Lower animals refer to animals with simple body structure, insignificant tissue and organ differentiation, and no spineinvertebrate。At least 90% of the animal species on the earth belong to lower animals.Some lower animals have soft bodies, and some creatures have protective shells.Lower animals include sponges, coelenterates,Echinoderma, insects, etc.In addition to insects, most of the animal phyla live in the sea, such asStarfish, sea urchin, etc.In the process of evolution of lower animals, their body structure has undergone great changes, from low to high, from simple to complex.
Chinese name
lower animal
Quantity
At least 90%
Classification
Nearly 20 species of sponges and insects
Features
Simple body structure, no significant differentiation of tissues and organs
Lower animals include about 20 phyla, including sponges, coelenterates, echinoderms and molluscs.In the process of evolution, the body structure of lower animals has undergone great changes, from low to high, from simple to complex.
There are more than 900 kinds of coelenterates, most of which breed by oviposition.Jellyfish use the wrist hanging in the water around the "bell jar" (umbrella) to catch small fish, and pull the prey into the mouth after anesthesia with ruffled tentacles.aurelia aurita Although its sting will not kill people, it can cause a tingling sensation.
There are 2000 species of echinoderms, which reproduce in vitro.Sea cucumbers protect themselves through their rough skin and bones with white spines. If the enemy gets too close, the threatened sea cucumbers will release a sticky, threadlike substance to ensure their own safety[1]。
There are 35000 species of single shell gastropod mollusks.The spire of Spirillum giganteum is high, the body layer is large, the shell top is often defective, the shell mouth is large, and the screw shaft is smooth.There are large spirally arranged tumors around the spiral layer and on the shoulder of the body layer[1]。
Sponge animal
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Basic Introduction
Primitive multicellular animals, this phylum is also calledSponifera(Spongiatia), commonly called Sponge.Sponges are important reef building organisms, and a few genera and species also have certain stratigraphic significance.
Sponge animal tissue is primitive, without real digestive cavity and nervous system.Although the cells of sponge animals have division of labor, they have little cooperation with each other. For example, if the sponge is ground and screened, the separated cells can still survive for several days (equivalent to protozoa).However, if they do not combine with each other, they cannot continue to survive. Sponges, which are both independent and cooperative, indicate that their organic structure is still at the cellular level, showing the characteristics of primitive multicellular animals.
Sponge animals are mostly groups with few monomers.The body is radially symmetrical or asymmetrical.The shape of the group varies greatly.The monomer is generally angular cone, disc, goblet, spherical, etc.The size varies from a few millimeters to 2m.Most have calcareous, siliceous or horny bones.The bones of sponge animals are spicules (sponge needles), sponge filaments (bone filaments) and non spicule minerals.The bone needle is composed of calcium (calcite, aragonite) or silica (opal).Bone needle can be divided into big bone needle and small bone needle according to size.The components of sponge silk are horny organic compounds, which are filiform, branched or connected together.Sponge silk is easy to decay and not easy to form fossils.
Sponge reproduction includes asexual and sexual.Modern sponge removalCommon sponge classMiddle and minority generaFreshwater spongeIn addition, most of them are marine animals, which live on the bottom.Modern stone sponges and calcareous sponges are mostly distributed in shallow sea areas, butGlass spongeIt can live in the deep sea up to 6000m deep.Fossil sponges also generally require similar water depths.Sponges often form reefs together with stromatoporoids, bryozoans and algae in different geological times.
Classification of sponge animals
It is known that there are about 10000 species of sponges, which are mainly divided into the following three classes according to the composition and shape of bone needles.
oneCalcifera(Calcarea), with calcareous spicules, small body, grayish white, with single or double ditches.It often lives in shallow sea areas.Such as Leucosolenia, Grantia, etc.
twoHexaradio sponge(Hexactinellida), with hexagonal silicon spicules, large body, double ditch ditch system, and large flagellum chamber.Most of them live in the deep sea.Such as Hyalonema andAged at the same acupoint(Euplectella)。
threeVulgaris(Demopongiae), with non hexagonal silicon spicules or sponge filaments, the ditch is of double ditch type, the flagellar chamber is small, and the body shape is irregular.Live in fresh water or sea water.Seawater dwelling, such as Cliona and Euspongia;They live in fresh water, such as the freshwater sponge (Spongilla).Most of the medicinal species belong to the class of sponge.Common ones are Spongilla fragilis Lecidy, Ephydatia mulleri var. Japonica (Higendorf), etc.
