Mollusca

One kind of biological classification

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Mollusca is a large group of invertebrates, second only to Arthropoda and the second largest in the animal kingdom, with no less than 130000 species. It is generally divided into 5 classes: Amphineiar, Gastropoda, Scaphopoda, Izrnellibranchia and Cephalopoda.^[1]

Chinese name: Mollusca
Foreign name: Mollusca
circles: Animal kingdom
door: Mollusca
Outline: Anamorpha, Polymorpha, Dignopoda, Gastropoda, Bivalvia, Cephalopoda

Distribution area: From cold zone, temperate zone to tropical zone, from ocean to river, from plain to mountain, lakes can be seen everywhere
Habitat environment: Distributed on land, fresh water and sea
Latin name: Mollusca
Protection level: Included in Appendix I, Appendix II and Appendix III of the Convention on International Trade in Endangered Species of Wild Fauna and Flora

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Species evolution

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The marine species of molluscs are characterized by spiral cleavage in ontogenesis, trochlear larvae, and metanephric ducts as excretory organs, which are similar to those of annelids, especially polychaetes. Therefore, it is reasonable to think that molluscs and annelids have a common origin in phylogeny. In the long-term evolution, they are developing towards an inactive lifestyle, so the somites disappear, producing shells, and the motor organs and nerve senses tend to degenerate. Among mollusks, the unilamella, anecta and polylamella are relatively primitive, and the secondary body cavities of these classes are developed, similar to ladder nerves; Some bodies are worm like and without shells, and many organs such as gills, kidneys, shells, etc. do not show segmental arrangement. The existence of these primitive traits suggests that they are close to the primitive ancestors of mollusks, and each of them develops a branch independently.

Gastropods are relatively primitive, with an active lifestyle and developed head. The life style of Lambranchia is inactive and headless, but the primitive species has shield gills and toes, which is similar to that of gastropoda. The head of the class Echinopoda is not obvious, the mantle is two pieces in the embryo, and then it heals into a tube. The paired kidneys, brain ganglia and lateral ganglia are separated, which indicates that they are close to the original Lamellibranchia , but Excavator It has no gills, no heart, and shell shape, which also shows that it is relatively distant from other animals in evolution, and may be an early branch. Cephalopoda is an ancient group with early origin and many fossil species. Their reproductive cavity is connected with the body cavity, which is similar to that of Anachezia; In ontogeny, there is no kidney in the early embryo, which is similar to polylamella and anecta; The reproductive duct originates from the coelomic duct and is similar to the polyplate class. Because Cephalopods have the characteristics of primitive molluscs, they are close to the original species of molluscs. But Cephalopoda has organic structure, complex nervous system is highly concentrated and surrounded by soft bone; The structure of the eye is similar vertebrate , basically closed pipe circulation system; It occurs directly without larval stage. Because Cephalopods have both primitive traits and highly evolutionary characteristics, it is speculated that they may be an early branch, an independent branch developing along a more active lifestyle.

morphological character

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head

Located at the front of the body. Some primitive species with slow movement have underdeveloped head, only mouth, and no obvious boundary with body, such as Chiton Etc; The body of some cave dwelling or fixed living species is completely enclosed in the mantle and shell, and their heads degenerate, such as mussels, oysters, etc; Some more evolved and agile species have well-developed heads and distinct differentiation, and have sensory organs such as antennae and eyes, such as snails, snails and cuttlefish.

foot

The foot is a motor organ located on the ventral side of the body, which presents different forms according to different lifestyles: some species have smooth plantar surface, suitable for crawling on land or under water, such as Gastropoda ； Some species have axe shaped feet, which are conducive to digging silt, such as Lambranchia ； Some sedentary species have degenerated, such as Oystidae ； Some species have atrophied feet and lost their motor function, but have foot silk glands that can secrete Filum pedis , used to attach to foreign objects for living, such as Mytillidae 、 Fanbeike Etc. stay Cephalopoda , feet are born on the head, some are specialized into wrists, on which many suction cups are born, which are predatory organs, and some are metamorphosed into funnels, suitable for swimming life, such as squid and octopus. The side of a few species of foot (i.e Lateral foot Parapodium) specialized into sheets, which can swim, called wings or fins, such as Pteropoda （Pteropoda）。 The foot usually has a balancer, and some species have many tentacles on the upper part of the foot.^[2]

Growth and reproduction

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Model Diagram of Mollusca Classes

The reproductive system of mollusks consists of gonads, reproductive ducts, connectors and some accessory glands. The gonad is formed by the body cavity wall. The inner end of the reproductive duct leads to the gonad cavity, and the outer end opens in the mantle cavity or directly outside the body. Molluscs can be divided into hermaphrodite and hermaphrodite. The species of hermaphroditism include Polyphylla, the vast majority of Procambranchia, Lambranchia, Cephalopoda, etc. Some of them are fertilized by mating, and some of them are fertilized by releasing their reproductive products into water respectively. Androgynous species include Anamorpha , metabranchia, pulmona, and a few probranchia and petabranchia, which are mostly fertilized by copulation.

