which enters the stomach through the esophagus.The front end of the stomach is hemispherical. One side of the inner wall of the stomach has a chitin plate, called a gastric shield. The opposite side forms many small ridges and grooves, in which there are cilia, called a sorting region.The back half of the stomach is saclike, calledCrystal rodThe style sac is called crystalline because it has a colloidal rod structure. The inner wall of the crystalline sac also has wrinkles and cilia, which also formCiliated sulcus。After food is stuck by mucus in the mouth to form food cord, it continuously enters the stomach. The viscosity of food cord is removed by gastric acid (the pH value of existing gastric juice is generally 5-6), so that food particles in the cord are free. At the same time, food is screened by the cilia in the stomach screening area, and small food particles are passed through the upper stomachDigestive glandThe tube is sent into the digestive gland, which is the place where food is digested and absorbed in cells;The larger food particles are digested outside the cell in the stomach;The undigested food enters the intestinal tract through the folds of the stomach wall, and then partially digests from the intestinal tract, finally forming fecal particles in the intestinal tract.The anal opening is at the rear end of the mantle cavity. The formation of fecal particles reduces the pollution of the external mantle cavity, and fecal particles can be discharged from the body by water flow.The body cavity of protomollusks is located in the middle back of the body. It surrounds the heart and part of the intestine, so it actually represents the surrounding heart cavity and the surrounding visceral cavity.The heart consists of a ventricle at the front end and a pair of auricles at the rear end. The arteries flowing from the ventricles form small blood vessels through branches, and finally form blood sinuses into tissue spaces, which then converge into veins through blood sinuses, and flow back to the auricles and ventricles through kidneys, gills and other blood. This is an open circulation.The blood contains deformable cells and respiratory pigments, which are mainlyHemocyanin（hemocyanin）。
The excretory organ is the posterior kidney, which is located on both sides of the pericardial cavity. One end of the posterior kidney is connected with the pericardial cavity, which is called the inner nephrostome, and the other end is connected with the outside world, which is called the outer nephropore.The pericardial cavity receives the metabolites released from the heart and the glands in the pericardial cavity, which enter the kidney from the renal orifice along with the pericardial cavity fluid. The kidney has a certain capacity of re absorption. It recycles the useful salts, and the useless wastes become urine, which is discharged out of the body through the external renal pores and the mantle cavity.
The neural structure of protomollusks is very simple. It forms a nerve ring around the esophagus, and two pairs of nerve rings are separated from the nerve ringNerve cord, a symmetrical foot cord on the ventral surfaceMuscle contraction；A symmetry on the backVisceral nerveVisceral cord, which controls the movement of viscera and coat.According to the speculation of existing mollusks, their senses may also include a pair of eyes and a pair of feetBalancing bursa(statocyst), and a pairOlfactometerOsphoradia, located on the lower gill membrane, is aChemoreceptor。
Schematic Diagram of Noodle Larvae
The reproductive system consists of a pair of gonads, which are located in the front, middle and back of the pericardium,Hermaphrodite, no reproductive ducts,spermOr the egg is released to the perinatal cavity when it is mature. Because the gonadal cavity is also part of the body cavity, the germ cells are discharged out of the body through the perinatal cavity and then through the kidney.Fertilization occurs in sea waterembryonic developmentIt may be very similar to the existing molluscs, and typicalSpiral cleavageThe blastocyst hole forms an opening, and then forms trochophore after passing through the gastrula.The trochlear is pear shaped, with a typical prototroch, which surrounds the body at the upper part of the mouth. The first four groups at the time of cleavage are at the top, with bundles of cilia at the top.Trombola larvae not only appear in molluscs, but also inAnnelida。
Schematic Diagram of Roller Larvae
The original species only passed through the trochlear larva in their development. Most of the existing molluscs had a very short trochlear larva period, and then entered the veliger larva stage.Flour dish larvaThere are foot, shell, viscera and other structures.It is speculated that protomolluscs do not have the dough disk larva stage. They lose the preoral cilia wheel from the trochophore larva, metamorphose into adults, and begin to camp on the bottom of the sea.
