Shellfish, i.eMolluscsClass of.It is three germ layers, symmetrical on both sides, and has trueCoelomOf animals.The true coelom of mollusks is formed by the split coelom method, that is, the coelom formed by the mesoderm.However, the true coelom of mollusks is not developed, and only exists in the pericardiocoelom and gonadal coelom.Molluscs vary greatly in morphology, but can be divided into four parts in structure: head, foot, visceral sac and mantle.The head is located at the front of the body, and the feet are located at the back of the head and the ventral surface of the body. It is a multi muscular and fleshy motor organ protruding from the body wall. The visceral sac is located at the back of the body, and is an internal organ surrounded by the soft body wall. The mantle is a membrane or a pair of membranes formed by the extension and drooping of the body wall on the back of the body. The cavity between the mantle and the visceral sac is the mantle cavity.Calcium carbonate is secreted from the mantle to the body surface, forming one or two shells around the whole body. A few kinds of shells are surrounded by the body wall or completely disappear.These basic structures have great changes and differences in different classes.Molluscs have complete digestive tract, respiratory and circulatory systems, and more evolved than the original kidneyMetanephric（metanephridium）。Molluscs are various and widely distributed.There are more than 110000 existing species and 35000 fossil species, which is the second largest category in the animal kingdom after arthropods.In particular, some molluscs use "lungs" to breathe, and their bodies have the ability to regulate water, making molluscs and arthropods the only terrestrial animals suitable for living on the groundinvertebrate。
Due to the different waters where they live, shellfish can be divided into different species - some live in salt water, such as the sea, or live in fresh water, such as rivers and streams.The most famous marine shellfish is the grayish black mussel, and the pearl mussel is widely distributed in the freshwater basin.The breeding season of shellfish is between spring and summer.The female shellfish lays eggs in the water, and correspondingly, the male shellfish also discharges mature sperm into the water.A certain chemical signal is used between them to ensure simultaneous emission.Sperm and egg meet in water and fertilize.Soon, the fertilized eggs hatch out small larvae.These larvae gradually develop into young shellfish.After about four weeks, they grew out of their shells and fell to the bottom.They stay there and gradually attach themselves to passing fish like a ship throwing an anchor.This "anchor" is some thin flagella, which is also called "beard"[1]。
There are many kinds of shellfish, such as red shellfish, salamander, snail, conch, clam, clam, etc[2]。
Form and structure
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The body of shellfish is soft, symmetrical and undividedAxe foot、Visceral sac, mantle andshell5 parts.The head has sensory organs such as mouth, eyes and antennae.The foot of the axe is on the belly of the body and is composed of strong muscles. It is an organ for crawling, digging mud or swimming.Visceral vesicles are located on the back of the body, includingheart, kidney, stomach, intestineDigestive glandandGonadAnd other internal organs.The mantle is wrapped around the outside of the body and consists of two layers of epidermis, connective tissue and a few muscles.MantleEpidermal cellSecretory shells, mantle and shells are protective organs of shellfish.
The nervous system of shellfish consists of four pairs of ganglia of brain, foot, side and viscera and the nerves connected with them.Cerebral nerveThe ganglion is located on the dorsal side of the esophagus and sends nerves to the head and front of the body;Ganglia pedisLocated in the front of the foot, it sends nerves to the foot;Lateral gangliaLocated in the front of the body, it sends nerves to the coat and gills;The visceral ganglia are located in the back of the body and send nerves to the visceral organs.The primitive species of shellfish have simple nervous system and no significant ganglia. The more evolved species form ganglia, while the more evolved species form "brain" with ganglia concentrated in the head.sense organIt mainly includes antenna, eyeBalancing bursa、OlfactometerEtc.
The digestive system includes the mouth, lingual esophagusGastrointestinal, anus and accessoryDigestive glandAmong them, the tongue is a special organ for licking and grinding food of shellfish, and is one of the important basis for classification.
ShellfishgillAnd lung breathing.The aquatic species have gills, which are usually formed by the extension of the skin inside the mantle, and are called the gills.There are gill filaments on both sides or one side of the gill axis of each gill slice, and cilia on the gill.According to the movement of cilia, the respiratory water flows through the gills in a certain waygas exchange。Some species lose their own gills and breathe with the secondary gills (posterior gills) formed on the skin surface or on the skin surface.A part of the mantle of terrestrial species forms a pulmonary chamber with dense pulse network, so as to breathe in the air.
