CATDOLL : CATDOLL: What does amphibians and molluscs mean?

Molluscs

【Overview】

The cephalopod group includes squids, octopuses, nautiluses, and the extinct ammonites and belemnites. It has long tentacles near its mouth to grab prey and moves by siphoning water.

Molluscs (Mollusca) are the second largest group in the animal kingdom. Molluscs are triploblastic, bilaterally symmetrical animals with true coelom. The true coelom of molluscs is formed by the schizocoelom method, that is, the coelom formed by the mesoderm. The body is soft and generally bilaterally symmetrical. Some species have various strange shapes due to twisting and bending. Usually they have shells, no body segments, fleshy feet or arms, and some have degenerate feet. The outer layer of skin is wrinkled from the back into the so-called mantle, which surrounds the body and secretes a calcareous shell for protection. The gills for breathing are born in the cavity between the mantle and the body. They are distributed all over the land and water. They include Dineura (such as chitons), Gastropoda (such as abalone and snails); Excavatoria (such as horned shells), Bivalvia (such as clams and oysters), Cephalopoda (such as squid and nautilus), etc.

One of the phyla of coelomates. It belongs to protostomes, true coelomates, and terminal cell stems. Gastropods have a second left-right asymmetry due to the twisting of the visceral mass, while the systems of other classes are bilaterally symmetrical. The body consists of a head (not in the Bivalvia), a foot, and a visceral mass. The epidermis of the visceral mass extends to form a mantle, which wraps around the main part of the body. The space between it and the visceral mass is the mantle cavity, which contains gills, and the end of the digestive organs and the excretory organs open here. There is no body segment structure at all. The irregular gaps in the body are the blood cavity. The true coelom becomes narrower due to the development of mesenchyme, forming only the pericardial cavity, the inner cavity of the kidney duct, and the inner cavity of the gonads (genital cavity). The head has sensory organs such as eyes and a mouth. The foot, which is an organ of locomotion, has smooth muscles, which is related to its slow movement. There are oblique striated muscles on the wall of the ink sac. The mantle secretes shells outward. The number and shape of shells vary in different classes. The digestive tract is long, and there are generally radulas and salivary glands in the mouth. The midgut gland opens into the stomach. Generally, there is one pair of gills, and the heart has two atria and one ventricle. However, species with two pairs of gills in the cephalopod class have four atria. Due to the twisting of the internal organs, some gastropods have only one gill (the heart has one atria and one ventricle), and some species have completely lost their gills and replaced by mantle gills. In the terrestrial gastropod class, the mantle of the pulmonary snails has evolved into lungs. It is an open vascular system, and the respiratory pigment is mostly hemocyanin. Clams, flat spiral snails, etc. have hemoglobin. The excretory organ is mainly the nephric duct (Boyena's organ), but there are also species with Kieber's organ (branchi). The nervous system is a special type. In addition to the one pair of head ganglia on the esophagus, there are also one pair of lateral ganglia, foot ganglia and visceral ganglia. Each pair of similar ganglia is connected by a transverse nerve system, and the head ganglia are connected to other ganglia by longitudinal nerves. Hermaphroditic or heterosexual. In oysters, shipworms and abalone, sexual conversion is seen. Except for cephalopods, all other species undergo spiral cleavage, going through the trochophore larvae and veliger larvae stages; however, the cleavage of cephalopods is bilaterally symmetrical and develops directly.

【Main Features】

Molluscs vary greatly in morphology and structure, but their basic structure is the same. They have a soft body with a hard shell, which they hide in for protection. Since the hard shell hinders their movements, they move very slowly. They are not segmented, but can be divided into three parts: head, foot, and visceral mass. They are covered with a mantle and often secrete shells. Their foot is shaped like an axe and has two shells, like an oyster.

It is speculated that the protomollusk appeared in the Precambrian period and lived in shallow seas. Its body was oval, no longer than 1 cm, and bilaterally symmetrical. Its head was located at the front and had a pair of antennae with eyes at the base of the antennae. The ventral side of the body was flat and muscular, with a rough shape and feet suitable for crawling. The back of the body was covered with a shield-shaped, convex shell to protect the entire body. The shell was probably originally formed only of keratin, called conchiolin, and later calcium carbonate was deposited on the conchiolin, increasing its hardness. Under the shell is a double-layer cell structure membrane extending from the body wall to the ventral side, called the mantle (or pallium), which has a strong secretory ability. The shell is formed by the mantle. The mantle covers the visceral sac. A cavity appears between the posterior end of the body, above the foot and the visceral sac, which is the mantle cavity, which communicates with the outside world. There are many gills for respiration in the mantle cavity, as well as openings for the metanephros, anus, and genital pores.

