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Scientific classificationAnimaliaInvertebrate_header_cell_0_1_0
(unranked):Invertebrate_cell_0_2_0 FilozoaInvertebrate_cell_0_2_1
Kingdom:Invertebrate_cell_0_3_0 AnimaliaInvertebrate_cell_0_3_1
Groups includedInvertebrate_header_cell_0_4_0
Cladistically included but traditionally excluded taxaInvertebrate_header_cell_0_5_0

Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a backbone or spine), derived from the notochord. Invertebrate_sentence_0

This includes all animals apart from the subphylum Vertebrata. Invertebrate_sentence_1

Familiar examples of invertebrates include arthropods (insects, arachnids, crustaceans, and myriapods), mollusks (chitons, snail, bivalves, squids, and octopuses), annelid (earthworms and leeches), and cnidarians (hydras, jellyfishes, sea anemones, and corals). Invertebrate_sentence_2

The majority of animal species are invertebrates; one estimate puts the figure at 97%. Invertebrate_sentence_3

Many invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata. Invertebrate_sentence_4

Invertebrates vary widely in size, from 50 μm (0.002 in) rotifers to the 9–10 m (30–33 ft) colossal squid. Invertebrate_sentence_5

Some so-called invertebrates, such as the Tunicata and Cephalochordata, are more closely related to vertebrates than to other invertebrates. Invertebrate_sentence_6

This makes the invertebrates paraphyletic, so the term has little meaning in taxonomy. Invertebrate_sentence_7

Etymology Invertebrate_section_0

The word "invertebrate" comes from the Latin word vertebra, which means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate. Invertebrate_sentence_8

The jointed aspect of vertebra is derived from the concept of turning, expressed in the root verto or vorto, to turn. Invertebrate_sentence_9

The prefix in- means "not" or "without". Invertebrate_sentence_10

Taxonomic significance Invertebrate_section_1

The term invertebrates is not always precise among non-biologists since it does not accurately describe a taxon in the same way that Arthropoda, Vertebrata or Manidae do. Invertebrate_sentence_11

Each of these terms describes a valid taxon, phylum, subphylum or family. Invertebrate_sentence_12

"Invertebrata" is a term of convenience, not a taxon; it has very little circumscriptional significance except within the Chordata. Invertebrate_sentence_13

The Vertebrata as a subphylum comprises such a small proportion of the Metazoa that to speak of the kingdom Animalia in terms of "Vertebrata" and "Invertebrata" has limited practicality. Invertebrate_sentence_14

In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing a cladogram, for example, the presence of a notochord. Invertebrate_sentence_15

That would at least circumscribe the Chordata. Invertebrate_sentence_16

However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry or perhaps bauplan. Invertebrate_sentence_17

Despite this, the concept of invertebrates as a taxon of animals has persisted for over a century among the laity, 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. Invertebrate_sentence_18

The following text reflects earlier scientific understanding of the term and of those animals which have constituted it. Invertebrate_sentence_19

According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. Invertebrate_sentence_20

They include hugely varied body plans. Invertebrate_sentence_21

Many have fluid-filled, hydrostatic skeletons, like jellyfish or worms. Invertebrate_sentence_22

Others have hard exoskeletons, outer shells like those of insects and crustaceans. Invertebrate_sentence_23

The most familiar invertebrates include the Protozoa, Porifera, Coelenterata, Platyhelminthes, Nematoda, Annelida, Echinodermata, Mollusca and Arthropoda. Invertebrate_sentence_24

Arthropoda include insects, crustaceans and arachnids. Invertebrate_sentence_25

Number of extant species Invertebrate_section_2

By far the largest number of described invertebrate species are insects. Invertebrate_sentence_26

The following table lists the number of described extant species for major invertebrate groups as estimated in the IUCN Red List of Threatened Species, 2014.3. Invertebrate_sentence_27