The circulating system is generally open tube type, but in advancedCephalopodsArterial and venous tubes are connected by microvessels to form closed tubes.The center of the shellfish circulatory system is the heart, which has one ventricle, one, two or four auricles.The blood contains hemocyanin, which is generally colorless. Only a few species, such as bivalve clams andGastropodsOfSnailThere is heme, and the blood is red.
The main organ of the excretory system is the kidney.Kidney constitutionciliaOfnephridiumForming, one end andPericentric cavityAnd the other end is opened in the outer sleeve cavity.The number of kidneys varies according to the type, including 6 pairs, 2 pairs, 1 pair or only 1 pair.In addition to the kidney, some kinds of glands around the heart wall or part of the liver also have excretion function.
The reproductive system includes gonads, reproductive ducts, connectors and some accessory glands.HermaphroditeorHermaphrodite。
Biological characteristics
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The lifestyle of shellfish varies from species to species.Terrestrial species belong to gastropods, which use muscular feet to crawl on land.
The aquatic life style includes floating, swimming, crawling, fixation, perforation and parasitism.The species of planktonic life drift in the water.Generally, they are small and have thin or no shells. Some species specialize into fins, such as many species of Pteropoda and Heteropoda;Some species can secrete oneVesicle, carrying animals floating on the ocean surface, such as the seasnail（Janthina）。Swimming species can migrate long distances in the ocean, such as squidSquid、Squid（Ommastrephes）Their feet are specialized into wrists andfunnel，CarcassThere are fins on both sides. You can swim quickly and steadily by spraying water from the funnel and swinging the fins.Some bivalves such asScallop in Shell, Chlamys farreri（Chlamys）、Sun moon shellfish（Amussium）, filing clams（Lima）Although they are not species of swimming life, they can perform butterfly swimming in the sea by virtue of the rapid opening and closing of shells and the role of mantle tentacles when necessary.Most aquatic shellfish live on the bottom, or crawl and crawl on the bottom, or dig holes in the bottom, or attach to other objects.For example, jade snail and mud snail（Bullacta）Wait and crawl at the bottom of the sand, abaloneHorseshoe snail（Trochus）、Salamander（Turbo）Wait to crawl on the rock, someNudibranch asManatee（Doris）、 Radish snail living in fresh water（Radix）、Snail（Planorbis）Waitingaquatic plantClimb up.Their foot muscles are particularly developed, and the plantar surface is wide and flat, which is suitable for crawling.Many benthic shellfish live in burying habitats, and most bivalves belong to this type.Their feet have well-developed muscles and are shaped like axes. They are suitable for digging mud and sand on muddy sand beaches and burying their bodies underground, such as clams（Venus）Cherry clam（Tellina）、Razor clam（Solen）, Hailang（Mya）Wait, they are developedWater inlet pipeAnd the outlet pipe communicates with the bottom surface for feeding and breathing.Some benthic shellfish live in camps, such asMussel, scallops, clams（Anomia）The foot can secreteFilum pedisFor attaching to rocksCoral reef, other shells or objects.Oysters、Chama （Chama）、spondylus （Spondylus）Wait for oneScallop shellThese species generally do not move after being fixed to live on foreign objects.Some benthic shellfish live in holes on rocks, coral reefs, shells, bamboo, wood and other foreign objects, also known as perforated organisms, such asLithophaga （Lithophaga），Asteraceae（Pholadidae）Some species, rock clams（Saxicava）, shipworm（Teredo）, Matt sea bamboo shoots（Martesia）, Wood eating Sea Bamboo Shoots（Xylo-phaga）They all rely on developed water pipes and traffic outside the tunnel to draw seawater for breathing and water intakeMicroorganismAnd organic debris as food.Some shellfish also live as parasites.ExoparasitismSuch as cylindrical snail（Stilifer）, parasitic onEchinodermaCarpalStep grooveMedium;Internal parasitismSuch as inner shell screw（Entovalva）Parasitic on anchorsea cucumberIn the esophagus.
