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Invertebrates are a
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

paraphyletic
group of
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...

animal
s that neither possess nor develop a
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...

vertebral column
(commonly known as a ''backbone'' or ''spine''), derived from the
notochord In Developmental anatomy, anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its Biological life cycle, life cycle (along with 4 other features), it is, b ...
. This is a grouping including all
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...

animal
s apart from the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five Apomorphy and synapomorphy, synapomorphies, or primary physical characteristics, that distinguish them from al ...

chordate
subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of "subdivision (rank), subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists ...
Vertebrata Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...
. Familiar examples of invertebrates include
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s,
mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
s,
annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant taxon, extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to v ...
s,
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the Crinoi ...
s and
cnidarian Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, predominantly the latter. Their distinguishing feature is cnidocyt ...

cnidarian
s. The majority of animal species are invertebrates; one estimate puts the figure at 97%. Many invertebrate
taxa In biology, a taxon (back-formation from ''Taxonomy (biology), taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known ...
have a greater number and variety of species than the entire subphylum of Vertebrata. Invertebrates vary widely in size, from 50  μm (0.002 in)
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first d ...

rotifer
s to the 9–10 m (30–33 ft)
colossal squid The colossal squid (''Mesonychoteuthis hamiltoni'') is part of the family Cranchiidae. It is sometimes called the Antarctic squid or giant cranch squid and is believed to be the largest squid species in terms of mass. It is the only recognize ...
. Some so-called invertebrates, such as the Tunicata and
Cephalochordata A cephalochordate (from Greek language, Greek: κεφαλή ''kephalé'', "head" and χορδή ''khordé'', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called Lancelet, lancelets. Cephalochordates possess ...
, are more closely related to vertebrates than to other invertebrates. This makes the invertebrates
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
, so the term has little meaning in
taxonomy Taxonomy is the practice and science of categorization or classification (general theory), classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are ...
.


Etymology

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. The jointed aspect of ''vertebra'' is derived from the concept of turning, expressed in the root ''verto'' or ''vorto'', to turn. The prefix ''in-'' means "not" or "without".


Taxonomic significance

The term ''invertebrates'' is not always precise among non-biologists since it does not accurately describe a
taxon In biology, a taxon (back-formation from ''Taxonomy (biology), taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known ...
in the same way that
Arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
a,
Vertebrata Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

Vertebrata
or
Manidae Manidae is the only Neontology, extant family of pangolins from superfamily Manoidea. This family comprises three genera (''Manis'' from subfamily Maninae, ''Phataginus'' from subfamily Phatagininae, and ''Smutsia'' from subfamily Smutsiinae), as ...

Manidae
do. Each of these terms describes a valid taxon,
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (biology), division has been used instead of ph ...
,
subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of "subdivision (rank), subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists ...
or
family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
. "Invertebrata" is a term of convenience, not a taxon; it has very little circumscriptional significance except within the
Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five Apomorphy and synapomorphy, synapomorphies, or primary physical characteristics, that distinguish them from al ...
. The Vertebrata as a
subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of "subdivision (rank), subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists ...
comprises such a small proportion of the
Metazoa Animals are multicellular A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellula ...

Metazoa
that to speak of the kingdom
Animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...

Animal
ia 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 a
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not ...

cladogram
, for example, the presence of a
notochord In Developmental anatomy, anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its Biological life cycle, life cycle (along with 4 other features), it is, b ...
. That would at least circumscribe the Chordata. However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry or perhaps
bauplan A body plan, ( ), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually ap ...
. Despite this, the concept of ''invertebrates'' as a taxon of animals has persisted for over a century among the
laity In religious organizations, the laity () consists of all Church membership, members who are not part of the clergy, usually including any non-Ordination, ordained members of religious orders, e.g. a nun or a lay brother. In both religious and wi ...
, 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. 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 varied
body plan A body plan, ( ), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually ap ...
s. Many have fluid-filled, hydrostatic skeletons, like
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...

jellyfish
or worms. Others have hard
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of th ...

exoskeleton
s, outer shells like those of
insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, Thorax (ins ...

insect
s and
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacea ...
s. The most familiar invertebrates include the
Protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other microorganisms or o ...