The structure of the gills of protomolluscs may be similar to the gills of existing gastropods. It is composed of triangular gill filaments extending alternately from both sides of a long gill axis. This type of gill is called a ctenidium. The gill axis extends outward from the mantle or body wall, and contains blood vessels, muscles and nerves. The front edge (i.e., the ventral edge) of the gill filaments is supported by chitinous bone rods to increase the hardness of the gills. The gills are fixed in position on both sides of the mantle cavity by the dorsal and ventral membranes, so the gills divide the mantle cavity into upper and lower chambers. Water flows into the lower chamber at the rear end of the mantle cavity and flows out of the mantle cavity through the surface of the gill filaments and the upper chamber. The front edge and surface of the gill filaments are covered with cilia, and the movement of cilia causes the flow of water in the mantle cavity. There are two blood vessels on the gill axis, the afferent blood vessel on the dorsal side and the efferent blood vessel on the ventral side. Blood flows from the afferent blood vessel to the efferent blood vessel, and also flows directly from the dorsal edge to the ventral edge through the microvessels on the surface of the gill filaments. In this way, the direction of blood flow is exactly opposite to the direction of water flow on the gill surface, which can achieve more efficient gas exchange.

Like many existing species, the protomolluscs not only have cilia on the surface of their gills, but also on their mantle and skin (including the skin of their feet). The swaying of these cilia causes water to flow continuously, which is beneficial to gas exchange and predation. Therefore, the respiration of the skin is very important in protomolluscs. The movement of cilia in the feet and the contraction of muscles also constitute the movement of the body. Protomolluscs may be herbivorous, feeding on algae growing on shallow sea rocks. They have the same feeding structure as existing mollusks. The mouth is located at the front of the head, behind the mouth is the buccal cavity, and at the back of the mouth is a bag-shaped radulasac. The bottom of the radula sac is a membrane band that can move forward and backward. The membrane band is distributed with rows of neatly arranged chitinous fine teeth, with the tips of the teeth facing backward. The membrane band and the teeth form the radula. The bottom of the radula sac has radula cartilage (odontophora). The radula and cartilage are attached with protractor muscles and retractor muscles. The muscles, cartilage and membrane band can be extended out of the mouth to scrape food. Due to the wear and tear of feeding, the fine teeth at the front gradually age and disappear. The rear end of the membrane band can continuously secrete new teeth to supplement them. The replacement rate is several rows per day in many mollusks. This radula structure exists in most existing mollusks. There is a pair of salivary gland openings on the back of the mouth. Their secretions can lubricate the radula 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, with a chitinous plate on one side of the stomach inner wall, called the gastric shield, and many tiny ridges and grooves on the opposite side. The grooves have cilia, called the sorting region. The back half of the stomach is sac-like, called the style sac, because there is a colloid rod-like structure in it, called the crystalline rod. The inner wall of the style sac also has wrinkles and cilia, forming ciliary grooves. After the food is adhered by mucus in the mouth to form a food cord, it continues to enter the stomach. The viscosity of the food cord is removed by gastric acid (the pH value of gastric juice of existing species is generally 5-6), making the food particles in the cord free. At the same time, the cilia in the gastric screening area screen the food, and send the small food particles into the digestive gland through the digestive gland duct at the upper end of the stomach. The digestive gland is the place where food is digested and absorbed intracellularly; larger food particles are digested extracellularly in the stomach; undigested food enters the intestine through the folds of the stomach wall, and is partially digested by the intestine, and finally forms fecal particles in the intestine. The anus opens at the rear end of the mantle cavity. The formation of fecal particles reduces the pollution of the mantle cavity, and the fecal particles can be discharged from the body by water. The body cavity of the protomollusk is located in the middle and back of the body. It surrounds the heart and part of the intestine, so it actually represents the pericardial cavity and the perivisceral cavity. The heart consists of a ventricle at the front and a pair of auricles at the back. The arteries from the ventricles branch out to form small blood vessels, and finally enter the interstitial space to form sinusoids. The sinusoids then gather into veins, and the blood flows back to the auricles and ventricles through the kidneys and gills. This is an open circulation. The blood contains amebocytes and respiratory pigments, the main respiratory pigment being hemocyanin.