The breeding methods of shellfish vary from species to species.monoplacophora 、Polyphylla、Excavatopoda、CephalopodaAnd most of the front branchials are dioecious, while the back branchialsAnamorpha, probranchia andBivalveA small part and all lungs ofsnails BothHermaphrodite。There are also some species with sexual changes, such as some speciesOysters、shipwormandConch(Calyptraea）Etc.Some types of hermaphrodite are fertilized by mating, and some are fertilized by releasing sperm and eggs into water or the gill cavity or "uterus" of the mother.In hermaphroditic species, two individuals fertilize each other. In some species, many individuals mate in a row. The first individual only plays a female role, the last individual only plays a male role, and the middle individual plays both a female and a male role, such asHornless snails(Acera）、Sea hare(Aplysia）Etc.Some eggs are scattered into grains, and some areeggWrapped in the egg sheath, many egg sheaths stick together to form an egg group.The shape and structure of egg group vary with species, such asNatica （Natica）The egg and sediment are bonded into a collar,Red snail（Rapana）The egg sheath ofChrysanthemum；The eggs of sea hares are vermicelli, commonly known asSea flour。Each egg of cuttlefish is wrapped in a round capsule, which looks like a bunch of grapes.The number of eggs laid by shellfish varies greatly due to the protection it receives in the process of fertilization and hatching. Some species that produce eggs for fertilization and hatching in water have a particularly high number of eggs, such as the egg laying American oyster（Crassostrea virgnica）It lays tens of millions to more than 100 million eggs.Juvenile foodOysters（Ostrea edulis）Only hundreds of thousands to 1 million capsules will be produced.A kind of bay clam（Nucula）Capable of protecting the egg behind a shellchitinIn the cyst, only 20-70 eggs were laid.Primitive gastropodsThe eggs are directly laid in seawater for fertilization, and the number of eggs laid is large. For example, abalone can produce more than 100000 eggs, while the eggs are laid in the "womb" for fertilized and hatched species such as snails（Viviparus）Only dozens of grains are produced,Snail(Margarya）Only 3-5 were produced.Some species of shellfish will reachSexual maturity, the service life is only 1 year, such asNudibranch And some species of cephalopods.Some species have a long life span, but most of them reach sexual maturity within one year.The growth rate varies with species and environmental conditions.If the temperature is appropriate and the food is sufficient, the growth will be fast, otherwise, the growth will be slow.The life span of shellfish is the highest in bivalves, and that of mussels andMya （Mya arenaria）Can live for 10 years,Pinctada martensii（Pinctadamartensi）It can live for 12 years,ClamClass has a long life,pearl mussel（Margari-tana margaritifera）Can live for 80 years,Tridacna（Tridacna）It can even live for one century.
shellfish
The feeding mode of shellfish can be divided into predation and filter feeding.Predatory species can be divided into herbivorous and carnivorous.Many primitive gastropods, such as abaloneHorseshoe snail、Li BeiAnd many lungssnails , such asConch（Lymnaea）、snail、 Smoke pipe screw（Clausilia）They are herbivorous.MesogastropodaSome of them have water pipes, such as conch（Strombus）AndNeogastropodaMost of them are carnivorous. Some of them eat animal carcasses, and some eatHydra, ring worm, bivalvecrabs , fish, etc.Cephalopods are also carnivorous. They prey on bivalves, crabs, and even chase schools of fish.Most bivalves are filter feeding.
Population distribution
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It is mainly distributed in the ocean, and a few species live in freshwater lakes.
Physiological characteristics
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Digestion: In the original sediment feeding type, the food digestion and the structure of the digestive tract still maintain the shape and function of the original mollusk.For example, the stomach wall of protobranchials is very thin, in which the stomach projection, crystal rod and other structures are still retained. Food is digested extracellular in the stomach, and digested and absorbed intracellular in the digestive blind sac.In the advanced filter feeding species, there are no tentacles around the digestive tract mouth.The gastric projection and screening area on the gastric wall are not developed, while the rod capsule is developed and protrudes beyond the gastric wall.There is mucus secretion in the capsule, which is solidified to form a crystal rod. Digestive enzymes (amylase, lipase) are adsorbed on the crystal rod. The cilia in the capsule wall make the crystal rod rotate constantly, and the top of the crystal rod is dissolved to release digestive enzymes for extracellular digestion. The rotation of the crystal rod also plays a role in mixing food and enzymes.The top of the crystal rod is constantly worn by food, and the back end can be continuously supplemented. The rotation of the crystal rod also enables small food particles to enter the gastric caecum for intracellular digestion and absorption.Indigestible food residues are discharged into the body through the intestines, anus and outlet holes.A slow but continuous food flow in the digestive tract is unique to Lamellibranchial filter feeders.The septibranch is a carnivorous animal. Its muscular stomach wall is surrounded by chitin, forming a millstone. Its crystal rod is underdeveloped and extends into the stomach in a small stick shape, with relatively developed digestive enzymes.