Body division
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brief introduction
The body of mollusks can generally be divided into head, foot andVisceral massThree parts.
head
Position at the front of the body.The species that are quick in movement, with obvious differentiation of head, on which there are eyes, antennae and other sensory organs, such as snails, snails and cuttlefish;The head of slow moving species is underdeveloped, such as chitons;Cave dwellingOr the head of the species with fixed life has disappeared, such as clams, oysters, etc.
foot
It is usually located on the ventral side of the body and is a motor organ. It often has different shapes due to different lifestyles of animals.Some feet are developed in leaf, axe or column shape, and can crawl or dig sediment;Some feet degenerate and lose sports function, such as scallops;There is no fixed living species, such as oysters;Some feet have been specialized into wrists, born on the head, and are prey organs, such as squid and octopus. They are calledCephalopod；The side of a few species of foot (i.eLateral footParapodium) is specialized into sheets and can swim, called wings or fins, such as Pteropoda.
Visceral mass
（visceral mass）
It is the part of internal organs, often located on the dorsal side of the foot.Most kinds of viscera are symmetrical from left to right, but some are twisted into spirals, losing their symmetry, such assnails 。
Mantle
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（mantle）
shell
It is made of skin folds on the back of the body extending downwards, often covering the whole visceral mass.The cavity formed between the mantle and visceral massMantle cavity（mantle cavity）。There are often gills in the cavity, enough to reach the anusRenal foramenThe planting hole is opened in the mantle cavity.The outer mantle is composed of two layers of epithelium. The secretions of the outer epithelium can form shells. The inner epithelium has cilia,ciliaSwing, causing water flow, which circulates in the jacket cavity to complete breathing, excretion, feeding, etc.One or two of the left and right sheaths often heal at the rear edge, forming an outlet siphon and an inlet siphon.Some kinds of water inlet and outlet holes are extended into tubes, which are called outlet pipes andWater inlet pipe。
shell
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（Shell）
shell
It is an important feature of molluscs to have shells outside the body, so the discipline of mollusc research is also calledMolluscology（Malacology）。Most molluscs have one or two or more shells with different shapes.Some are hat shaped;The snail is spiral;Tunipeds are tubular;Lamellibranchia are petaloid.Some kinds of shells degenerate into inner shells, and some have no shells.Shells have the function of protecting the soft body.
Pearl oyster
The shell is mainly composed of calcium carbonate and a small amount of shell matrixShellatin）These substances are composed of mantleepithelial cellsSecreted.The shell structure can generally be divided into three layers), and the outermost layer iscuticle(periodicum), very thin, transparent, shiny, composed of shell matrix, free from acid and alkali erosion, can protect shells.The middle layer is the ostracum, also known as the prismatic layer, which occupies most of the shell and is composed of angular calcite.The innermost layer is the bottom of the shell, that is, the pearl layer, which is rich in luster and composed of aragonite.The outer and middle layers are secreted from the edge of the mantle, which can gradually increase with the growth of animals, but not thicken;The inner layer is secreted by the entire mantle and can increase in thickness with individual growth.Pearl is formed from nacreous layer.When the outer mantle is stimulated by the invasion of small sand particles and other foreign bodies, the epithelial cells at the stimulated site will sink into the epithelium of the outer mantle with the foreign body as the nucleusconnective tissueThe trapped epithelial cells divide themselves to form pearl sacs. The sacs secrete nacre, layer by layer wrapping the nuclei to gradually form pearls.According to historical records, freshwater pearl cultivation has been recorded in China for more than 2200 BC (Book of Books, Yu Gong Chapter). Guangxi Hepu Yuzhu has been famous since ancient times, and its picking began in the Han Dynasty.