Important representative fossils
According to bone composition and drainage system type, it can be divided into the following:
The body of coelenterates has a fixed shape and is radially symmetrical, that is, the body is divided into two symmetrical halves through many sections on the central axis of the body.There are only two sides: mouth side and counter mouth side.Some species, such as sea anemones, divide their bodies into two equal halves through only two sections of the central axis, which are called two radially symmetrical.This is a form between radial symmetry and bilateral symmetry.
Erliang Embryo and Original Digestive Cavity
The coelenterate is an animal with true endoderm and ectodermCell secretionThe middle glue layer of.The cavity in the body is the original intestinal cavity, which has the function of extracellular digestion. The residue is discharged from the mouth, so it is also called the digestive circulatory cavity, or coelenterate, from which coelenterates get their name.
Cell and tissue differentiation
The cells of coelenterate have differentiated into skin muscle cells, glandular cells, mesenchymal cells, prickle cells, sensory cells, etc.Skin muscle cells are the main cells in the endoderm and ectoderm, which can perform the physiological functions of epithelium and muscle, so they are called skin muscle cells, which also indicates that the coelenterates began to have primitive epithelium and muscle tissue.The base of skin muscle cells extends one or several slender projections, in which myofibrils are distributed. Its composition and contraction mechanism are similar to those of higher animals.
nerve net
The nervous system begins to appear in coelenterates.Nerve cells are connected with each other by neurites to form a network, so they are callednerve net 。Nerve cells are connected with sensory cells and skin muscle cells in the endoderm and ectoderm, and can effectively respond to various external stimuli, but there is no nerve center.Nerve conduction generally has no fixed direction, so it is called decentralized nervous system.
The coelenterates have no respiratory and excretory organs and mainly rely oncell surfaceGet oxygen from water andcarbon dioxideSuch wastes are directly discharged into water, or discharged into the digestive circulation cavity and discharged from the mouth.
Reproductive methods
yesAsexual reproductionAnd sexual reproduction.Asexual reproduction often forms colonies by budding.Sexual reproduction is mostlyHermaphroditeThe gonad of Hydra is formed from the ectoderm, but the gonad of Medusae and Coral is from the endoderm, and the germ cell is from the interstitial cell.Marine productsembryonic developmentThere areFloating wave larvaPeriod.
Flatfish
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Basic Introduction
FlatfishIt plays an important role in the history of animal evolution.From this kind of animals, bilateral symmetry and mesoderm appeared, which further complicated, improved and developed the structure and function of animals and laid a necessary foundation for the transition of animals from aquatic to terrestrial.Related to this, primitiveexcretory systemAnd ladder like nervous system.
Bilateral symmetry
From flat animals, bilateral symmetry has emerged, that is, through the central axis of the animal body, there is only one symmetry plane (or section) to divide the animal body into two equal parts, so bilateral symmetry is also called bilateral symmetry.From the perspective of animal evolution, this body type is mainly due to the result of animals' floating life from water to underwater crawling life.The developed body type is of great significance to the evolution of animals.Because the body of any bilaterally symmetrical animal can be clearly divided into front and rear, left and right, back and abdomen.The back of the body has developed the function of protection, and the belly has developed the function of movement. The front end is always the first to contact with new external conditions, which promotes the nervous system and sensory organs to focus more and more towards the front of the body. The head gradually appears, making the animals move from non directional movement to directional movement, making the animal's sensing more accurate, rapid and effective, and making its scope of adaptation more extensive.Bilateral symmetry is not only suitable for swimming, but also suitable for crawling.It is possible to evolve from crawling in water to crawling on land.Therefore, bilateral symmetry is an important condition for animals to develop from aquatic to terrestrial.