Circulation and gas exchange: bivalve animals are open circulation, and the pericardium is located at the back of the body. There is a ventricle and two auricles in the pericardium, and there are valves between the ventricle and the auricle to prevent blood reflux.In protobranchials and filamentous branchials, the anterior great artery only flows forward from the ventricles, such as mussels; in lamellibranchials, in addition to the anterior great artery, the posterior great artery also flows backward from the ventricles, such as mussels.After flowing out of the artery, the blood will branch to the front of the body, feet and viscera, and form a blood sinus in the tissue. After passing through the blood sinus, it will converge into blood vessels, pass through the kidney and gills, and then flow back to the auricle and ventricle.The gill is its main gas exchange place. When water flows through, the oxygen absorbed by the gill is less than that of other molluscs, which may be related to the larger surface area of the gill than that of other molluscs.In addition, all bivalves have more or less developed mantle circulation. After the blood flows out of the artery, it directly flows into the mantle to form blood sinuses. The blood sinuses converge into blood vessels, or flow back to the auricle directly, or flow back to the auricle after the metabolic products are discharged from the kidney.The jacket circulation is also an auxiliary place for gas exchange.Most bivalves do not exist in the bloodRespiratory pigmentOnly a few species, such as clams and clams (Lima), have heme, which makes the mantle and other tissues red.The septibranch has disappeared, and the gas exchange is completely carried out by the mantle.
Excretion: The excretory system of bivalves is a pair of metanephras, located in the ventral side of the pericardium.The kidney is a long tube, but the opening of the inner renal orifice is at the front end of the pericardial cavity, and the opening of the outer renal orifice is below the inner renal orifice of the outflow, so the back end of the kidney is turned back. There are glands called glands in the front half of the kidney, where waste is filtered, followed by the bladder part, which is the storage place for metabolites. There is no distinction between the gland part and the bladder part in the protobranchial kidney.The kidney of freshwater species has the function of salt reabsorption, so the urine discharged is hypotonic.
Nerves and senses: The nervous system of bivalves is relatively simple. The original species has four pairs of ganglia: brain, side, foot, and Sheng. The more evolved species, brain, and side ganglia are combined, so there are only three pairs of ganglia. The brain side ganglia are located on both sides of the esophagus, controlling the anterior adductor muscle and coordinating the movement of the foot and the shell.The visceral ganglion is located on the muscle column of the posterior obturator muscle, which controls the contraction of the viscera and the posterior obturator muscle. The foot ganglion is located in the muscle at the front of the foot, which controls the movement of the foot. In addition, there are two pairs of nerve cords between the brain side ganglion and the visceral ganglion, and between the brain side ganglion and the brain foot ganglion to connect the ganglion.Sensory development is not developed. For some species with greater activity ability, such as scallops, there are pairs of small tentacles on the middle fold of the mantle edge, which contain tactile and chemical sensory cells. There are also many small eyes. The structure of the small eyes is relatively developed, and even contains crystals and omentum, which can sense the change of light intensity.In addition, there is a pair of balance capsules around the foot ganglion to control the balance of the body.Many species have some sensory epithelium under the posterior adductor muscle or around the water outlet, called olfactometer, which can sense changes in water quality and water flow.
