The majority of animal species are invertebrates; one estimate puts the figure at 97%.[1] Many invertebratetaxa have a greater number and diversity of species than the entire subphylum of Vertebrata.[2] Invertebrates vary widely in size, from 10 μm (0.0004 in)[3]myxozoans to the 9–10 m (30–33 ft)colossal squid.[4]
Some so-called invertebrates, such as theTunicata andCephalochordata, are actually sister chordate subphyla to Vertebrata, being more closely related to vertebrates than to other invertebrates. This makes the "invertebrates"paraphyletic, so the term has no significance intaxonomy.
Etymology
The word "invertebrate" comes from the Latin wordvertebra, which means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate. The jointed aspect ofvertebra is derived from the concept of turning, expressed in the rootverto orvorto, to turn.[5] The prefixin- means "not" or "without".[6]
Taxonomic significance
The terminvertebrates does not describe ataxon in the same way thatArthropoda,Vertebrata orManidae do. Each of those terms describes a valid taxon,phylum,subphylum orfamily. "Invertebrata" is a term of convenience, not a taxon; it has very littlecircumscriptional significance except within theChordata. The Vertebrata as asubphylum comprises such a small proportion of theMetazoa that to speak of thekingdomAnimalia in terms of "Vertebrata" and "Invertebrata" has limited practicality. In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing acladogram, for example, the presence of anotochord. That would at least circumscribe the Chordata. However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry[7] or perhapsBauplan.[8]
Despite this, the concept ofinvertebrates as a taxon of animals has persisted for over a century among thelaity,[9] and within the zoological community and in its literature it remains in use as a term of convenience for animals that are not members of the Vertebrata.[10] The following text reflects earlier scientific understanding of the term and of those animals which have constituted it. According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. They include hugely variedbody plans. Many have fluid-filled, hydrostatic skeletons, likejellyfish or worms. Others have hardexoskeletons, outer shells like those ofinsects andcrustaceans. The most familiar invertebrates include theProtozoa,Porifera,Coelenterata,Platyhelminthes,Nematoda,Annelida,Echinodermata,Mollusca andArthropoda. Arthropoda includeinsects,crustaceans andarachnids.
Number of extant species
By far the largest number of described invertebrate species are insects. The following table lists the number of describedextant species for major invertebrate groups as estimated in theIUCN Red List of Threatened Species, 2014.3.[11]
TheIUCN estimates that 66,178extant vertebrate species have been described,[11] which means that over 95% of the described animal species in the world are invertebrates.
Characteristics
The trait that is common to all invertebrates is the absence of avertebral column (backbone): this creates a distinction between invertebrates and vertebrates. The distinction is one of convenience only; it is not based on any clear biologicallyhomologous trait, any more than the common trait of having wings functionally unites insects, bats, and birds, or than not having wings unitestortoises,snails andsponges. Being animals, invertebrates are heterotrophs, and require sustenance in the form of the consumption of other organisms. With a few exceptions, such as thePorifera, invertebrates generally have bodies composed of differentiated tissues. There is also typically a digestive chamber with one or two openings to the exterior.
Morphology and symmetry
Thebody plans of mostmulticellular organisms exhibit some form ofsymmetry, whether radial, bilateral, or spherical. A minority, however, exhibit no symmetry. One example of asymmetric invertebrates includes allgastropod species. This is easily seen insnails andsea snails, which have helical shells.Slugs appear externally symmetrical, but theirpneumostome (breathing hole) is located on the right side. Other gastropods develop external asymmetry, such asGlaucus atlanticus that develops asymmetricalcerata as they mature. The origin of gastropod asymmetry is a subject of scientific debate.[12]
Other examples of asymmetry are found infiddler crabs andhermit crabs. They often have one claw much larger than the other. If a male fiddler loses its large claw, it will grow another on the opposite side aftermoulting.Sessile animals such assponges are asymmetrical[13] alongsidecoralcolonies (with the exception of the individualpolyps that exhibit radial symmetry);Alpheidae claws that lack pincers; and somecopepods,polyopisthocotyleans, andmonogeneans which parasitize by attachment or residency within thegill chamber of theirfishhosts).
Nervous system
Neurons differ in invertebrates from mammalian cells. Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH. The first invertebrate in which a neuron cell was identified was the medicinalleech,Hirudo medicinalis.[14][15]Learning and memory using nociceptors have been described in the sea hare,Aplysia.[16][17][18] Mollusk neurons are able to detect increasing pressures and tissue trauma.[19]
Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs,nematodes and arthropods.[20][21]
Respiratory system
Tracheal system of dissectedcockroach. The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm.The tracheal system branches into progressively smaller tubes, here supplying thecrop of the cockroach. Scale bar, 2.0 mm.