Protozoa
,
Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

Porifera
,
Coelenterata Coelenterata is a term encompassing the animal phyla Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, p ...
,
Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...
,
Nematoda The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below Kingdom (biology), kingdom and above Class (bio ...

Nematoda
,
Annelida
Annelida
,
Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the Crinoi ...

Echinodermata
,
Mollusca Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species In biology, a species is the basic unit of Taxonomy (bio ...

Mollusca
and
Arthropoda Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

Arthropoda
. Arthropoda include
insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, Thorax (ins ...

insect
s,
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacea ...
s and
arachnid Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, came ...

arachnid
s.


Number of extant species

By far the largest number of described invertebrate species are insects. The following table lists the number of described
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
species for major invertebrate groups as estimated in the
IUCN Red List of Threatened Species The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biol ...
'', 2014.3.The World Conservation Union. 2014. ''
IUCN Red List of Threatened Species The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biol ...
'', 2014.3. Summary Statistics for Globally Threatened Species
Table 1: Numbers of threatened species by major groups of organisms (1996–2014)
The
IUCN The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natura ...
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.


Characteristics

The trait that is common to all invertebrates is the absence of a
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...

vertebral column
(backbone): this creates a distinction between invertebrates and vertebrates. 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
tortoise Tortoises () are reptiles of the family Testudinidae of the order Testudines (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect ...

tortoise
s,
snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the m ...

snail
s and
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
s. Being animals, invertebrates are heterotrophs, and require sustenance in the form of the consumption of other organisms. With a few exceptions, such as the
Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

Porifera
, 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

The
body plan A body plan, ( ), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually ap ...
s of most
multicellular organism A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
s exhibit some form of
symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
, whether radial, bilateral, or spherical. A minority, however, exhibit no symmetry. One example of asymmetric invertebrates includes all
gastropod The gastropods (), commonly known as snails and slugs, belong to a large Taxonomy (biology), taxonomic class (taxonomy), class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater ...
species. This is easily seen in
snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the m ...

snail
s and
sea snail Sea snail is a common name for slow-moving marine (ocean), marine gastropod Mollusca, molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from sna ...

sea snail
s, which have helical shells.
Slug Slug, or land slug, is a common name for any apparently shell-less terrestrial molluscs, terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced ...

Slug
s appear externally symmetrical, but their
pneumostome The pneumostome or breathing pore is a respiratory opening of the external body anatomy of an air-breathing land slug or land snail. It is a part of the respiratory system of gastropods. It is an opening in the right side of the Mantle (mollu ...

pneumostome
(breathing hole) is located on the right side. Other gastropods develop external asymmetry, such as that develops asymmetrical cerata as they mature. The origin of gastropod asymmetry is a subject of scientific debate. Other examples of asymmetry are found in s and
hermit crab Hermit crabs are anomuran Decapoda, decapod crustaceans of the superfamily (taxonomy), superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit c ...

hermit crab
s. 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 after
moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
.
Sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...
animals such as
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
s are asymmetricalSymmetry, biological
cited at FactMonster.com from ''
The Columbia Electronic Encyclopedia The ''Columbia Encyclopedia'' is a one-volume encyclopedia produced by Columbia University Press and, in the last edition, sold by the Gale Group. First published in 1935, and continuing its relationship with Columbia University, the encyclopedi ...
'' (2007).
alongside
coral Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important C ...
colonies In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the ''metropole, metropolit ...
(with the exception of the individual polyps that exhibit radial symmetry); alpheidae claws that lack pincers; and some
copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s,
polyopisthocotylea Polyopisthocotylea is a subclass of parasitic flatworms in the class Monogenea. WoRMS (2019). Polyopisthocotylea. Accessed at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=119220 on 2019-02-08Yamaguti, S. (1963). Systema Helminthum Volum ...
ns, and
monogenea Monogeneans are a group of ectoparasitic flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', me ...
ns which parasitize by attachment or residency within the
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
chamber of their
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
hosts).