economic value
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Most species of shellfish canedibleThe meat of many shellfish is tender, delicious and nutritious.The squid, spear squidSquid、octopus（Octopus）And other marine organisms, abalone, conchNeptunea cumingi（Neptunea）、Babylonia （Babylonia）、Snails（Voluta）、Red snail, and many terrestrialsnailThey are fishing objects, while abalone and others are breeding objects.Many kinds of bivalves such asArcidae （Arcidae）, Fanbeike（Pectinidae）、Mytillidae（Mytilidae）、Pearl fritillary（Pteriidae）、Oystidae（Ostreidae）、ClamSection（Mactridae）、Muridae（Veneridae）、Muscidae（Unionidae）、Solenidae （Solenidae）There are many kinds of resources in many families, such as, etc., which have developed into important objects of mariculture, and the output is also very considerable. In addition to fresh food, they can also be dried, salted or canned. The products include mussel (dried mussel)dried scallop(ScallopAdductor muscle）, dried oysters (dried oysters), dried razor clams, dried clamsDried squid, squid eggs (squid egg glands) and various canned shellfish.Many shellfish are indispensable fine Chinese medicinal materials, such asPearlandNacre powder, abalone shellsCassia obtusifolia, precious shellfish, squid's inner shellSnail meat, the eggs of sea hares, etc.Small shellfish with large yield can be used as farmland fertilizer and feed for poultry and livestock.The main component of shells is calcium carbonate, which is the raw material for burning lime. It can also be used to make paint conditioners, shell carvings and other arts and crafts, and pearls are valuable decorations.
But shellfish is also harmful to human beings.Some shellfish are poisonous, and will be poisoned after human consumption or contact.Some freshwater and terrestrial gastropods are human and livestock parasitesIntermediate host, such asSchistosoma japonicum The larvae ofOncomelania hupensisIn vivo.Shipworms in the seaAsparagusThey are the species that specially drill through wood or rocks and live in caves, which do great harm to wooden boats, wooden piles and wood and stone buildings in the harbor.
Edible method
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In order to avoid the potential safety hazards of shellfish and enjoy the delicious food with confidence, we must start with cleaning and cooking:
First, do not eat viscera.Heavy metals and other pollutants are easily concentrated in the visceral mass of shellfish, while the content of heavy metals in muscle is the lowest.Therefore, only the muscle of shellfish can be eaten."Look at the color" is a simple judgment method.After opening the shell, the slightly blackened meat is mostly visceral mass.Some shellfish have a black sand line inside and can't be eaten.
Second, the shell is smooth。PimpledOysters、Scallop in ShellEtc,Razor clam、MusselThe surface is clean and smooth, with less dirt attached and less pollution.
Third, salt water immersion and full heating。Before cooking, shellfish should be put into salt water for a period of time.This can help shellfish expel all kinds of toxins and sand.Then, carefully clean the surface of the shell with a small brush.Cooking methods such as steaming and boiling are preferred, which can thoroughly heat and kill bacteria.When processing, it should be boiled in cold water to ensure the consistency of raw and cooked inside and outside.The method of barbecue is easy to cause uneven heating, and it is recommended to use less.
From the 21st century BC to the 2nd century BC, it was mainly used in the Central Plains. Later, it was gradually replaced by metal coins. The unit is "Peng", and every ten coins is "Peng".In the pre Qin period, shellfish played both the role of coin and ornament.Until the end of the Ming Dynasty and the beginning of the Qing Dynasty, the shell was still used as currency in the minority areas of China, which was called "ba (left and right structures, with 'shell' on the left and 'ba' on the right)".
Natural seashells
In the 11th century BC, it was produced on the seashore, and gradually evolved intoPhysical currency。
Artificial shellfish (stone shellfish)
From the 16th century BC to the 2nd century BC, during the Shang and Zhou dynasties, the commodity economy continued to develop, and the demand for money continued to increase. The imitation jade shells, bone shells, pottery shells, stone shells, etc., which were used to make up for the shortage of natural currency circulation, were collectively called artificial shellsShellfish currency。Their form is largely modeled on natural seashells, and their exchange value is about equal to or slightly lower than that of natural cargo seashells.
In the 11th century BC, in the middle and late Shang Dynasty, with the development of society, human beings mastered the smelting technology, and then metal shellfish coins appeared.Shaped like natural seashells.yesJinbei, silver shell, copper shell, etc.The unwritten copper shell cast with bronze is the first one to appear in ChinaMetal coinage。
Relevant regulations
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In October 2023, the latest version of the list of precautions for carrying goods on trains will be released. Shellfish will be carried on the train, but it is required to be sealed and cannot be eaten in the train.[3]
