HOME

TheInfoList




Invertebrates are
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

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 anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any in ...
. This includes all
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s apart from the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess 5 Apomorphy and synapomorphy , synapomorphies, or primary characteristics, at some point during their larval or adulthood stages that distinguish them from all other ta ...

chordate
subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is als ...
Vertebrata Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
. Familiar examples of invertebrates include
arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart fr ...
s (
insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

insect
s,
arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytica ...

arachnid
s,
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), der ...
s, and
myriapod Myriapoda (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycen ...
s),
mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil species is es ...
(
chitons Chitons are marine Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.snail A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the pr ...

snail
s,
bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged p ...
,
squid Squid are cephalopod A cephalopod is any member of the mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is ...

squid
s, and
octopus An octopus (pl. octopuses/octopi, see below for variants) is a soft-bodied, eight- limbed mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). A ...

octopus
es),
annelid The annelids (Annelida , from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the p ...
(
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to man ...

earthworm
s and
leech Leeches are segmented parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it so ...

leech
es), and
cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

cnidaria
ns (
hydras ''Hydra'' ( ) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including t ...
,
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

jellyfish
es,
sea anemone Sea anemones are the marine, predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms ...

sea anemone
s, and
coral Corals are 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 ver ...

coral
s). The majority of animal species are invertebrates; one estimate puts the figure at 97%. Many invertebrate
taxa In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
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 Latin ''wikt:rota#Latin, rota'' "wheel" and ''wikt:-fer#Latin, -fer'' "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, ...

rotifer
s to the 9–10 m (30–33 ft)
colossal squid The colossal squid (''Mesonychoteuthis hamiltoni'') is part of the family Cranchiidae The family Cranchiidae comprises the approximately 60 species of glass squid, also known as cockatoo squid, cranchiid, cranch squid, or bathyscaphoid squid. ...
. Some so-called invertebrates, such as the Tunicata and
Cephalochordata A cephalochordate (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10 ...
, 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 all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...
, so the term has little meaning in
taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
.


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 Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
in the same way that
Arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
a,
Vertebrata Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...

Vertebrata
or
Manidae Manidae is the only 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 ...

Manidae
do. Each of these terms describes a valid taxon,
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
,
subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is als ...
or
family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same Politic ...
. "Invertebrata" is a term of convenience, not a taxon; it has very little circumscriptional significance except within the Chordata. The Vertebrata as a
subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is als ...
comprises such a small proportion of the
Metazoa Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the L ...

Metazoa
that to speak of the
kingdom Kingdom may refer to: Monarchy * A type of monarchy * A realm ruled by: **A king, during the reign of a male monarch **A queen regnant, during the reign of a female monarch Taxonomy * Kingdom (biology), a category in biological taxonomy Arts an ...
Animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

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 ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics Cladistics (, from Greek language, Greek , ''kládos'', "branch") is an approach to Taxonomy (biology), biological classification in whi ...

cladogram
, for example, the presence of a
notochord In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any in ...
. 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, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
. 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 members who are not part of the clergy Clergy are formal leaders within established religions. Their roles and functions vary in different religious traditions, but usually involve presidin ...
, 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, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
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 In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

jellyfish
or worms. Others have hard
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

exoskeleton
s, outer shells like those of
insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

insect
s and
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), der ...
s. The most familiar invertebrates include the
Protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

Protozoa
,
Porifera Sponges, the members of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. Th ...

Porifera
,
Coelenterata Coelenterata is a term encompassing the animal phyla Cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic a ...
,
Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in S ...
,
Nematoda
Nematoda
,
Annelida
Annelida
,
Echinodermata An echinoderm is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of num ...

Echinodermata
,
Mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...

Mollusca
and
Arthropoda An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart ...

Arthropoda
. Arthropoda include
insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

insect
s,
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), der ...
s and
arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytica ...

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 Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual o ...
species for major invertebrate groups as estimated in the
IUCN Red List of Threatened Species The International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization An internationa ...
'', 2014.3.The World Conservation Union. 2014. ''
IUCN Red List of Threatened Species The International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization An internationa ...
'', 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 An international organization (also known as an international institut ...
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: tortoise). They are particularly distinguished from other Turtle, turtles (which includes the order Chelonia) by being exclusively land-dwelling, while many (t ...

tortoise
s,
snail A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the pr ...

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 In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. Th ...

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, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
s of most
multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biol ...
s exhibit some form of
symmetry Symmetry (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...
, 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 taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may ...
species. This is easily seen in
snail A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the pr ...

snail
s and
sea snail Sea snail is a common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It is some ...

sea snail
s, which have helical shells.
Slug Slug, or land slug, is a common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. ...