Mesoderm
From flat animals, mesoderm appeared between ectoderm and endoderm.The appearance of mesoderm is of great significance to the further development of animal structure and function.On the one hand, the formation of mesoderm alleviates the burden of the endoderm and ectoderm, causing the differentiation of a series of tissues, organs and systems, providing necessary material conditions for the further complexity and completeness of the animal body structure, so that the flat animals reach the level of organ system.On the other hand, the formation of mesoderm promotes the strengthening of metabolism.For example, the mesoderm forms a complex muscle layer, which enhances the motor function. In addition, the bilaterally symmetrical body shape makes it possible for animals to eat more food in a larger range.At the same time, because there are muscles on the wall of the digestive tube, the ability of the digestive tube to wriggle is also strengthened.These undoubtedly promoted the strengthening of metabolic function. As a result of the strengthening of metabolic function, the amount of metabolic waste generated also increased, thus promoting the formation of the excretory system.Flats began to have a primitive excretory system, the protonephric system.Moreover, due to the improvement of animals' motor function, they often contact with the changing external environment, which promotes the further development of the nervous system and sensory organs.The nervous system of flat animals has made remarkable progress compared with that of coelenterates, and has begun to focus on the ladder shaped nervous system.In addition, the parenchyma formed by mesoderm has the function of storing nutrients and water, and animals can tolerate hunger and drought to some extent. Therefore, the formation of mesoderm is also one of the basic conditions for animals to evolve from aquatic to terrestrial.
Skin muscle capsule
Due to the formation of mesoderm, there are complex muscle structures, such as ring muscle, longitudinal muscle and oblique muscle.The body wall formed by the epidermis formed from the ectoderm is called "skin muscle sac", which formsMuscle systemIn addition to the protective function, it also strengthens the motor function. In addition, it is symmetrical on both sides, so that animals can take food faster and more effectively, which is more conducive to the survival and development of animals.
In the dermatomuscular sac, it is filled with parenchyma, in which all organs in the body are embedded.
digestive system
Similar to common coelenterates, the opening to the outside body is both mouth and anus. Only monopharyngeal planarians, such as Haplopharynx, have temporary anus, so it is called incomplete digestive system.There is no large body cavity except the intestines.Intestine is a blind tube formed by endoderm, which is a kind of parasitic life. The digestive system tends to degenerate (such as trematodes) or disappear completely (tapeworms).
excretory system
The excretory system of the protonephric tube began to appear in the flat animals.It exists in all groups of this phylum (except Anentera).The pronephric tube is formed by invading the ectoderm on both sides of the body, and is usually composed of many branched excretory tubes with excretory holes passing through the body.At the extreme end of each small branch, the blind tube is composed of flame cells.The actual flame cells are composed of cap cells and tube cells.
The cap cell is located at the top of the small branch and covers the tube cell. The cap cell has two or more flagella, which hang in the center of the tube cell.Flagella move like fire, so it is called flame cell.Under the electron microscope, there are numerous pores between the two cells or on the tube cells, and the tube cells connect to the small branches of the excretory tube.The function of the protonephridial tube may be to generate negative pressure at the end of the tube through the constant movement of the flagellum of the flame cell, causing the fluid in the substance to filter through the cell membrane on the tube cell, and the Cl -, K+plasma is reabsorbed at the tube cell, generating hypotonic liquid or water, entering the tube cell through numerous pores on the tube cell membrane, and the excretory tube is discharged out of the body through the excretory hole.The function of the protonephric duct is mainly to regulate the osmotic pressure of water in the body and discharge some metabolic wastes.Some real excreta, such as nitrogenous waste, are discharged through the body surface.
nervous system
The nervous system of flat animals is significantly improved than that of coelenterates.The nerve cells gradually concentrate forward to form a "brain" and separate several longitudinal nerve cords from the "brain" backward. There are transverse nerves connected between the longitudinal nerve cords.In higher species, the number of longitudinal nerve cords is reduced, and only one pair of abdominal nerve cords are developed, including transverse nerve connections such as trapezoid (orLadder nervous system）There are nerve fibers in the brain and nerve cords that connect with various parts of the body.It can be said that primitive flat animalscentral nervous system。Although this nervous system is better than that of coelenteratesnerve net Advanced, but it is primitive, because nerve cells are not completely concentrated in the "brain", but also scattered in the nerve cord.
reproductive system
majorityHermaphroditeDue to the appearance of mesoderm, fixed gonads and certain reproductive ducts, such as fallopian tube, vas deferens, and a series of accessory glands, such as prostate, vitelline gland, have been formed to produce male and female germ cells.This allows the germ cells to reach the outside of the body for mating and in vivo fertilization.
Platyhelminthes live freely or parasitically. Free living species (such as planarian) are distributed in seawater, fresh water or moist soil, and are carnivorous.Parasitic living species (such as trematodes and tapeworms) parasitize on the surface or body of other animals to absorb the nutrition of the animals.