The excretory organ is the metanephros, located on both sides of the pericardium. One end of the metanephros is connected to the pericardium, called the nephrostome, and the other end is connected to the outside world, called the nephridiopore. The pericardium receives metabolites released by the heart and pericardial glands, which enter the kidneys through the nephrostome along with the pericardial fluid. The kidneys have a certain reabsorption capacity, which recycles useful salts and converts useless waste into urine, which is then excreted from the body through the nephrostome and the mantle cavity.

The neural structure of protomolluscs is very simple, forming a nerve ring around the esophagus, from which two pairs of nerve cords are divided. The symmetrical pedal cord on the ventral side controls the muscle contraction of the foot, and the symmetrical visceral cord on the dorsal side controls the movement of the internal organs and coat. Based on the speculation of existing mollusks, their sense organs may also include a pair of eyes, a pair of statocysts located in the feet, and a pair of osphradia, which are located on the lower branchial membrane and are a kind of chemical receptors.

The reproductive system includes a pair of gonads, located in the middle and back of the front end of the pericardium. The sexes are different, and there is no reproductive duct. Sperm or eggs are released into the pericardium after maturity. Because the gonad cavity is also part of the body cavity, the reproductive cells pass through the pericardium and then pass through the kidneys to be discharged from the body. Fertilization occurs in seawater. The embryonic development of protomolluscs may be very similar to that of existing mollusks. They also undergo typical spiral cleavage, and the blastocyst pore forms a mouth. After gastrulation, trochophores are formed. Trochophore larvae are pear-shaped, with a typical prototroch, which surrounds the body at the top of the mouth. The first four groups during cleavage are all located at the top, and there are bundles of cilia at the top. Trochophore larvae appear not only in mollusks, but also in annelids.

The primitive species only went through the trochophore larvae during their development. Most existing mollusks have a very short trochophore larvae period, after which they enter the veliger larvae stage. During the veliger larvae, structures such as legs, shells, and internal organs appear. It is speculated that the primitive mollusk did not have a veliger larvae stage. It lost the ciliary whorl before the mouth from the trochophore larvae, metamorphosed into an adult, and began to live on the seabed.

【Division of the body】

The body of a mollusk can generally be divided into three parts: the head, the foot, and the visceral mass.

①Head

The head of a species that moves quickly is clearly differentiated, with eyes, tentacles and other sensory organs on it, such as snails, snails and squids; the head of a species that moves slowly is not well developed, such as chitons; the head of a species that lives in caves or is fixed has disappeared, such as clams and oysters.

②Foot

Usually located on the ventral side of the body, it is an organ of locomotion, and its shape varies depending on the animal's lifestyle. Some have well-developed feet that are leaf-shaped, axe-shaped, or columnar, and can crawl or dig mud and sand; some have degenerated feet that have lost their locomotion function, such as scallops; species that live in a fixed state have no feet, such as oysters; some have specialized feet that have become arms and grow on the head, and are organs for hunting, such as squid and octopus, and are called cephalopods; a few species have specialized lateral parts of their feet (i.e., parapodiums) that can swim, and are called wings or fins, such as Pteropoda.

③Visceral mass

The part where the internal organs are located is usually located on the back of the foot. The internal organs of most species are bilaterally symmetrical, but some are twisted into a spiral shape and lose their symmetry, such as snails.

Mantle

It is formed by the skin folds on the back of the body extending downward, often wrapping the entire visceral mass. The cavity formed between the mantle and the visceral mass is called the mantle cavity. There are often gills, feet, anus, kidney holes, implant holes, etc. opening into the mantle cavity.

The mantle is composed of two layers of epithelium, the secretions of the outer epithelium can form shells, and the inner epithelial cells have cilia, which swing to create water flow, so that water circulates in the mantle cavity, thereby completing breathing, excretion, feeding, etc. The left and right mantles often fuse at one or two places at the rear edge to form the exhalant siphon and inhalant siphon. In some species, the inhalant siphon is extended into a tube shape, extending out of the shell and called the exhalant tube and inhalant tube.

Shell

Having a shell outside the body is an important feature of molluscs, so the discipline that studies molluscs is also called conchology. Most molluscs have one, two, or more shells, which vary in shape. Some are cap-shaped; snails are spiral-shaped; shovel-foots are tubular; and lobate-branchs are flap-shaped. The shells of some species have degenerated into inner shells, and some have no shells. The shells have the function of protecting the soft body.