Slug
s appear externally symmetrical, but their
pneumostome The pneumostome of ''Triboniophorus graeffei'' is on its dorsal surface. 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 ...

pneumostome
(breathing hole) is located on the right side. Other gastropods develop external asymmetry, such as that develops asymmetrical
cerata :''The tortrix moth genus ''Cerata'' is considered a junior synonym of ''Cydia (genus), Cydia. Cerata, singular ceras, are anatomical structures found externally in nudibranch sea slugs, especially in aeolid nudibranchs, marine animal, marine opi ...
as they mature. The origin of gastropod asymmetry is a subject of scientific debate. Other examples of asymmetry are found in
fiddler crab A fiddler crab, sometimes known as a ''calling crab'', may be any of more than one hundred species of semiterrestrial marine crab Crabs are decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes suc ...

fiddler crab
s and
hermit crab Hermit crabs are anomuran decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (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 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 An encyclopedia or encyclopaedia (British English) is a reference work or compendium providing summaries of knowledge either from all branches or from a particular field or discip ...
'' (2007).
alongside
coral Corals are 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 ver ...

coral
colonies In political science, 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, metropolitan ...
(with the exception of the individual polyps that exhibit radial symmetry);
alpheidae Alpheidae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the well ...
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), ...

copepod
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#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

monogenea
ns which parasitize by attachment or residency within the
gill A gill () is a respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
chamber of their
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host (1 ...
).


Nervous system

Neurons A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapse In the nervous system In biology Biology is the natural science that studies life and living ...

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 medicinal
leech Leeches are segmented parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it so ...

leech
, ''
Hirudo medicinalis ''Hirudo medicinalis'', the European medicinal leech, is one of several species of leeches used as "medicinal leeches". Other species of ''Hirudo'' sometimes also used as medicinal leeches include ''Hirudo orientalis, H. orientalis'', ''H. troct ...

Hirudo medicinalis
.''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 of medium-sized to extremely large sea slugs, specifically sea hares, which are one clade of large Opisthobranchia, sea slugs, marine (ocean), marine gastropod mollusks. These benthic herbivorous creatures can become ...
'' 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-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

nematode
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 A biological system is a complex network Network and networking may refer to: Arts, entertainment, and media * ''Network'' (1976 film), a 1976 Ame ...

respiratory system
composed of spiracles, tracheae, and
tracheole Tracheole (trā'kē-ōl') is a fine respiratory tube of the trachea The trachea, also known as the windpipe, is a cartilaginous Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue, rubber-like padding that covers and prot ...
s that
terrestrial Terrestrial refers to things related to land Land is the solid surface of the Earth that is not permanently covered by water. The vast majority of human activity throughout history has occurred in land areas that support agriculture A ...
arthropods have to transport
metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as Cell signaling, signaling and self-sustaining ...

metabolic
gases to and from tissues. The distribution of spiracles can vary greatly among the many
orders Orders is a surname In some cultures, a surname, family name, or last name is the portion of one's personal name that indicates their family, tribe or community. Practices vary by culture. The family name may be placed at either the start of ...
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 έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

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, or streams. Such a connection may be normal ...
) 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 In chemistry Chemistry is the study of the properties and behavior of . It is a that covers ...

diffusion
for
water Water (chemical formula H2O) 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 li ...

water
,
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, and
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
. Gas may be conducted through the respiratory system by means of active
ventilation Ventilation may refer to: * Ventilation (physiology), the movement of air between the environment and the lungs via inhalation and exhalation ** Mechanical ventilation, in medicine, using artificial methods to assist breathing *** Ventilator, a mac ...
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 humans and other animals that delivers necessary substances such as nutrient A nutrient is a substance used by an organism to survive, grow, and reprod ...
. A tracheal tube may contain ridge-like circumferential rings of taenidia in various such as loops or
helices The right-handed helix (cos ''t'', sin ''t'', ''t'') from ''t'' = 0 to 4π with arrowheads showing direction of increasing ''t'' A helix (), plural helixes or helices (), is a shape like a corkscrew or spiral staircase. It is a type of smooth sp ...

helices
. In the
head Head Sport GmbH is an American-Austrian manufacturing company Manufacturing is the creation or production Production may be: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in th ...

head
,
thorax The thorax or chest is a part of the anatomy of humans, mammals, 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), divisions ...

thorax
, or
abdomen The abdomen (colloquially called the belly, tummy, midriff 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 Trunk (anatomy) ...

abdomen
, tracheae may also be connected to air sacs. Many insects, such as
grasshopper Grasshoppers are a group 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 exoskele ...

grasshopper
s and
bee Bees are flying insects closely related to wasps and ants, known for their role in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyly, monophyletic lineage within the ...

bee
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 respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
, or function essentially as normal, via a
plastron The turtle shell is a shield for the ventral and dorsal parts of turtles (the Order (biology), order Testudines), completely enclosing all the vital organs of the turtle and in some cases even the head. It is constructed of modified bony elements ...
. Note that despite being internal, the tracheae of arthropods are shed during moulting (ecdysis).