The growth of cuticle and prismatic layer is discontinuous. Because food, temperature and other factors affect the secretion function of mantle, the growth rate of shell is different, so theGrowth line, indicating the growth rate.
digestive system
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MolluskAlimentary canalDeveloped, a few parasitic species (Entocolax) degenerated.Most species have mandibles and radulas in their mouths, one or two of which can assist in predation.Tooth tongue is a unique organ of mollusks. It is located on the surface of the odontophore at the bottom of the oral cavity. It is composed of horizontal horny teeth, like a knife.When taking food, the teeth and tongue make fore and aft stretching movement to scrape food.The shape and number of small teeth on the lingua differ from species to species, which is one of the important characteristics for species identification.The small teeth form a horizontal row, and many rows of small teeth form a tongue.Each horizontal row has one central tooth, one or more pairs of left and right teeth, and one or more pairs of marginal teeth.The arrangement of the small teeth on the tongueTooth typeRepresent ascipangopaludina chinensis (Cipangopaludina chinensis) is 2.1.1.1.2.
circulatory system
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MolluskSecondary coelomExtremely degenerated, with residual pericardial cavity and internal cavity of gonad and excretory organ.The primary body cavity is located in the space between various tissues and organs, where there is blood flow, forming blood sinuses.
circulatory systemIt is composed of heart, blood vessels, blood sinuses and blood.The heart is generally located in the heart cavity around the back of the visceral mass and consists of the auricle and ventricle.There is one ventricle with thick wall, which can move and is the power of blood circulation;The auricle is one or in pairs, often consistent with the number of gills.There are valves between the auricle and the ventricle to prevent blood reflux.Blood vessels differentiate into arteries and veins.The blood flows from the ventricle through the artery to all parts of the body, then flows into the blood sinus, and returns to the auricle from the vein, so the mollusk is open tube circulation.Some types of fast swimming areClosed loop。The blood is colorless and containsAmoebaLike cells.Some kinds of plasma contain hemoglobin orHemocyanin(haemocyanin), so the blood is red or blue.
respiratory organ
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For aquatic speciesGill respirationThe gills are formed by the extension of the epithelium on the inner surface of the mantle, which is located in the inner cavity.The shapes of gills are different. There are gill filaments on both sides of the gill axis, which are feathery and called shield gills;Only one side of the gill axis has gill filaments, which are in the form of combs, called ctenidium;Some gills are petaloid, called lamellibranch;In some species, gills grow into filaments.It is called filibranch.Some of the gills disappear and grow on the surface of the dorsal skinSecondary gill(secondary branchium), and some species have no gills.The gills are paired or single, with different numbers, ranging from one or a pair to dozens of pairs.All species living on land have no gills.The tiny blood vessels in a certain area inside the outer lumen form a dense lung, which can directly absorb oxygen in the air.This is an adaptation to life on land.
Excretory organ
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Molluscs
The excretory organs of mollusks are basicallyMetanephric ductIts number is generally consistent with the number of gills, and only a few species of larvae areProtonephric duct。The metanephric duct is composed of glandular part and tubular part, and the glandular part is rich in blood vessels,NephrostomeCiliate, opening in the pericardium;The tubular part is a thin-walled tube with cilia on the inner wall, and the renal pore opens in the mantle cavity.The metanephric duct can exclude not only the metabolites in the heart, but also the metabolites in the blood.In addition, the surrounding visceral glands on the inner wall of the heart are densely covered with microvessels, which can exclude metabolites from the surrounding heart and excrete them from the body through the metanephric duct.
nervous system
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The primitive nervous system has no differentiation of the god meridian segment, only the parapharyngeal nerve ring and a pair of foot cords and a pair of lateral cords protruding backward.Higher species mainly have 4 pairs of ganglia, each of which is connected by nerves.The cerebral ganglion is located at the dorsal side of the esophagus and sends out nerves to the head and front of the body to sense;Ganglia pedis(foot ganglion) Position the front of the foot, extend the nerve to the foot, and master the movement and sensation;The lateral ganglion sends out nerves to the mantle and gills;The visceral ganglion sends out nerves to the internal organs.These ganglia tend to be concentrated. Some main ganglia are concentrated together to form a brain surrounded by cartilage, such as cephalopods.Molluscs have differentiated sensory organs such as antenna, eye, olfactory detector and balance capsule, which are sensitive.