Echinoderma
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Basic Introduction
(Echinodermata) A phylum of coelomorphs in the animal kingdom.Radial symmetry, with uniqueWater pipe system。In the body, there is a true body cavity separated from the digestive tract, and the body wall has an endoskeleton from the mesoderm. The larva is symmetrical on both sides, and develops through complex metamorphosis.Mouth occurs from the opposite end of the blastopore, which belongs to the hind mouth animalinvertebrateIt has a high position in evolution.Including starfish, snake tail, sea urchin, sea cucumber and sea lily, etc.It is named because the epidermis is generally spiny.All are marine products.There are about 5900 species,ChinaMore than 500 have been found.
form
Appearance varies greatly, including star shaped, ball shaped, cylinder shaped and flower shaped.The radiation symmetry of adult five radiators is shown by the arrangement of tube feet.According to the presence or absence of tube feet, the body can be divided into the spokes or gait bands with tube feet and the interproximal spokes or gait bands without tube feet.The internal organs, including the water pipe system, nervous system, blood system and reproductive system, are radially symmetric, except for the digestive tract.Due to radial symmetry, the body can be divided into oral and anti oral surfaces.
Although the shapes of each class of animals in this phylum are very different, their basic structures are very consistent.Starfish and snaketails are star shaped, flat from top to bottom, with short body axis, mouth facing down, and tube feet arranged radially along the wrist (spoke).Sea urchins and sea cucumbers have extended body axes, and the spokes and the intermediate spokes are combined. The body is spherical or cylindrical, and the tube feet are arranged in meridian.The mouth of the crinoid is facing upward, and the opposite mouth is covered with long handles or twigs for attachment.
The bones of echinoderms are very developed, which are composed of many separate calcium carbonate bone plates, each of which is composed of a single crystal of calcite.The skeleton is covered with epidermis, and the skin is usually covered with spines.Sea urchins and starfish have different fork spines.Sea urchin bones are the most developed, and the bone plates are closely fused into shells.The carpal plates of starfish, snake tail and crinoid are vertebral.The skeleton of sea cucumber is the least developed, becoming tiny scattered bone needles or pieces.
structure
The adult coelom of echinoderms is mainly occupied by the digestive system and reproductive system, but the coelom of crinoids is filled with membranes and cords of connective tissue.
The water pipe system evolved from a part of the body cavity, the water cavity.It's all about moving, feeding, breathing and feeling.The typical water pipe system starts from the sieve plate and passes through the stone pipe to the ring pipe around the mouth and the spoke pipe extending to the spoke.There are small branches on the radial tube to the tube foot.There is an altar sac at the base of the tube foot, a sucker at the end, filled with seawater, and the body moves by stretching and contracting the muscles.
The nervous system, blood system and water pipe system all have a ring around the esophagus and a main branch from the ring to each spoke.There are three kinds of nervous systems in echinoderms: ① the orofacial nervous system or the external nervous system, which is in or under the epidermis, including the nerve ring around the esophagus and the radial nerve extending into each radiation. It is the main nerve of modern echinoderms, and it exists in all classes, and most of them are developed (excluding crinoids). ②The lower nerve system is located inside the oral nerve, and its arrangement is exactly the same as that of the oral nerve. ③The contraoral facial nerve system or internal nerve system is under the contraoral facial skinCrinoidsRelatively developed.The blood system is located between the nervous system and the water pipe system, and is relatively developed in sea cucumbers and sea urchins.
The digestive system of starfish and snake tail has a large cystic stomach, and the intestines go straight without bending;Starfish often have a digestive gland that extends from the stomach to the wrists and is symmetrical as the pyloric caecum.The digestive tubes of other classes are mostly a long and curved tube, which is connected to the body wall by the intestines.Snakes' tails and some starfish have no anus, and anything that cannot be digested is spit out from the mouth.The mouth and anus of sea lilies are all upward, and the anus of many crooked sea urchins is often at the back of the body or on the oral surface.There is no obvious excretory organ, and the excretory function is deformed fromWandering cellbe responsible for.The respiratory system is also underdeveloped.The skin gills of starfish and the perioral gills of sea urchins can increase the capacity and area of respiration.Some sea cucumbers have special respiratory organs, called sucking trees.
Most echinoderms are dioecious, and there is no difference between male and female in appearance.Germ cells are released into seawater for fertilization.At the beginning, the larvae have the same shape, all of which have ciliary rings, and the digestive tract is divided into three parts. Later, they vary with the class, and develop into four different types of larvae: ear shaped larvae (sea cucumber), feather wrist larvae (starfish), snake tail larvae, and sea urchin larvae.CrinoidsThe larvae are barrel shaped, called thallus larvae, which are very similar to the thallus of planktonic Neobranchia.