Nervous system

Neurons A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...
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 medicinal
leech Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...
, ''
Hirudo medicinalis ''Hirudo medicinalis'', the European medicinal leech, is one of several species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species ...
.''Nicholls, J.G. and Baylor, D.A., (1968). Specific modalities and receptive fields of sensory neurons in CNS of the leech. Journal of Neurophysiology, 31: 740–756Pastor, J., Soria, B. and Belmonte, C., (1996). Properties of the nociceptive neurons of the leech segmental ganglion. Journal of Neurophysiology, 75: 2268–2279 Learning and memory using nociceptors in the sea hare, ''
Aplysia ''Aplysia'' () is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biolo ...
'' has been described.Byrne, J.H., Castellucci, V.F. and Kandel, E.R., (1978). Contribution of individual mechanoreceptor sensory neurons to defensive gill-withdrawal reflex in Aplysia. Journal of Neurophysiology, 41: 418–431Castellucci, V., Pinsker, H., Kupfermann, I. and Kandel, E.R., (1970). Neuronal mechanisms of habituation and dishabituation of the gill-withdrawal reflex in Aplysia. Science, 167: 1745–1748Fischer, T.M., Jacobson, D.A., Counsell, A.N., et al., (2011). Regulation of low-threshold afferent activity may contribute to short-term habituation in Aplysia californica. Neurobiology of Learning and Memory, 95: 248-259 Mollusk neurons are able to detect increasing pressures and tissue trauma.Illich, P.A and Walters, E.T., (1997). Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization. The Journal of Neuroscience, 17: 459-469 Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs,
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
s and arthropods.Eisemann, C.H., Jorgensen, W.K., Merritt, D.J., Rice, M.J., Cribb, B.W., Webb, P.D. and Zalucki, M.P., (1984). "Do insects feel pain? — A biological view". Cellular and Molecular Life Sciences, 40: 1420–1423St John Smith, E. and Lewin, G.R., (2009). Nociceptors: a phylogenetic view. Journal of Comparative Physiology A, 195: 1089-1106


Respiratory system

One type of invertebrate respiratory system is the open
respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
composed of spiracles, tracheae, and
tracheole Tracheole (trā'kē-ōl') is a fine respiratory tube of the Invertebrate trachea, trachea of an insect or a spider, part of the respiratory system of insects, respiratory system. Tracheoles are about 1 µm in diameter, and they convey oxygen ...
s that terrestrial arthropods have to transport
metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
gases to and from tissues. 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. The tracheae are invaginations of the cuticular
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of th ...

exoskeleton
that branch (
anastomose An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf#Veins, leaf veins, or streams. Such a connection m ...
) 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 of
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
for
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
,
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
, and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
. Gas may be conducted through the respiratory system by means of active
ventilation Ventilation may refer to: * Ventilation (physiology) Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the Milieu intérieur, internal environment, mostly to flush out carbon dio ...
or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their
haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the Cell (biology), cells, and ...
. A tracheal tube may contain ridge-like circumferential rings of
taenidia Taenidia (singular: taenidium) are circumferential thickenings of the cuticle inside a Invertebrate trachea, trachea or tracheole in an insect's respiratory system. The geometry of the Taenidiae varies across different orders of insects and even ...
in various geometries such as loops or
helices A helix () is a shape like a corkscrew or spiral staircase. It is a type of smoothness (mathematics), smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as ...
. In the
head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as visual perception, sight, hearing, olfaction, smell, and taste. Some ...
,
thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main Tagma (biology), divis ...
, or
abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
, tracheae may also be connected to air sacs. Many insects, such as
grasshopper Grasshoppers are a group of insects belonging to the suborder Caelifera. They are among what is possibly the most ancient living group of chewing herbivorous insects, dating back to the early Triassic around 250 million years ago. Grasshoppe ...
s and
bee Bees are winged insects closely related to wasp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which lo ...
s, 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 a
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
, or function essentially as normal, via a plastron. Note that despite being internal, the tracheae of arthropods are shed during moulting (
ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnants ...
).


Reproduction

Like vertebrates, most invertebrates reproduce at least partly through
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...
. They produce specialized
reproductive cells A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling dif ...
that undergo
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
to produce smaller, motile
spermatozoa A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and ...
or larger, non-motile ova. These fuse to form
zygote A zygote (, ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual ...
s, which develop into new individuals. Others are capable of asexual reproduction, or sometimes, both methods of reproduction.