Reproductive development
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Most molluscs are dioecious, and many species are dioecious;Some are hermaphrodite.Most of the cleavage types are completely unequal, many of which are spiral.The minority isIncomplete cleavage。OntogenyTwo stage larvae of middle and veliger larva, morphology of trombola and annelidaPolychaeteIn the early stage of the development of the larvae of the facial disc, there is a mantle primordium on the back and a secretory shell, and a foot primordium on the ventral side. The preoral ciliary ring develops into a velum or facial disc.Some species are directly developed.Freshwater mussels have specialHookworm larva（glochidium）。
type
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brief introduction
Molluscs have a wide range of species and live in a wide range, including sea water, fresh water and land.More than 130000 have been recorded, second only toArthropoda。The structure of molluscs is more complex and their functions are more perfect. They have some characteristics similar to annelids: secondary coelom, metanephric canal,Spiral cleavageIt is believed that molluscs evolved from annelids and differentiated early towards an inactive lifestyle.
Molluscs are mostly covered with various shells outside their bodies, so they are often called shellfish.Because most of their shells are gorgeous, delicious, nutritious and easy to catch, they have been used by humans since the ancient fishing and hunting period.Many of them can be used for food, medicine, agriculture, arts and crafts, and some of them are poisonous, which can spread diseases, harm crops, damage harbor buildings and transportation facilities, and be harmful to human beings.
Molluscs include gastropods that people are familiar with in life, such assnail, Snailslug；BivalveMussel、Scapharca subcrenataEtc;Cephalopod squid（Cuttlefish）、octopusEtc;And the multi plate chitons attached to the rocks in the coastal intertidal zone.They are very different in morphology, for example, their systems are either symmetrical or asymmetrical;The body surface either has a shell or has no shell;The shell is either one or two or more.However, based on the comparative morphology of existing speciesembryologyAnd the research ofFossilOfPaleontologyThe research found that all molluscs are built on a basic model structure, which is the protomolluscs, that is, the ancestor model of molluscs, which evolved from protomolluscs to different classes.Therefore, protomolluscs represent the basic characteristics of all molluscs.
According to the research on existing animals, it is assumed that protomolluscs have formed the structural characteristics of existing animals of various classes through the changes of the anteroposterior axis and the dorsal ventral axis of the body, and the displacement of the foot, visceral sac, and mantle cavity.
Molluscs can be divided into 8 classes:monoplacophora （Monoplacophora）、Polyphylla（Polyplacophora）、Anamorpha（Aplacophora）、Gastropoda（Gastropoda）、Bivalvia（Bivalvia）、Excavatopoda（Scaphopoda）、Cephalopoda(Cephalopoda), (Caudofoveata), sometimes people will.Among them, only gastropoda and bivalves have freshwater living species, and gastropoda also hasTerrestrial Species, these two classes contain more than 95% of the mollusc species, and other classes are marine life.
Gastropoda
(Gastropoda) 1 spiral shell with operculum at the shell mouth;Obvious head, eyes and antennae, teeth and tongue in mouth;The visceral mass is generally helical with the twist of the spiral shell, and is asymmetric from left to right;Some species are viviparous;Marine species have trochlear larva stage and noodle larva stage;There are about 75000 species in existence, which are widely distributed in the oceans, freshwater rivers and land, and can be divided intoPrebranchia、Retrobranchia、Pulmonary SpiraeaClass 3.
Bivalvia
(Bivalvia) The body is flat and symmetrical;There are two shells on the body surface, so it is called bivalve;The head is degenerated, without tooth tongue;The feet are developed and shaped like axes, hence the nameAxopods；There are 1~2 pairs of gills in petal shape, so they are called Lamellibranchia;The nervous system is relatively simple, with three pairs of ganglia: brain, viscera and foot;Marine species often have trochlear larvae and pasta larvae, while freshwater mussels have hookworm larvae;There are about 30000 existing species.