Invertebrate groupInvertebrate_header_cell_1_0_0 Latin nameInvertebrate_header_cell_1_0_1 ImageInvertebrate_header_cell_1_0_2 Estimated number of

described speciesInvertebrate_header_cell_1_0_3

InsectsInvertebrate_cell_1_1_0 InsectaInvertebrate_cell_1_1_1 Invertebrate_cell_1_1_2 1,000,000Invertebrate_cell_1_1_3
ArachnidsInvertebrate_cell_1_2_0 ArachnidaInvertebrate_cell_1_2_1 Invertebrate_cell_1_2_2 102,248Invertebrate_cell_1_2_3
MolluscsInvertebrate_cell_1_3_0 MolluscaInvertebrate_cell_1_3_1 Invertebrate_cell_1_3_2 85,000Invertebrate_cell_1_3_3
CrustaceansInvertebrate_cell_1_4_0 CrustaceaInvertebrate_cell_1_4_1 Invertebrate_cell_1_4_2 47,000Invertebrate_cell_1_4_3
CoralsInvertebrate_cell_1_5_0 AnthozoaInvertebrate_cell_1_5_1 Invertebrate_cell_1_5_2 2,175Invertebrate_cell_1_5_3
Velvet wormsInvertebrate_cell_1_6_0 OnychophoraInvertebrate_cell_1_6_1 Invertebrate_cell_1_6_2 165Invertebrate_cell_1_6_3
Horseshoe crabsInvertebrate_cell_1_7_0 XiphosuraInvertebrate_cell_1_7_1 Invertebrate_cell_1_7_2 4Invertebrate_cell_1_7_3

jellyfish, echinoderms, sponges, other worms etc.Invertebrate_cell_1_8_0

Invertebrate_cell_1_8_1 Invertebrate_cell_1_8_2 68,658Invertebrate_cell_1_8_3
Invertebrate_cell_1_9_0 Invertebrate_cell_1_9_1 Total:Invertebrate_cell_1_9_2 ~1,300,000Invertebrate_cell_1_9_3

The IUCN estimates that 66,178 extant vertebrate species have been described, which means that over 95% of the described animal species in the world are invertebrates. Invertebrate_sentence_28

Characteristics Invertebrate_section_3

The trait that is common to all invertebrates is the absence of a vertebral column (backbone): this creates a distinction between invertebrates and vertebrates. Invertebrate_sentence_29

The distinction is one of convenience only; it is not based on any clear biologically homologous trait, any more than the common trait of having wings functionally unites insects, bats, and birds, or than not having wings unites tortoises, snails and sponges. Invertebrate_sentence_30

Being animals, invertebrates are heterotrophs, and require sustenance in the form of the consumption of other organisms. Invertebrate_sentence_31

With a few exceptions, such as the Porifera, invertebrates generally have bodies composed of differentiated tissues. Invertebrate_sentence_32

There is also typically a digestive chamber with one or two openings to the exterior. Invertebrate_sentence_33

Morphology and symmetry Invertebrate_section_4

The body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical. Invertebrate_sentence_34

A minority, however, exhibit no symmetry. Invertebrate_sentence_35

One example of asymmetric invertebrates includes all gastropod species. Invertebrate_sentence_36

This is easily seen in snails and sea snails, which have helical shells. Invertebrate_sentence_37

Slugs appear externally symmetrical, but their pneumostome (breathing hole) is located on the right side. Invertebrate_sentence_38

Other gastropods develop external asymmetry, such as Glaucus atlanticus that develops asymmetrical cerata as they mature. Invertebrate_sentence_39

The origin of gastropod asymmetry is a subject of scientific debate. Invertebrate_sentence_40

Other examples of asymmetry are found in fiddler crabs and hermit crabs. Invertebrate_sentence_41

They often have one claw much larger than the other. Invertebrate_sentence_42

If a male fiddler loses its large claw, it will grow another on the opposite side after moulting. Invertebrate_sentence_43

Sessile animals such as sponges are asymmetrical alongside coral colonies (with the exception of the individual polyps that exhibit radial symmetry); alpheidae claws that lack pincers; and some copepods, polyopisthocotyleans, and monogeneans which parasitize by attachment or residency within the gill chamber of their fish hosts). Invertebrate_sentence_44

Nervous system Invertebrate_section_5

Neurons differ in invertebrates from mammalian cells. Invertebrate_sentence_45

Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH. Invertebrate_sentence_46

The first invertebrate in which a neuron cell was identified was the medicinal leech, Hirudo medicinalis. Invertebrate_sentence_47

Learning and memory using nociceptors in the sea hare, Aplysia has been described. Invertebrate_sentence_48