The main components of shells are calcium carbonate and a small amount of conchiolin, which are secreted by the epithelial cells of the mantle. The structure of the shell can generally be divided into three layers. The outermost layer is the cuticle (periostracum), which is very thin, transparent, and shiny. It is composed of the shell matrix and is not corroded by acids and alkalis, and can protect the shell. The middle layer is the ostracum, also known as the primatic layer, which occupies most of the shell and is composed of angular calcite (cacalcite). The innermost layer is the hypostracum, that is, the peral layer, which is shiny and composed of leaf-shaped aragonite (aragonite). The outer and middle layers are secreted by the edge of the mantle and can gradually increase in size as the animal grows, but not thicker; the inner layer is secreted by the entire mantle and can increase in thickness as the individual grows. Pearls are formed by the nacre layer. When the mantle is stimulated by tiny sand particles or other foreign bodies, the epithelial cells at the stimulated site will use the foreign bodies as nuclei and sink into the connective tissue between the epithelial cells of the mantle. The trapped epithelial cells will divide and form pearl sacs, which will secrete nacre and gradually form pearls by encapsulating the nuclei layer by layer. According to historical records, there are records of freshwater pearl cultivation in my country more than 2200 BC (Book of Documents. Yu Gong Chapter). Pearl cultivation in Hepu, Guangxi has been famous since ancient times, and picking began in the Han Dynasty.

The growth of the cuticle and prismatic layer is not continuous. Because factors such as food and temperature affect the secretory function of the mantle, the growth rate of the shell is different. Therefore, growth lines are formed on the surface of the shell to indicate the speed of growth.

Digestive system

The digestive tract of molluscs is well developed, but it is degenerate in a few parasitic species (Entocolax). Most species have mandibles and radulas in their mouths. The mandibles are single or in pairs, which can assist in predation. The radula is a unique organ of molluscs, located on the surface of the odontophore at the bottom of the mouth. It is composed of horizontal rows of keratinous teeth, which are like a file. When feeding, the radula is used to scrape food by moving back and forth. The shape and number of the small teeth on the radula vary among different species, which is one of the important characteristics for identifying species. The small teeth are arranged in horizontal rows, and many rows of small teeth constitute the radula. Each horizontal row has one central tooth, one or several pairs of left and right side teeth, and one or more pairs of marginal teeth on the edge. The arrangement of the small teeth on the radula is represented by the tooth formula, such as the tooth formula of the Chinese round field snail (Cipangopaludina chinensis) is 2.1.1.1.2.

Body cavity and circulatory system

The secondary body cavity of molluscs is extremely degenerate, with the pericardinal cavity and the inner cavity of the gonads and excretory organs remaining. The primary body cavity exists in the gaps between tissues and organs, with blood flowing in it, forming blood sinuses.

The circulatory system consists of the heart, blood vessels, sinusoids and blood. The heart is generally located in the dorsal pericardial cavity of the visceral mass and is composed of the auricles and ventricles. There is one ventricle with thick walls that can pulsate and is the driving force of blood circulation; there is one auricle or a pair of auricles, which are often the same number as the gills. There is a membrane between the auricle and the ventricle to prevent blood from flowing back. Blood vessels are divided into arteries and veins. Blood flows from the ventricles through the arteries to various parts of the body, then flows into the sinusoids and returns to the auricles through the veins, so mollusks have an open-tube circulation. Some fast-swimming species have a closed-tube circulation. The blood is colorless and contains amoeba-like cells. Some species contain hemoglobin (haemoglobin) or hemocyanin (haemocyanin) in their plasma, so the blood is red or blue.

Respiratory organs

Aquatic species breathe with gills, which are formed by the extension of the epithelium on the inner surface of the mantle cavity. The gills have different shapes. There are gill filaments on both sides of the gill axis, which are feather-like and called shield gills; there are gill filaments on only one side of the gill axis, which are comb-like and called ctenidium; some gills are flap-like, called lamellibranch; some species have gills extended into filaments. Called filibranch. Some of the primary gills disappear, and secondary gills (secondary branchium) grow on the dorsal skin surface, and some species have no gills. The gills are in pairs or singles, and the number varies, ranging from one or a pair to dozens of pairs. Species living on land have no gills. The micro-blood vessels in a certain area inside the mantle cavity are densely packed to form lungs, which can directly absorb oxygen from the air. This is an adaptation to life on land.