Social interaction

Social behavior is widespread in invertebrates, including cockroaches, termites, aphids,
thrips Thrips (Order (biology), order Thysanoptera) are minute (mostly long or less), slender insects with fringed wings and unique asymmetrical mouthparts. Different thrips species feed mostly on plants by puncturing and sucking up the contents, al ...
, ants, bees, Passalidae,
Acari Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...
, spiders, and more. Social interaction is particularly salient in
eusocial Eusociality (from Greek εὖ ''eu'' "good" and social Social organisms, including human(s), live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, and whether the exchange is ...
species but applies to other invertebrates as well. Insects recognize information transmitted by other insects.Frisch, Karl von. (1967) The Dance Language and Orientation of Bees. Cambridge, Massachusetts: The Belknap Press of Harvard University Press.


Phyla

The term invertebrates covers several phyla. One of these are the sponges (
Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

Porifera
). They were long thought to have diverged from other animals early. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges typically feed by drawing in water through pores. Some speculate that sponges are not so primitive, but may instead be secondarily simplified.Dunn ''et al.'' 2008. "Broad phylogenomic sampling improves resolution of the animal tree of life". ''Nature'' 06614. The
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine life, marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly ...
and the
Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, predominantly the latter. Their distinguishing feature is cnidocyt ...
, which includes
sea anemone Sea anemones are a group of predation, predatory marine invertebrates of the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemones are classifi ...
s,
coral Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important C ...
s, and
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...

jellyfish
, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Both have distinct tissues, but they are not organized into
organs In biology, an organ is a collection of Tissue (biology), tissues joined in a structural unit to serve a common function. In the biological organization, hierarchy of life, an organ lies between Tissue (biology), tissue and an organ system. Tiss ...
. There are only two main germ layers, the
ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
and
endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastru ...
, with only scattered cells between them. As such, they are sometimes called
diploblastic Diploblasty is a condition of the blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and ...
. The
Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the Crinoi ...

Echinodermata
are radially symmetric and exclusively marine, including
starfish Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...
(Asteroidea),
sea urchin Sea urchins () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard s ...
s, (Echinoidea),
brittle star Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-poin ...
s (Ophiuroidea),
sea cucumbers Sea cucumbers are echinoderms from the class (biology), class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The numb ...
(Holothuroidea) and feather stars (Crinoidea). The largest animal phylum is also included within invertebrates: the Arthropoda, including insects,
spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and rank ...
s,
crab Crabs are Decapoda, decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen#Other animals, abdomen) ( el, :wikt:βραχύς, βραχύς , translit=brachys = short, / = tail), usually hid ...
s, 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. Two smaller phyla, the
Onychophora Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus' ...
and
Tardigrada Tardigrades (), known colloquially as water bears or moss piglets, are a phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below Kingdom (biology), kingdom and above Class (biology), class. Tradi ...
, are close relatives of the arthropods and share some traits with them, excluding the hardened exoskeleton. The
Nematoda The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below Kingdom (biology), kingdom and above Class (bio ...

Nematoda
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. A number are important parasites. Smaller phyla related to them are the
Kinorhyncha Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a phylum (biology), phylum of small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. Modern spec ...
,
Priapulida Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' 'Priapus In Greek mythology, Priapus (; grc, Πρίαπος, ) is a minor rustic fertility god, protector of livestock, fruit plants, gardens and male genitalia. Priapus is ma ...
, and
Loricifera Loricifera (from Latin, ''wikt:lorica, lorica'', corselet (armour) + ''ferre'', to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals that had been determined to be 37 described species, in 9&nbs ...
. These groups have a reduced coelom, called a pseudocoelom. Other invertebrates include the
Nemertea Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many h ...
or ribbon worms, and the
Sipuncula The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of segmentation (biology), unsegmented sea, marine Annelid, annelid worms. The name ''Sipuncula'' is from the genus name ''Sipu ...
. Another phylum is
Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...
, the flatworms. These were originally considered primitive, but it now appears they developed from more complex ancestors. Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic
Gastrotricha The gastrotrichs (phylum Gastrotricha), common name, commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, acoelomate animals, and are widely distributed and abundant in freshwater and Marin ...
. The
Rotifera The rotifers (, from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) a ...
or rotifers, are common in aqueous environments. Invertebrates also include the
Acanthocephala Acanthocephala ( Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses ...
or spiny-headed worms, the
Gnathostomulida Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively Hypoxia (environmental), anoxic environments. They were first recognise ...
,
Micrognathozoa ''Limnognathia maerski'' is a microscopic freshwater animal, discovered living in warm springs on Disko Island, Greenland, in 1994. Since then, it was also found in Crozet Islands of Antarctica.). With an average length of 100 micrometre, ...
, and the
Cycliophora ''Symbion'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biologic ...
. Also included are two of the most successful animal phyla, the Mollusca and Annelida. The former, which is the second-largest animal phylum by number of described species, includes animals such as
snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the m ...