Mollusk neurons are able to detect increasing pressures and tissue trauma. Invertebrate_sentence_49

Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods. Invertebrate_sentence_50

Respiratory system Invertebrate_section_6

One type of invertebrate respiratory system is the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues. Invertebrate_sentence_51

The distribution of spiracles can vary greatly among the many orders 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. Invertebrate_sentence_52

The tracheae are invaginations of the cuticular exoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm. Invertebrate_sentence_53

The smallest tubes, tracheoles, penetrate cells and serve as sites of diffusion for water, oxygen, and carbon dioxide. Invertebrate_sentence_54

Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion. Invertebrate_sentence_55

Unlike vertebrates, insects do not generally carry oxygen in their haemolymph. Invertebrate_sentence_56

A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices. Invertebrate_sentence_57

In the head, thorax, or abdomen, tracheae may also be connected to air sacs. Invertebrate_sentence_58

Many insects, such as grasshoppers and bees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. Invertebrate_sentence_59

In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a gill, or function essentially as normal, via a plastron. Invertebrate_sentence_60

Note that despite being internal, the tracheae of arthropods are shed during moulting (ecdysis). Invertebrate_sentence_61

Reproduction Invertebrate_section_7

Like vertebrates, most invertebrates reproduce at least partly through sexual reproduction. Invertebrate_sentence_62

They produce specialized reproductive cells that undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova. Invertebrate_sentence_63

These fuse to form zygotes, which develop into new individuals. Invertebrate_sentence_64

Others are capable of asexual reproduction, or sometimes, both methods of reproduction. Invertebrate_sentence_65

Social interaction Invertebrate_section_8

Social behavior is widespread in invertebrates, including cockroaches, termites, aphids, thrips, ants, bees, Passalidae, Acari, spiders, and more. Invertebrate_sentence_66

Social interaction is particularly salient in eusocial species but applies to other invertebrates as well. Invertebrate_sentence_67

Insects recognize information transmitted by other insects. Invertebrate_sentence_68

Phyla Invertebrate_section_9

The term invertebrates covers several phyla. Invertebrate_sentence_69

One of these are the sponges (Porifera). Invertebrate_sentence_70

They were long thought to have diverged from other animals early. Invertebrate_sentence_71

They lack the complex organization found in most other phyla. Invertebrate_sentence_72

Their cells are differentiated, but in most cases not organized into distinct tissues. Invertebrate_sentence_73

Sponges typically feed by drawing in water through pores. Invertebrate_sentence_74

Some speculate that sponges are not so primitive, but may instead be secondarily simplified. Invertebrate_sentence_75

The Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Invertebrate_sentence_76

Both have distinct tissues, but they are not organized into organs. Invertebrate_sentence_77

There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. Invertebrate_sentence_78

As such, they are sometimes called diploblastic. Invertebrate_sentence_79

The Echinodermata are radially symmetric and exclusively marine, including starfish (Asteroidea), sea urchins, (Echinoidea), brittle stars (Ophiuroidea), sea cucumbers (Holothuroidea) and feather stars (Crinoidea). Invertebrate_sentence_80

The largest animal phylum is also included within invertebrates: the Arthropoda, including insects, spiders, crabs, and their kin. Invertebrate_sentence_81

All these organisms have a body divided into repeating segments, typically with paired appendages. Invertebrate_sentence_82

In addition, they possess a hardened exoskeleton that is periodically shed during growth. Invertebrate_sentence_83

Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. Invertebrate_sentence_84

The Nematoda or roundworms, are perhaps the second largest animal phylum, and are also invertebrates. Invertebrate_sentence_85

Roundworms are typically microscopic, and occur in nearly every environment where there is water. Invertebrate_sentence_86

A number are important parasites. Invertebrate_sentence_87

Smaller phyla related to them are the Kinorhyncha, Priapulida, and Loricifera. Invertebrate_sentence_88

These groups have a reduced coelom, called a pseudocoelom. Invertebrate_sentence_89

Other invertebrates include the Nemertea or ribbon worms, and the Sipuncula. Invertebrate_sentence_90

Another phylum is Platyhelminthes, the flatworms. Invertebrate_sentence_91

These were originally considered primitive, but it now appears they developed from more complex ancestors. Invertebrate_sentence_92

Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha. Invertebrate_sentence_93

The Rotifera or rotifers, are common in aqueous environments. Invertebrate_sentence_94

Invertebrates also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and the Cycliophora. Invertebrate_sentence_95

Also included are two of the most successful animal phyla, the Mollusca and Annelida. Invertebrate_sentence_96

The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. Invertebrate_sentence_97

These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented. Invertebrate_sentence_98

Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla. Invertebrate_sentence_99

Among lesser phyla of invertebrates are the Hemichordata, or acorn worms, and the Chaetognatha, or arrow worms. Invertebrate_sentence_100

Other phyla include Acoelomorpha, Brachiopoda, Bryozoa, Entoprocta, Phoronida, and Xenoturbellida. Invertebrate_sentence_101

Classification of invertebrates Invertebrate_section_10

Invertebrates can be classified into several main categories, some of which are taxonomically obsolescent or debatable, but still used as terms of convenience. Invertebrate_sentence_102

Each however appears in its own article at the following links. Invertebrate_sentence_103

Macroinvertebrates Invertebrate_section_11


Microscopic invertebrates Invertebrate_section_12


History Invertebrate_section_13

The earliest animal fossils appear to be those of invertebrates. Invertebrate_sentence_104

665-million-year-old fossils in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia have been interpreted as being early sponges. Invertebrate_sentence_105

Some paleontologists suggest that animals appeared much earlier, possibly as early as 1 billion years ago. Invertebrate_sentence_106

Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms. Invertebrate_sentence_107

Around 453 MYA, animals began diversifying, and many of the important groups of invertebrates diverged from one another. Invertebrate_sentence_108

Fossils of invertebrates are found in various types of sediment from the Phanerozoic. Invertebrate_sentence_109

Fossils of invertebrates are commonly used in stratigraphy. Invertebrate_sentence_110

Classification Invertebrate_section_14

Carl Linnaeus divided these animals into only two groups, the Insecta and the now-obsolete Vermes (worms). Invertebrate_sentence_111

Jean-Baptiste Lamarck, who was appointed to the position of "Curator of Insecta and Vermes" at the Muséum 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 and Infusoria from the Linnean Vermes. Invertebrate_sentence_112

They are now classified into over 30 phyla, from simple organisms such as sea sponges and flatworms to complex animals such as arthropods and molluscs. Invertebrate_sentence_113

Significance of the group Invertebrate_section_15

Invertebrates are animals without a vertebral column. Invertebrate_sentence_114

This has led to the conclusion that invertebrates are a group that deviates from the normal, vertebrates. Invertebrate_sentence_115

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. Invertebrate_sentence_116

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." Invertebrate_sentence_117

Another reason cited for this continued distinction is that Lamarck created a precedent through his classifications which is now difficult to escape from. Invertebrate_sentence_118

It is also possible that some humans believe that, they themselves being vertebrates, the group deserves more attention than invertebrates. Invertebrate_sentence_119

In any event, in the 1968 edition of Invertebrate 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." Invertebrate_sentence_120

The book also points out that the group lumps a vast number of species together, so that no one characteristic describes all invertebrates. Invertebrate_sentence_121

In addition, some species included are only remotely related to one another, with some more related to vertebrates than other invertebrates (see Paraphyly). Invertebrate_sentence_122

In research Invertebrate_section_16

For many centuries, invertebrates were neglected by biologists, in favor of big vertebrates and "useful" or charismatic species. Invertebrate_sentence_123

Invertebrate biology was not a major field of study until the work of Linnaeus and Lamarck in the 18th century. Invertebrate_sentence_124

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. Invertebrate_sentence_125

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. Invertebrate_sentence_126

Two of the most commonly studied model organisms nowadays are invertebrates: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Invertebrate_sentence_127

They have long been the most intensively studied model organisms, and were among the first life-forms to be genetically sequenced. Invertebrate_sentence_128

This was facilitated by the severely reduced state of their genomes, but many genes, introns, and linkages have been lost. Invertebrate_sentence_129

Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to animals. Invertebrate_sentence_130

Invertebrates are also used by scientists in the field of aquatic biomonitoring to evaluate the effects of water pollution and climate change. Invertebrate_sentence_131

See also Invertebrate_section_17


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