Excretory organs

The excretory organs of molluscs are basically metanephric ducts, the number of which is generally consistent with the number of gills. Only the larvae of a few species have protonephric ducts. The metanephric duct is composed of glandular part and tubular part. The glandular part is rich in blood vessels, with cilia on the renal opening and opening into the pericardial cavity; the tubular part is a thin-walled tube with cilia on the inner wall and renal pores opening into the mantle cavity. The metanephric duct can not only eliminate metabolic products from the heart, but also eliminate metabolic products from the blood. In addition, the pericardial glands on the inner wall of the pericardium are densely covered with microvessels, which can eliminate metabolic products in the pericardium and excrete them out of the body through the metanephric duct.

Nervous system

The nervous system of primitive species has no differentiated ganglia, only the peripharyngeal nerve ring and a pair of pedal cords and pleural cords extending to the back of the body. Higher-level species mainly have 4 pairs of ganglia, and nerves are connected between each ganglion. The cerebral ganglion is located on the dorsal side of the esophagus, and sends nerves to the head and the front of the body, responsible for sensation; the pedal ganglion is located in the front of the foot, and extends nerves to the foot, responsible for movement and sensation; the pleural ganglion sends nerves to the mantle and gills, etc.; the visceral ganglion sends nerves to various internal organs. These ganglia tend to be concentrated, and the main ganglia of some species are concentrated together to form a brain surrounded by cartilage, such as cephalopods. Molluscs have differentiated sensory organs such as antennae, eyes, olfactory detectors and balance sacs, and are sensitive.

Reproduction and development

Most mollusks are dioecious, and many species are dioecious; some are hermaphroditic. The cleavage is mostly completely unequal, and many are spiral. A few are incomplete cleavage. During individual development, there are two larval stages: trochophore larvae and veliger larvae. The morphology of trochophore larvae is similar to that of polychaete larvae of annelids. In the early stage of development, veliger larvae have the primordium of the mantle on the dorsal side and secrete the outer shell, the primordium of the foot on the ventral side, and the ciliary ring in front of the mouth develops into the velum or veliger. Some species also develop directly. Freshwater mussels have special glochidium larvae.

【type】

Molluscs are of various types and live in a wide range of areas, including seawater, freshwater and land. More than 130,000 species have been recorded, second only to arthropods. Molluscs have a more complex structure and more perfect functions. They have some of the same characteristics as annelids: secondary body cavity, metanephric duct, spiral cleavage, trochophore larvae in individual development, etc. Therefore, it is believed that molluscs evolved from annelids and differentiated earlier towards a less active lifestyle.

Most mollusks are covered with various shells, so they are usually called shellfish. Since most of them have gorgeous shells, delicious meat, rich nutrition, and are easy to catch, they have been used by humans as early as the ancient fishing and hunting period. Many of them can be used for food, medicine, agriculture, and arts and crafts, but some species are poisonous, can spread diseases, harm crops, damage port buildings and transportation facilities, and are harmful to humans.

Molluscs include gastropods such as snails, field snails and slugs that are familiar to people in life; bivalves such as river clams and hairy clams; cephalopods such as squid (cuttlefish) and octopus; and multi-plate chitons attached to rocks in coastal intertidal zones. They have great differences in morphology, such as their body structure is either symmetrical or asymmetrical; the body surface may have a shell or no shell; the shell may be one, two or more. However, according to the comparative morphological research of existing species, embryological research, and paleontological research on fossils that appeared as early as the Cambrian period, it is found that all mollusks are built on a basic model structure. This model is the original molluscs that people imagine, that is, the ancestral model of mollusks, which then developed and evolved into different classes from the original molluscs. Therefore, the original molluscs represent the basic characteristics of all mollusks.

Based on the study of existing animals, people imagine that the structural characteristics of animals in various existing classes were formed from proto-molluscs through changes in the anterior-posterior axis and dorsal-ventral axis of the body, and the displacement of the feet, visceral sacs and mantle cavity.

Molluscs can be divided into seven classes: Monoplacophora, Polyplacophora, Aplacophora, Gastropoda, Bivalvia, Scaphopoda, and Cephalopoda. Among them, only Gastropoda and Bivalvia have species that live in freshwater, and Gastropoda also has terrestrial species. These two classes contain more than 95% of the species of molluscs, and the other classes are all marine.