One type of invertebrate respiratory system is the openrespiratory system composed ofspiracles, tracheae, andtracheoles thatterrestrial arthropods have to transportmetabolic gases to and from tissues.[22] The distribution of spiracles can vary greatly among the manyorders of insects, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticularexoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm. The smallest tubes, tracheoles, penetrate cells and serve as sites ofdiffusion forwater,oxygen, andcarbon dioxide. Gas may be conducted through the respiratory system by means of activeventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in theirhaemolymph.[23]
A tracheal tube may contain ridge-like circumferential rings oftaenidia in variousgeometries such as loops orhelices. In thehead,thorax, orabdomen, tracheae may also be connected to air sacs. Many insects, such asgrasshoppers andbees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of agill, or function essentially as normal, via aplastron. Despite being internal, the tracheae of arthropods are shed during moulting (ecdysis).[24]
Only vertebrate animals have ears, though many invertebrates detect sound using other kinds of sense organs. In insects,tympanal organs are used to hear distant sounds. They are located either on the head or elsewhere, depending on the insectfamily.[25] The tympanal organs of some insects are extremely sensitive, offering acute hearing beyond that of most other animals. The female cricket flyOrmia ochracea has tympanal organs on each side of her abdomen. They are connected by a thin bridge of exoskeleton and they function like a tiny pair of eardrums, but, because they are linked, they provide acute directional information. The fly uses her "ears" to detect the call of her host, a male cricket. Depending on where the song of the cricket is coming from, the fly's hearing organs will reverberate at slightly different frequencies. This difference may be as little as 50 billionths of a second, but it is enough to allow the fly to home in directly on a singing male cricket and parasitise it.[26]
Simpler structures allow otherarthropods to detectnear-field sounds. Spiders and cockroaches, for example, have hairs on their legs, which are used for detecting sound. Caterpillars may also have hairs on their body that perceive vibrations[27] and allow them to respond to sound.
Reproduction
Like vertebrates, most invertebrates reproduce at least partly throughsexual reproduction. They produce specializedreproductive cells that undergomeiosis to produce smaller, motilespermatozoa or larger, non-motileova.[28] These fuse to formzygotes, which develop into new individuals.[29] Others are capable ofasexual reproduction, or sometimes, both methods of reproduction.
Extensive research with model invertebrate species such asDrosophila melanogaster andCaenorhabditis elegans has contributed much to our understanding ofmeiosis and reproduction. However, beyond the few model systems, the modes of reproduction found in invertebrates show incredible diversity.[30] In one extreme example, it is estimated that 10% oforbatid mite species have persisted without sexual reproduction and have reproduced asexually for more than 400 million years.[30]
Invertebrates have an extremely diverse array of reproductive systems, the only commonality may be that they all lay eggs. Also, aside fromcephalopods andarthropods, nearly all other invertebrates exhibitexternal fertilization.
Social interaction
Social behavior is widespread in invertebrates, including cockroaches, termites, aphids,thrips, ants, bees,Passalidae,Acari, spiders, and more.[31] Social interaction is particularly salient ineusocial species but applies to other invertebrates as well.
Insects recognize information transmitted by other insects.[32][33][34]
The term invertebrates covers several phyla. One of these are the sponges (Porifera). They were long thought to have diverged from other animals early.[35] They lack the complex organization found in most other phyla.[36] Their cells are differentiated, but in most cases not organized into distinct tissues.[37] Sponges typically feed by drawing in water through pores.[38] Some speculate that sponges are not so primitive, but may instead be secondarily simplified.[39] TheCtenophora and theCnidaria, which includessea anemones,corals, andjellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.[40] Both have distinct tissues, but they are not organized intoorgans.[41] There are only two main germ layers, theectoderm andendoderm, with only scattered cells between them. As such, they are sometimes calleddiploblastic.[42]
The largest animal phylum is also included within invertebrates: the Arthropoda, including insects,spiders,crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. In addition, they possess a hardened exoskeleton that is periodically shed during growth.[44] Two smaller phyla, theOnychophora andTardigrada, are close relatives of the arthropods and share some traits with them, excluding the hardened exoskeleton. TheNematoda, or roundworms, are perhaps the second largest animal phylum, and are also invertebrates. Roundworms are typically microscopic, and occur in nearly every environment where there is water.[45] A number are important parasites.[46] Smaller phyla related to them are theKinorhyncha,Priapulida, andLoricifera. These groups have a reduced coelom, called a pseudocoelom. Other invertebrates include theNemertea, or ribbon worms, and theSipuncula.