snail
s,
clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two s ...
s, and
squid True squid are molluscs with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not st ...
s, and the latter comprises the segmented worms, such as
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. The ...
s and
leech Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...
es. These two groups have long been considered close relatives because of the common presence of
trochophore A trochophore (; also spelled trocophore) is a type of free-swimming planktonic marine larva with several bands of cilia. By moving their cilia rapidly, they make a water eddy, to control their movement, and to bring their food closer, to captur ...
larvae, but the annelids were considered closer to the arthropods because they are both segmented. Now, this is generally considered
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
, owing to many morphological and genetic differences between the two phyla. Among lesser phyla of invertebrates are the Hemichordata, or acorn worms, and the Chaetognatha, or arrow worms. Other phyla include
Acoelomorpha Acoelomorpha is a subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of "subdivision (rank), subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological t ...
, Brachiopoda,
Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...
,
Entoprocta Entoprocta (), or Kamptozoa , is a phylum (biology), phylum of mostly Sessility (zoology), sessile aquatic animals, ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whos ...
, Phoronida, and
Xenoturbellida ''Xenoturbella'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biolog ...
.


Classification of invertebrates

Invertebrates can be classified into several main categories, some of which are taxonomically obsolescent or debatable, but still used as terms of convenience. Each however appears in its own article at the following links. *
Sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...
(''Porifera'') *
Comb jellies Ctenophora (; ctenophore ; ) comprise a phylum of marine life, marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly ...
(''Ctenophora'') * Medusozoans and corals (''Cnidaria'') *
Acoels Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order of ...
(''Xenacoelomorpha'') *
Flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...
s (''Platyhelminthes'') * Bristleworms, earthworms and leeches (''Annelida'') * Insects, springtails, crustaceans, myriapods, chelicerates (''Arthropoda'') * Chitons, snails, slugs, bivalves, tusk shells, cephalopods (''Mollusca'') * Roundworms or threadworms (''Nematoda'') *
Rotifers The rotifers (, from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) a ...
(''Rotifera'') *
Tardigrades Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged Segmentation (biology), segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called t ...
(''Tardigrada'') * Scalidophores (''Scalidophora'') * Lophophorates (''Lophophorata'') * Velvet worms (''Onychophora'') * Arrow worms (''Chaetognatha'') * Gordian worms or horsehair worms (''Nematomorpha'') * Ribbon worms (''Nemertea'') *''
Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical '' Trichoplax adhaerens'', '' Hoilungia hongkongensis'', ...
'' *''
Loricifera Loricifera (from Latin, ''wikt:lorica, lorica'', corselet (armour) + ''ferre'', to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals that had been determined to be 37 described species, in 9&nbs ...
'' * Starfishes, sea urchins, sea cucumbers, sea lilies and brittle stars (''Echinodermata'') * Acorn worms, cephalodiscids and graptolites (''Hemichordata'') *
Lancelets The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochorda ...
(''Amphioxiformes'') * Salps, pyrosomes, doliolids, larvaceans and sea squirts (''Tunicata'')


History

The earliest animal fossils appear to be those 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. Some paleontologists suggest that animals appeared much earlier, possibly as early as 1 billion years ago though they probably became multicellular in the
Tonian The Tonian (from grc, τόνος, tónos, meaning "stretch") is the first geologic period of the Neoproterozoic era (geology), Era. It lasted from to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by t ...
.
Trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of Earliest known life forms, biological activity but not the preserved remains of the plant or animal itself. Trace fossils ...
s such as tracks and burrows found in the late
Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth' ...
era indicate the presence of
triploblastic Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organogenes ...
worms, roughly as large (about 5 mm wide) and complex as
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. The ...
s. 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 the
Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the Geologic record, rock record of Earth. It is a system of chronolo ...
. Fossils of invertebrates are commonly used in stratigraphy.