【Origin and evolution】

There are two theories about the origin of mollusks: one is that mollusks originated from flatworms; the other is that mollusks and annelids evolved from a common ancestor, but because they developed different lifestyles during the long evolution process, they eventually formed two different types of animals. The latter theory is more reasonable, because many species of marine mollusks also have a larval stage in the embryonic development process like many annelids. In addition, both types of animals have cleavage during development, and have some common changes in adults. For example, the excretory organs are basically metanephric tubular type, and the body cavity is secondary.

Part of this common ancestor developed in a way that was suitable for movement, forming body segments, papillae, and a well-developed head, which is the annelid; the other part developed in a way that was suitable for a relatively inactive path, which produced a protective shell and many structures suitable for movement, such as segmentation and the head, which either did not appear or degenerated. At the same time, some structures unique to mollusks, the mantle, also developed. Because of the large differences between the various groups of mollusks, there are no more obvious differences to well explain their kinship.

Among the mollusks, the Dineura are relatively primitive because they have bilateral symmetry, a relatively developed secondary body cavity, and retain a primitive trapezoidal dineural system. The Gastropoda are relatively low-level groups because they have trochophore larvae or similar veliger larval stages similar to those of annelids. The most notable characteristic of the respiratory system of the Bivalvia is the flap-shaped gills. Taking the living river mussels as an example, each flap-shaped gill is a gill flap, which is composed of two gill flaps, the outer one is called the outer gill flap, and the inner one is called the inner gill flap. Each gill flap is composed of many gill filaments, with cilia on the surface of the gill filaments, blood vessels inside, and many small holes. The gaps between the gill flaps are separated by diaphragms with flaps, forming many gill water tubes. Due to the swinging of cilia, water enters the mantle from the water inlet, enters the gill water tube through the gill inlet, then rises to the supragillary cavity, and finally flows out of the body through the water outlet. Gas exchange is completed in the blood in the gill filaments during the process of water flowing through the gill filaments. This type of animal has two mantles and thus two-valve shells. Their lower species have wide and flat bottoms on their feet and crawl. Trochophore larvae also appear during their development, so they may have a common ancestor with gastropods. Cephalopods have highly developed body structures, and their brains, eyes, and circulatory systems are the most advanced among mollusks. The earliest mollusks found in the strata were also cephalopods. They may have evolved quickly toward specialization due to their adaptation to a fast-moving social style.

Relationship with humans

value

① Edible value

Marine abalone, jade conch, fragrant conch, red conch, Dongfeng conch, mud snail, clams, mussels, scallops, clams, oysters, clams, cockles, clams, razor clams, squid, calamari, octopus; freshwater snails, snails, clams, cockles; and land snails have delicious meat and high nutritional value.

②Medicinal value

The shells of abalone (called Shijueming in Chinese medicine), the shells of treasures are called sea ba, the shells of pearls and cuttlefish are called sea octopus, as well as the shells of clams, oysters, clams, and green clams are all commonly used in Chinese medicine. Antibiotics and anti-tumor drugs can be extracted from abalone, conch, sea snails, clams, oysters, and cuttlefish.

③Agricultural value

Small mollusks with high yields can be used as fertilizer or feed for farmland, and river clams can be used to feed freshwater fish. ④ Industrial use. Mollusk shells are good raw materials for burning lime. Shells with thicker pearl layers (such as clams, horseshoe snails, etc.) are raw materials for making buttons.

④For craft or decoration

The shells of many shellfish have unique shapes and patterns, are shiny and colorful, and are favorite collectibles for people from ancient times to the present, both in China and abroad.

⑤Geological value

Mollusca has many fossils that can be used as indicators of sedimentary environments in geological history. Monoplacophora and other mollusc fossils have appeared at the bottom of the Cambrian system in China and the world. Many ammonites in the Mesozoic Era have become zone fossils for dividing and comparing strata on an intercontinental scale, and some can be used to understand the temperature and salinity of ancient waters. Snail fossils can reflect the climate environment of the Quaternary Period.