Another phylum isPlatyhelminthes, the flatworms.[47] These were originally considered primitive, but it now appears they developed from more complex ancestors.[48] Flatworms areacoelomates, lacking a body cavity, as are their closest relatives, the microscopicGastrotricha.[49] TheRotifera, or rotifers, are common in aqueous environments. Invertebrates also include theAcanthocephala, or spiny-headed worms, theGnathostomulida,Micrognathozoa, and theCycliophora.[50]
Also included are two of the most successful animal phyla, the Mollusca and Annelida.[51][52] The former, which is the second-largest animal phylum by number of described species, includes animals such assnails,clams, andsquids, and the latter comprises the segmented worms, such asearthworms andleeches. These two groups have long been considered close relatives because of the common presence oftrochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented.[53] Now, this is generally consideredconvergent evolution, owing to many morphological and genetic differences between the two phyla.[54]
Invertebrates can be classified into several main categories, some of which aretaxonomically obsolescent or debatable, but still used as terms of convenience. Each however appears in its own article at the following links.[56]
The earliest animal fossils are of invertebrates. 665-million-year-old fossils in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia have been interpreted as being early sponges.[57] Some paleontologists suggest that animals appeared much earlier, possibly as early as 1 billion years ago[58] though they probably became multicellular in theTonian.Trace fossils such as tracks and burrows found in the lateNeoproterozoic Era indicate the presence oftriploblastic worms, roughly as large (about 5 mm wide) and complex asearthworms.[59]
Around 453 MYA, animals began diversifying, and many of the important groups of invertebrates diverged from one another. Fossils of invertebrates are found in various types of sediment from thePhanerozoic.[60] Fossils of invertebrates are commonly used in stratigraphy.[61]
Classification
Carl Linnaeus divided these animals into only two groups, the Insecta and the now-obsoleteVermes (worms).Jean-Baptiste Lamarck, who was appointed to the position of "Curator of Insecta and Vermes" at theMuséum National d'Histoire Naturelle in 1793, both coined the term "invertebrate" to describe such animals and divided the original two groups into ten, by splitting Arachnida and Crustacea from the Linnean Insecta, and Mollusca, Annelida,Cirripedia,Radiata,Coelenterata andInfusoria from the Linnean Vermes. They are now classified into over 30phyla, from simple organisms such assea sponges andflatworms to complex animals such as arthropods and molluscs.
Significance
Invertebrates are animals without a vertebral column. This has led to the conclusion thatinvertebrates are a group that deviates from the normal, vertebrates. This has been said to be because researchers in the past, such as Lamarck, viewed vertebrates as a "standard": in Lamarck's theory of evolution, he believed that characteristics acquired through the evolutionary process involved not only survival, but also progression toward a "higher form", to which humans and vertebrates were closer than invertebrates were. Although goal-directed evolution has been abandoned, the distinction of invertebrates and vertebrates persists to this day, even though the grouping has been noted to be "hardly natural or even very sharp." Another reason cited for this continued distinction is that Lamarck created a precedent through his classifications which is now difficult to escape from. It is also possible that some humans believe that, they themselves being vertebrates, the group deserves more attention than invertebrates.[62] In any event, in the 1968 edition ofInvertebrate Zoology, it is noted that "division of the Animal Kingdom into vertebrates and invertebrates is artificial and reflects human bias in favor of man's own relatives." The book also points out that the group lumps a vast number of species together, so that no one characteristic describes all invertebrates. In addition, some species included are only remotely related to one another, with some more related to vertebrates than other invertebrates (seeParaphyly).[63]
In research
For many centuries, invertebrates were neglected by biologists, in favor of big vertebrates and "useful" orcharismatic species.[64] Invertebrate biology was not a major field of study until the work ofLinnaeus andLamarck in the 18th century.[64] During the 20th century, invertebrate zoology became one of the major fields of natural sciences, with prominent discoveries in the fields of medicine, genetics, palaeontology, and ecology.[64] The study of invertebrates has also benefited law enforcement, as arthropods, and especially insects, were discovered to be a source of information for forensic investigators.[44]
Two of the most commonly studied model organisms nowadays are invertebrates: the fruit flyDrosophila melanogaster and the nematodeCaenorhabditis elegans. They have long been the most intensively studiedmodel organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of theirgenomes, but manygenes,introns, andlinkages have been lost. Analysis of thestarlet sea anemone genome has emphasised the importance of sponges, placozoans, andchoanoflagellates, also being sequenced, in explaining the arrival of 1,500 ancestral genes unique to animals.[65] Invertebrates are also used by scientists in the field ofaquatic biomonitoring to evaluate the effects of water pollution andclimate change.[66]
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