The fertilized eggs of mollusks are typical spiral cleavage, and the gastrula is formed from the outer or inner or both. After the gastrula is formed, it quickly develops into free swimming Trochlear larva 。 Some species develop into adults directly from trochlear larvae, but most species develop into adults from trochlear larvae Flour dish larva (veliger larva). The shape of the trochlear larva is similar to that of the annelid polychaete larva. At the early stage of the development of the facial disc larva, there is an outer covering 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. Most of the trochlear larvae of marine gastropods spend in egg bags, while some of the trochlear larvae of probranchia, freshwater gastropods, lung snails and dough plate larvae spend in egg bags. The mussels living in fresh water are specialized to adapt to parasitic life Hookworm larva (glochidium), which parasitizes on the gills, fins or other parts of fish and forms cysts on the fish body. Larvae obtain nutrition from the host body, gradually develop into adults, break out of the capsule, and sink into the benthic life of the underwater camp. The egg division of Cephalopoda belongs to the discoid division type of incomplete division, which is direct development.

Habitat environment

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Gastropods are distributed on land, fresh water (lakes and streams) and the sea. Lambranchia only lives in fresh water and the sea, while other groups live entirely in the sea.

Distribution range

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It can be seen everywhere from cold zone, temperate zone to tropical zone, from ocean to rivers, and lakes from plains to mountains.

Species classification

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monoplacophora

Lamellates have only one shell, no eyes, and their organs are repetitive. It is hermaphroditic and lives on the sea floor or parasitizes and feeds on stinging animals. It has a cap shaped shell and more than 5 pairs of gills, kidneys and muscles.^[3-4]

Anamorpha

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

The body is flat and oval; The head is not obvious; There are 8 shells arranged like tiles on the back; Trapezoidal nervous system; Generally, they feed on the seaweed on the reef scraped 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.

Excavatopoda

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.

Gastropoda

There is one 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, which are widely distributed in the oceans and freshwater rivers, and can be divided into three subclasses, namely, anterior branchia, posterior branchia, and pulmonary snail.

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 axe shaped, so they are called axopods; 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

The 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, and the ganglia are concentrated in the head, protected by the cartilage box formed by the mesoderm; There are about 756 species, all of which are marine products.

Key values

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Edible value

Seafood abalone, conch Neptunea cumingi , Red snail, Dongfeng snail Whelk , clams, mussels, scallops, oysters, clams, clams, clams, clams, razor clams, squid, squid, octopus, freshwater snails Snail , clams, clams, land dwelling snails and other meat tastes delicious, rich in protein, inorganic salts and vitamins, with high nutritional value.

Medicinal value

Abalone shell Cassia obtusifolia ）It can treat eye diseases; The precious shell is called "Haiba", which can clear the eyes and detoxify; Pearl is a valuable Chinese herbal medicine, which can calm the liver and suppress yang, clear heat and detoxify, calm the mind, relieve cough and phlegm, improve eyesight and pain, and astringe vitality; The shell of cuttlefish is called "cuttlebone", which can cure trauma, heart disease and stomach disease, and stop bleeding; Clams, oysters, clams Clam Shells are also commonly used in traditional Chinese medicine. Antibiotics and anti-tumor drugs can be extracted from abalone, chicken snail, sea snail, clam, oyster, squid, etc.

Agricultural value

Small molluscs with large production can be used as farmland fertilizer or feed. For example, the clams produced in the coastal areas of China, such as Xunmei clam, duck billed clam, basket clam, can be fed to pigs, ducks, fish, shrimp, freshwater snails Corbicula fluminea Freshwater fish can be raised.

Industrial value

Mollusc shells are good raw materials for burning lime. There are many shell burning kilns in the eastern and southern coastal areas of China, which provide a part of the source of construction lime. Shells with thick nacre (such as clams, horseshoe snails, etc.) are the raw materials for making buttons.

Decorative value

The shells of many shellfish have unique shapes and patterns, are full of luster, and are colorful. All kinds of treasures, taro shells, phoenix shells, ladder shells, bone shells, scallops, sea clams, pearl shells, etc. are favorite collections of people at all times and in all over the world. Some shellfish, such as clams, mussels, abalone, Tangguan Melon snail They are raw materials for mother of pearl, shell carving and arts and crafts.

Geoscience value

Molluscs have many finger fossils that can be used as indicators of sedimentary environment in geological history. At the bottom of Cambrian system in the world and China, fossils of veneer class and other molluscs have appeared. Many ammonites of Mesozoic group have become the zone fossils for division and correlation of strata within the intercontinental range, some of which can be used to understand the temperature and salinity of ancient waters; Snail fossils can reflect the Quaternary climatic environment.

Protection level

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Included in Appendix I, Appendix II and Appendix III of the Convention on International Trade in Endangered Species of Wild Fauna and Flora 。^[5]

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