Cephalopoda
Cephalopoda) body is divided into three parts: head, foot and trunk;Except for the primitive species such as nautilus, which have shells, the rest are inner shells or without shells;The head is developed, and there is a pair of well structured eyes on the head side;The foot specialized into 8 or 10 wrists and 1 funnel, and the wrist ring was born in front of the head;Closed pipe circulation system；The nervous system is complex, with ganglia concentrated in the head and mesoderm formedCartilaginous boxProtection;There are about 756 species, all of which are marine products.
Excavatopoda
(Scaphopoda) shells are long conical, slightly curved tubular or ivory shaped, with openings at both ends, so they are also called tracheids;The foot is developed and cylindrical;The head degenerates into a protuberance at the front end;All marine products, mostly live in the sand and burrow, filter and feed on plankton;There are about 350 species, including only two families of hornworts and phoebellidae.
Anamorpha
(Apacophora) is the original species;The body is worm like;No shell;The body length is generally about 5cm, slender or thick, with underdeveloped head and degenerated feet;Ventral sulcus;All marine products, most of which live in caves in soft mud, and a few can crawl in coral reefs. There are only about 300 species, most of which belong to crescent shellfish, and a few belong to fur shellfish.
Polyphylla
(Polyplacophora) The body is flat and oval;The head is not obvious;There are 8 shells arranged like tiles on the back;Trapezoidal nervous system;They generally feed on the seaweed scraped from the reef by the tongue;Indirectly developed, with trochlear larvae and dough plate larvae;There are about 600 kinds of marine products, which are divided into Lepidoptera and Chitosana.
monoplacophora
For a long time, people have always thought that veneer is an extinct mollusc, because only inCambriananddevonianTheir fossil species have been found in the strata of.However, in 1952, the Danish "Galathea Expedition" found 10 specimens of living veneer animals, Neoplina galatha, at the bottom of the sea 3350 m deep off the coast of Costa Rica.Thus, people have renewed their interest in the board class.Since then, seven different species have been found in many areas of the Pacific Ocean and the South Atlantic Ocean at depths of 2000~7000m, making this primitive mollusc of new research value.The new butterfly shell is 0.3~3cm long, and has a symmetrical, flat projection shell on both sides, or a low conical shell. The shell top points to the front, so it is called veneer.The external morphology of the new butterfly shell is similar to that of the chiton.The head is very underdeveloped, the abdomen * * has flat and wide feet, and the mantle is separated from the feet by a palial groove.The mouth is located in the front of the abdomen and foot, and the anus is located in the outer groove of the back end of the body.There is a pair of large ciliated whisker structures on both sides of the front of the mouth, called velum.Behind the mouth are a pair of pleats, called postoral tentacles.There are 5-6 pairs of single comb gills (gill filaments on one side of the gill axis) in the mantle groove.There are 8 pairs of foot retractor muscles on both sides of the body. There is a tooth tongue in the mouth. There are also developed digestive glands. There are also crystal rods and crystal rod capsules in the stomach. The contents of the stomach containdiatom, foraminifera, sponge spicules and other debris, so the new butterfly shell is also a sediment feeder.The intestine is highly convoluted.There is a pair of ventricles and two pairs of auricles on both sides of the rectum at the back end of the body, which are respectively enclosed in a pair of pericardial cavities.The blood vessels from the two ventricles combine to form the anterior great artery, which is also an open circulation.The new butterfly shell has 6 pairs of posterior kidneys. Except for the first pair, the other pairs open in the body cavity at one end and open to the outer sheath groove at the other end.The nervous system is also similar to that of chitons. There is a nerve ring around the mouth, and there are two pairs of nerve cords, namely foot nerve cords and lateral nerve cords, which are connected by transverse nerves.It is a hermaphrodite, with two pairs of gonads and reproductive ducts, which are connected to the two pairs of posterior kidneys in the middle, so the germ cells are still discharged into the body through the renal pores for in vitro fertilization.As the new butterfly shell lives in the deep sea, little is known about its ecology and development, but its projection shell, creeping feet, and cephalization are not obvious, and the repeated arrangement of tooth tongue, gills, kidneys, and muscles all indicate their primitivity.Today, many zoologists believe that it is very likely that the monotremes are the ancestors of existing gastropods, bivalves and cephalopods.