Ecological habits
Echinodermata are important benthos, which are distributed in all the oceans of the world.The vertical distribution range is very wide, from the intertidal zone to the trench with a depth of 10000 meters.Most of them areNarrow salt animal, rare or rare in brackish water or low salt seawater.There is no special coating outside the body, but there is a special water pipe system.The water contained in the body can freely permeate with the external salinity.Echinodermata are very sensitive to water pollution, and they are rarely seen in polluted seawater.The regenerative power is generally strong. If the wrist, disc or other external organs are damaged or broken, they can regenerate.Some species often have the phenomenon of self cutting.A few species can reproduce asexually.
The habitat environment varies from species to species.They crawl on the sea floor or burrow into the mud and sand floor to live.A few sea urchins can not diamond.CrinoidsFixed or temporarily fixed life.A few sea cucumbers are floating.Free living species can move slowly.There are various ways of feeding, some are swallowing, some are filter feeding, and some are carnivorous.
There are no parasitic species of echinoderms except for the tail of the elapidus.Sucker insects are special polychaetes that are parasitic on crinoids.A few snails are parasitic on echinoderms, some are exoparasites, some are endoparasites.Crustaceans and echinoderms are parasitic or symbiotic.Cryptocarp is a famous animal parasitic in the discharge cavity of sea cucumber.Ciliates, flatworms and roundworms also parasitize in echinoderms.
classification
Echinodermata has developed coelom, trigerm and endoskeleton from mesoderm, which is much more advanced than coelenterates.The radial symmetry of adult echinoderms is a secondary trait acquired from the fixation of their bilaterally symmetrical ancestors.Some primitive traits still exist, such as no head and higher nerve center, primitive sensory organs, no heart, sinus system of blood vessels, lower respiratory organs, and primitive amoeba cells for nutrient delivery and waste removal.
Among the existing animals, no groups directly related to echinoderms have been found.Basser (1900) believed that the imaginary ancestors of echinoderms were symmetrical, soft and free living symmetrical larvae;
Arthropoda--Insecta -- animals under this definition
The common characteristics are: six legged, boneless, the body is supported by the hard shell on the body surface, divided into three growth stages of eggs, larvae and adults, and the body is multi segmented.
behavior
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When lower animals act, they are not simply pushed and pulled by external forces;But if it actively resists the pushing and pulling force, then it is behavior.The behavior of lower animals is usually expressed in response to certain sensory impression, such as sound, touch or light.Some mechanists arbitrarily stipulate that all movements are responses to some similar impression, which they call "stimulus reflex activities";In many cases, movement cannot be traced back to an external impression. They assume that there is some internal stimulus, that is, an impression from the sensory nerve inside the body, so that this obvious exception can also be included in their "rules".It is not yet possible to say whether this assumption has a solid foundation.Even if it is correct, a specific action is initiated by "stimulation".The continuous movement of behavior, that is, the action chain, will go on continuously independent of the stimulus.A sudden sound, such as the crisp sound of a broken twig, may make the rabbit run back to its cave, making birds fly away in groups, or make a wary deer look back and forth in fear;Taking my hat off the hook or saying a word may stimulate my dog to excite and jump intensely and continuously, which will last for a long time and can be repeated several times[2]。
The most inferior animals
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1. The largest jellyfish in the world -Arctic JellyfishThe Arctic Jellyfish, which lives in the Atlantic Ocean and Arctic Ocean, is the largest jellyfish in the world.Their bodies are like red brown or yellow parachutes. The diameter of the umbrella cover can reach 2.5 meters. Eight groups of 1200 tentacles protrude from the edge of the umbrella cover, and each group of tentacles can reach more than 40 meters after extension.
2. The animal with the most teeth in the world - snails and snails are commonly known as "mountain snails": gastropod mollusks usually have two pairs of antennae, one pair is longer and the other pair is shorter.Snails can eat hard food because they have 135 rows of regular horny teeth in their mouths, 105 in each row, and 14175 teeth in total, which can be called the animal with the most teeth in the world.
3. The largest spider in the world - bird catching spider: It lives in South America. It weaves webs in the woods and feeds on birds who throw themselves into the net.Bird catching spiders are generally as big as palms, hairy on the body surface, and have 8 eyes, all of which grow in the front of the dorsal armor.Some bird catching spiders also prey on other small vertebrates, such as frogs or mice[3]。