Reproduction

Like vertebrates, most invertebrates reproduce at least partly through sexual reproduction. They produce specialized Gametogonium, reproductive cells that undergo meiosis to produce smaller, motile spermatozoon, spermatozoa or larger, non-motile ovum, ova. These fuse to form zygotes, 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, ants, bees, Passalidae, Acari, spiders, and more. Social interaction is particularly salient in eusocial 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 In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. Th ...

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 and the Cnidaria, which includes
sea anemone Sea anemones are the marine, predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms ...

sea anemone
s,
coral Corals are 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 ver ...

coral
s, and
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

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 organ (anatomy), organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, they are sometimes called diploblastic. The
Echinodermata An echinoderm is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of num ...

Echinodermata
are radially symmetric and exclusively marine, including starfish (Asteroidea), sea urchins, (Echinoidea), brittle stars (Ophiuroidea), Holothuroidea, sea cucumbers (Holothuroidea) and Crinoid, feather stars (Crinoidea). 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. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. The 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, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom. Other invertebrates include the Nemertea or ribbon worms, and the Sipuncula. Another phylum is
Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in S ...
, the flatworms. These were originally considered primitive, but it now appears they developed from more complex ancestors. Flatworms are Aceolomate, acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha. The Rotifera or rotifers, are common in aqueous environments. Invertebrates also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and the Cycliophora. 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 gastropods (), commonly known as snails and slugs, belong to a large taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the pr ...

snail
s, clams, and
squid Squid are cephalopod A cephalopod is any member of the mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is ...

squid
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 A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to man ...

earthworm
s and
leech Leeches are segmented parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it so ...

leech
es. 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. Now, this is generally considered convergent evolution, 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, Brachiopoda, Bryozoa, Entoprocta, Phoronida, and Xenoturbellida.


Classification of invertebrates

Invertebrates can be classified into several main categories, some of which are Biological classification, taxonomically obsolescent or debatable, but still used as terms of convenience. Each however appears in its own article at the following links. *Sponges (''Porifera'') *Comb jellies (''Ctenophora'') *Cnidaria, Medusozoans and corals (''Cnidaria'') * Xenacoelomorpha, Acoels (''Xenacoelomorpha'') *Flatworms (''Platyhelminthes'') *Annelid, Bristleworms, earthworms and leeches (''Annelida'') *Arthropod, Insects, springtails, crustaceans, myriapods, chelicerates (''Arthropoda'') *Mollusca, Chitons, snails, slugs, bivalves, tusk shells, cephalopods (''Mollusca'') *Nematodes, Roundworms or threadworms (''Nematoda'') *Rotifers (''Rotifera'') *Tardigrades (''Tardigrada'') *Scalidophora, Scalidophores (''Scalidophora'') *Lophophorata, Lophophorates (''Lophophorata'') *Velvet worms (''Onychophora'') *Arrow worms (''Chaetognatha'') *Nematomorph, Gordian worms or horsehair worms (''Nematomorpha'') *Nemertea, Ribbon worms (''Nemertea'') *''Placozoa'' *''Loricifera'' *Echinoderm, Starfishes, sea urchins, sea cucumbers, sea lilies and brittle stars (''Echinodermata'') *Hemichordata, Acorn worms, cephalodiscids and graptolites (''Hemichordata'') *Amphioxiformes, Lancelets (''Amphioxiformes'') *Tunicates, 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. Trace fossils such as tracks and burrows found in the late Neoproterozoic era indicate the presence of triploblastic worms, like metazoans, 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 A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to man ...

earthworm
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. Fossils of invertebrates are commonly used in stratigraphy.


Classification

Carl Linnaeus divided these animals into only two groups, the Insecta and the now-obsolete Vermes (worms). 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, Barnacle, Cirripedia, Radiata,
Coelenterata Coelenterata is a term encompassing the animal phyla Cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic a ...
and Infusoria from the Linnean Vermes. They are now classified into over 30 phylum, phyla, from simple organisms such as sea sponges and flatworms 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 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 Carl Linnaeus, Linnaeus 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'' and the nematode ''Caenorhabditis elegans''. They have long been the most intensively studied model organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but many genes, introns, and genetic linkage, linkages have been lost. 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. Invertebrates are also used by scientists in the field of aquatic biomonitoring to evaluate the effects of water pollution and climate change.


See also

* Invertebrate zoology * Invertebrate paleontology * Marine invertebrates * Pain in invertebrates


References


Further reading

* Libbie Hyman, 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

*
Buglife (UK)

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