Caudate
Caudate, English name: Caudofoveata.One of the classifications of molluscs.
Caudate
The body is in the shape of a fine worm. The head and trunk can be clearly distinguished. There is a excretory cavity at the back end of the body, in which there is a pair of pinnate gills.There are no calcareous plates and shells, but the cuticle with calcareous acicular spines is covered.It lives in the sea area with deep mud bottom environment, such as fur shellfish.
Origin and evolution
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There are two theories about the origin of molluscs: one is that molluscs originated fromFlatfish；The other believes that molluscs and annelids evolved from a common ancestor, but because they developed towards different lifestyles in the long-term evolution process, they finally formed two different types of animals.The reason for the latter statement is quite sufficient, because there are manyMarine productsThe species of molluscs also have a trochlear larva stage during embryonic development like many annelids[3]。In addition, the two types of animals have cleavage in their development, and they share some common features in some modifications in adults.For example, the excretory organs basically belong to the metanephric type, and the body cavity is secondary.
Part of this common ancestor developed towards a way suitable for activities, forming a segment, warty foot and developed head, which are annelids;The other part is developing towards a relatively inactive road, which produces a protective shell and many structures suitable for movement, such as segmentation and head, or no appearance or degradation.At the same time, some unique structures of mollusks - mantle have also been developed.There is no more obvious difference between various groups of molluscs to explain the genetic relationship between each other.
In molluscs, the double nerve class is relatively primitive, because its system is symmetrical, the secondary body cavity is relatively developed, and the original trapezoidal double nerve system is retained.Gastropoda is a lower class group, because it has a similar trochlear larva stage of annelids or a similar facial disc larva stage.The most prominent characteristic of the petal branchia is the petal gill of the respiratory system.Take the living clam as an example, each petaloid gill is a gill flap, which is composed of two small gill flaps, and the outer one is called the outer oneBranchial lobuleThe inner flap is called the inner cheek flap.Each gill flap is composed of many gill filaments. There are cilia on the surface of the gill filaments, blood vessels inside, and many holes.In the space between the branchial leaflets, there are septal diaphragms, which form many branchial water tubes.Due to the oscillation of cilia, water enters the mantle from the inlet pipe, and then enters the gill water pipe through the gill hole, and then rises toSuprabranchial cavityFinally, it flows out of the body through the outlet pipe.In the process of water flowing through the gill filament, gas exchange is completed in the blood in the gill filament.This kind of animal has two mantle membranes, so it has two petal shells. Their lower species have wide and flat feet at the bottom, crawling, and the development process also presents trochlear larvae, so they may share a common ancestor with gastropods.The body of a cephalopodStructure heightDeveloped. Brain, eyes and circulatory system are the most evolved among mollusks. The mollusks found in the stratum are also cephalopods. It is also possible that due to adapting to the social mode of rapid activity, evolution has developed rapidly towards specialization[2]。
Abalone shellCassia obtusifolia）Baby's shell is called Haiba, pearl and squid's shell is calledCuttleboneAs well as clams, oysters, clams, clams and other shells are commonly used in traditional Chinese medicine.fromAbalone, chicken snailSea snail, clams, oysters, cuttlefish, etc. can extract antibiotics and anti-tumor drugs.
Agricultural value
Small mollusks with high yield can be used as farmland fertilizer or feed,Corbicula flumineaFreshwater fish can be raised.Industrial use.Mollusc shells are good raw materials for burning lime.Shells with thick nacre (such asClam、Horseshoe snailIt is the raw material for making buttons.
Craft or decoration
The shells of many shellfish have unique shapes and patterns, rich in luster, and are colorful, which is a favorite collection for people at all times and in all over the world.
Geological value
MolluscaDuring the geological history, there are manyFinger fossil。In the world and ChinaCambrianAt the bottom of,MesozoicMany of the ammonites have become zone fossils for stratigraphic division and correlation within the intercontinental range, and some can be used to understand the temperature and salinity of ancient waters;Snail fossilIt can reflect the Quaternary climate environment.