Classification

Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his Nobility#Ennoblement, ennoblement in 1761 as Carl von Linné#Blunt, Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalise ...
divided these animals into only two groups, the Insecta and the now-obsolete Vermes (
worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length ...
s).
Jean-Baptiste Lamarck Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist, biologist, academic, and soldier. He was an early proponent of the idea that biologi ...
, who was appointed to the position of "Curator of Insecta and Vermes" at the
Muséum National d'Histoire Naturelle The French National Museum of Natural History, known in French language, French as the ' (abbreviation MNHN), is the national natural history museum of France and a ' of higher education part of Sorbonne University (alliance), Sorbonne Universities ...
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 A barnacle is a type of arthropod constituting the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tid ...
,
Radiata Radiata or Radiates is a historical taxonomic rank that was used to classify animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe ...
,
Coelenterata Coelenterata is a term encompassing the animal phyla Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, p ...
and
Infusoria Infusoria are minute freshwater life forms including ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are ...
from the Linnean Vermes. They are now classified into over 30 phyla, from simple organisms such as
sea sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...
s and
flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...
s to complex animals such as arthropods and molluscs.


Significance of the group

Invertebrates are animals ''without'' a vertebral column. This has led to the conclusion that ''in''vertebrates 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. 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." 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 (see
Paraphyly In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
).


In research

For many centuries, invertebrates were neglected by biologists, in favor of big vertebrates and "useful" or charismatic species. Invertebrate biology was not a major field of study until the work of
Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his Nobility#Ennoblement, ennoblement in 1761 as Carl von Linné#Blunt, Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalise ...
and Lamarck in the 18th century. 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. 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. Two of the most commonly studied model organisms nowadays are invertebrates: the fruit fly ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
'' and the nematode ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a Hybrid word, blend of the Greek ''caeno-'' (recent), ''rh ...
''. They have long been the most intensively studied
model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biology, biological phenomena, with the expectation that discoveries made in the model organism will provide insight into th ...
s, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gene ...
s, but many
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanin ...
s,
intron An intron is any Nucleic acid sequence, nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of ...
s, and linkages have been lost. Analysis of the
starlet sea anemone The starlet sea anemone (''Nematostella vectensis'') is a species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defin ...
genome has emphasised the importance of sponges, placozoans, and
choanoflagellate The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconne ...
s, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to animals. Invertebrates are also used by scientists in the field of aquatic biomonitoring to evaluate the effects of water pollution and climate change.


See also

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Invertebrate zoology Invertebrate zoology is the subdiscipline of zoology Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anato ...
*
Invertebrate paleontology Invertebrate paleontology (also spelled invertebrate palaeontology) is sometimes described as invertebrate paleozoology or invertebrate paleobiology. Whether it is considered to be a subfield of paleontology, paleozoology, or paleobiology, this d ...
*
Marine invertebrates Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evo ...
*
Pain in invertebrates Pain in invertebrates is a contentious issue. Although there are numerous definitions of pain, almost all involve two key components. First, nociception is required. This is the ability to detect noxious stimuli which evokes a reflex response th ...


References


Further reading

* Hyman, L. H. 1940. ''The Invertebrates'' (6 volumes) New York : McGraw-Hill. A classic work. * Anderson, D. T. (Ed.). (2001). ''Invertebrate zoology'' (2nd ed.). Oxford: Oxford University Press. * Brusca, R. C., & Brusca, G. J. (2003). ''Invertebrates'' (2nd ed.). Sunderland, Mass. : Sinauer Associates. * Miller, S.A., & Harley, J.P. (1996). ''Zoology'' (4th ed.). Boston: WCB/McGraw-Hill. * * Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). ''Invertebrate zoology: a functional evolutionary approach''. Belmont, CA: Thomas-Brooks/Cole. * Adiyodi, K.G. & Adyiodi, R.G. (Eds) 1983- . ''Reproductive Biology of Invertebrates''. Wiley, New York. (Many volumes.) * Giese, A.G. & Pearse, J.S. (Eds) 1974- . ''Reproduction of Marine Invertebrates''. Academic Press, New York. (Many volumes.) * ''Advances in Invertebrate Reproduction''. Elsevier Science, Amsterdam. (Five volumes.)


External links

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Buglife (UK)

''African Invertebrates''
{{Use dmy dates, date=April 2017 01 Zoology Paraphyletic groups Obsolete animal taxa