Danger to humans

Land snails and slugs eat leaves and buds of plants, harming vegetables, fruit trees, tobacco, etc.; some carnivorous species in the ocean can kill the seedlings of oysters, mud clams, etc., causing losses to farmed bivalves; some herbivorous species often eat the seedlings of kelp and laver, and are enemies of algae farming. Among freshwater and terrestrial mollusks, Lymnaea is the intermediate host of liver flukes, bean snails are the intermediate host of Chinese clonorchis, flat spiral snails are the intermediate host of ginger fasciola, short groove curled snails are the intermediate host of lung flukes, and Oncomelania hupensis is the intermediate host of Japanese schistosoma, which are very harmful to humans. Shipworms and sea asparagus in the ocean are species that specialize in drilling holes in wood or rocks, which are harmful to wooden boats, wooden piles, and wooden and stone buildings in harbors in the ocean. Species that attach or settle often attach to the bottom of ships in large numbers, which can affect the speed of ships. Some species that attach can block water pipes and affect production.

[Differences between Mollusc and Crustacean Shells]

First, the shells of molluscs are formed by the lime secreted by the mantle. Except for the joints, they do not shed their shells throughout their lives. The outer body of arthropods is covered with a chitinous exoskeleton, also known as the epidermis or cuticle. The cuticle on the joint membrane between adjacent body segments is very thin and easy to bend and move. The joints of the appendages are also movable. Arthropods shed their skin regularly during their growth.

Second, molluscs are a type of invertebrate with a soft body. Molluscs generally have bilaterally symmetrical bodies, but some molluscs have various strange shapes due to the twisting of their bodies. They often have an outer shell and no body segments, and most of them can be divided into three parts: head, foot, and visceral sac. The outer skin is wrinkled from the back into a layer of membrane, called the mantle. The mantle surrounds the body and secretes lime.

Crustaceans belong to the phylum Arthropoda. Arthropods are composed of one row of segments, which are segmented irregularly and can be divided into three parts: head, thorax, and abdomen. The head and thorax are fused into the cephalothorax, or the thorax and abdomen are fused into the trunk. Each segment has a pair of segmented appendages. There are two types of appendages: double-branched and single-branched. The circulatory system is an open tube type. The respiratory organs of aquatic species are gills or book gills, and those of terrestrial species are trachea or book lungs or both. Primitive arthropods exchange gases through the body surface. The nervous system is a centralized chain nervous system. There are sensory organs such as touch, taste, smell, hearing, balance and vision. There are two types of eyes: simple eyes and compound eyes. Compound eyes are 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, such as the fierce water flea, is less than 1 mm long. The largest giant crab can reach a width of 4 meters when its claws are extended. The body is long and cylindrical, with distinct segments, and the whole body is divided into three parts: head, thorax, and abdomen. The head is formed by the fusion of 6 segments. The exoskeleton of each segment of crustaceans consists of two parts: the back is the carapace, and the ventral is the plastron. The two sides of the carapace often extend outward (downward) to form the side armor, and the appendages are generally attached to the two sides of the plastron. It can be seen that arthropods are higher than molluscs.

Reference Baidu Encyclopedia

Amphibians are the first air-breathing terrestrial vertebrates, which appeared in the late Devonian period 360 million years ago, as determined by fossils. Evolving directly from fish, these animals represent a transition from aquatic to terrestrial life. At the beginning of their lives, amphibians have gills, which gradually develop into lungs as they grow into adults. The word "amphibian" comes from the Greek words for "two amphi" and "life bios." This is because amphibians can live both on land and in water.

Molluscs (Mollusca) are the second largest group in the animal kingdom. Molluscs are triploblastic, bilaterally symmetrical animals with true coelom. The true coelom of molluscs is formed by the schizocoelom method, that is, the coelom formed by the mesoderm. The body is soft and generally bilaterally symmetrical. Some species have various strange shapes due to twisting and bending. Usually they have shells, no body segments, fleshy feet or arms, and some have degenerate feet. The outer layer of skin is wrinkled from the back into the so-called mantle, which surrounds the body and secretes a calcareous shell for protection. The gills for breathing are born in the cavity between the mantle and the body. They are distributed everywhere on land and water. They include Dineura (such as chitons), Gastropoda (such as abalones and snails); Excavators (such as horned clams), Bivalvia (such as clams and oysters), Cephalopoda (such as squids and nautiluses), etc.

Amphibians: live on land and water, with their larvae living in water and their adults living near water sources. Examples: frogs, toads

Molluscs: have no spines in their bodies, have soft bodies, and are protected by shells, such as snails, squids, and octopuses.

Amphibians are animals that live in water when they are young and live on land when they grow up. They are vertebrates.

Molluscs are invertebrates with an external skeleton.

Simply put, amphibians refer to aquatic and terrestrial animals.

Molluscs are invertebrates