Harm to people
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Terrestrial snails, slugs, etc. eat leaves and buds of plants, and harm vegetables, fruit trees, tobacco, etc;Some carnivorous species in the sea can kill the seedlings of oysters, clams, etc., resulting in the loss of cultured bivalves;Some herbivorous species often eat kelp and laver seedlingsAlgae cultureThe enemy of.In freshwater and terrestrial molluscs,ConchyesFasciola hepaticaThe intermediate host of the bean snail isClonorchis sinensisIs the intermediate host of fasciola, short furrow curl is the intermediate host of paragonimus, and oncomelania is the intermediate host of fasciolaSchistosoma japonicum It is the intermediate host for human beings, which is very harmful to human beings.Shipworms in the seaAsparagusIt is a kind of cave dwelling species specialized in drilling wood or rock, which is harmful to wooden boats and piles in the sea and wood and stone buildings in the harbor.A large number of camp attached or fixed living species often attach to the bottom of the ship, which can affect the speed of the ship.Some species attached to life can block water pipes and affect production.
Related differences
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First, mollusks' shells are formed by the calcareous matter secreted by their coats. Except that there are no joints at the joints, mollusks will not molt their shells all their lives.The body of an arthropod is covered with chitinExoskeleton, also known as epidermis or cuticle.On the articular membrane between adjacent segments, the stratum corneum is very thin and easy to flex.The joints of the appendages can also be moved.Arthropods should molt regularly during their growth.
Second, mollusks are softinvertebrate。Molluscs generally have a symmetrical body shape, but some molluscs have a variety of strange shapes due to body twisting.They often have a shell, no joints, and most of them can be divided into three parts: head, foot, visceral sac, etc.The outer layer of skin folds from the back into a skin film, called a coat.The coat surrounds the body and secretes limestone.
Crustaceabelong toArthropodaArthropods are composed of one line of segments, which can be divided into three parts: head, chest and abdomen, or the head and chest are fused intoCephalothorax, or the chest and abdomen are fused to form a trunk, each of which has a pair of segmented appendages.There are two types of appendages: double branch type and single branch type.The circulating system is open tube type.Aquaticrespiratory organFor gills or book gills,Terrestrial The trachea or the book lung or both.Primitive arthropods exchange gases on the body surface.The nervous system is centralizednerve cord 。There are touch, taste, smell, hearing, balance andvisionAnd other sensory organs.Eyes haveMonocularAnd compound eyes.The compound eye is composed of individual eyes, which can sense the movement and shape of external objects, adapt to light intensity and distinguish colors.
In addition, the morphology of crustaceans varies greatly. The smallest one, such as Daphnia magna, is less than 1mm long.maximalGiant crabThe width can reach 4m when the two chelates are extended.The body is long tube shaped, with distinct segments, and the whole body is divided into three parts: head, chest, and abdomen.The head is composed of 6 individual segments.CrustaceaThe exoskeleton of each body segment is composed of two parts: one on the back is the dorsal armor, and the other on the ventral side is the abdominal armor.The two sides of the dorsal armor often extend outward (downward), which is the lateral armor, and appendages are usually born on both sides of the abdominal armor.It can be seen that arthropods are higher than molluscs[2]。
Protect wild animals
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In February 2021, the State Forestry and Grassland Administration of China and the Ministry of Agriculture and Rural Affairs jointly released《Catalog of Wildlife under Key State Protection》The scope of wildlife under national key protection was clarified, which played a positive role in strengthening species rescue and protection, combating indiscriminate hunting and illegal trade, and improving public awareness of protection.In the List of National Key Protected Wildlife, two molluscs, giant tridacna and nautilus, are classified as national first-class protected animals;Nineteen kinds of molluscs, including giant pearl mussel, scaleless tridacna, scale tridacna, long tridacna, saffron tridacna, triclam, pearl mussel, frederie mussel, silk mussel, mussel with back tumor, mussel with multiple tumors, clam with carved cracks, Chinese freshwater clam, clam with dragon bone, spiral snail, noctilucent salamander, babe tiger spot, Tangguan snail, and French snail, have been designated as national secondary protected animals.[5-8]
