Editing Invertebrate (section)

== Characteristics ==

The trait that is common to all invertebrates is the absence of a [[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 [[Homology (biology)|homologous]] trait, any more than the common trait of having wings functionally unites insects, bats, and birds, or than not having wings unites [[tortoise]]s, [[snail]]s and [[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]], 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]]s of most [[multicellular organism]]s exhibit some form of [[Symmetry in biology|symmetry]], whether radial, bilateral, or spherical. A minority, however, exhibit no symmetry. One example of asymmetric invertebrates includes all [[gastropod]] species. This is easily seen in [[snail]]s and [[sea snail]]s, which have helical shells. [[Slug]]s appear externally symmetrical, but their [[pneumostome]] (breathing hole) is located on the right side. Other gastropods develop external asymmetry, such as ''[[Glaucus atlanticus]]'' that develops asymmetrical [[cerata]] as they mature. The origin of gastropod asymmetry is a subject of scientific debate.<ref name="gastropod development">{{cite journal| author=Louise R. Page| title=Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan| journal=Integrative and Comparative Biology| year=2006| volume=46| issue=2| pages=134–143| doi=10.1093/icb/icj018| pmid=21672730| df=dmy-all| doi-access=free}}</ref>

Other examples of asymmetry are found in [[fiddler crab]]s and [[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]]. [[Sessility (zoology)|Sessile]] animals such as [[sponge]]s are asymmetrical<ref name=Columbia>[http://www.factmonster.com/ce6/sci/A0847482.html Symmetry, biological] {{Webarchive|url=https://web.archive.org/web/20121113143415/http://www.factmonster.com/ce6/sci/A0847482.html |date=13 November 2012 }}, cited at FactMonster.com from ''[[The Columbia Electronic Encyclopedia]]'' (2007).</ref> alongside [[coral]] [[Colony (biology)|colonies]] (with the exception of the individual [[polyp (zoology)|polyp]]s that exhibit radial symmetry); [[Alpheidae]] claws that lack pincers; and some [[copepod]]s, [[polyopisthocotylea]]ns, and [[monogenea]]ns which parasitize by attachment or residency within the [[gill]] chamber of their [[fish]] [[host (biology)|hosts]]).

==== Nervous system ====

[[Neurons]] differ in invertebrates from mammalian cells. Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH. The first invertebrate in which a neuron cell was identified was the medicinal [[leech]], ''[[Hirudo medicinalis]]''.<ref name="Nicholls and Baylor, 1968">{{cite journal |vauthors=Nicholls JG, Baylor DA |title=Specific modalities and receptive fields of sensory neurons in CNS of the leech |journal=Journal of Neurophysiology |volume=31 |issue=5 |pages=740–756 |date=September 1968 |pmid=5711143 |doi=10.1152/jn.1968.31.5.740 }}</ref><ref name="Pastor et al., 1996">{{cite journal |vauthors=Pastor J, Soria B, Belmonte C |title=Properties of the nociceptive neurons of the leech segmental ganglion |journal=Journal of Neurophysiology |volume=75 |issue=6 |pages=2268–2279 |date=June 1996 |pmid=8793740 |doi=10.1152/jn.1996.75.6.2268 }}</ref>
Learning and memory using nociceptors have been described in the sea hare, ''[[Aplysia]]''.<ref name="Byrne et al., 1978">{{cite journal |vauthors=Byrne JH, Castellucci VF, Kandel ER |title=Contribution of individual mechanoreceptor sensory neurons to defensive gill-withdrawal reflex in Aplysia |journal=Journal of Neurophysiology |volume=41 |issue=2 |pages=418–431 |date=March 1978 |pmid=650275 |doi=10.1152/jn.1978.41.2.418 }}</ref><ref name="Castellucci et al., 1970">{{cite journal |vauthors=Castellucci V, Pinsker H, Kupfermann I, Kandel ER |title=Neuronal mechanisms of habituation and dishabituation of the gill-withdrawal reflex in Aplysia |journal=Science |volume=167 |issue=3926 |pages=1745–1748 |date=March 1970 |pmid=5416543 |doi=10.1126/science.167.3926.1745 |bibcode=1970Sci...167.1745C }}</ref><ref name="Fischer et al., 2011">{{cite journal |vauthors=Fischer TM, Jacobson DA, Counsell AN, Pelot MA, Demorest K |title=Regulation of low-threshold afferent activity may contribute to short-term habituation in Aplysia californica |journal=Neurobiology of Learning and Memory |volume=95 |issue=3 |pages=248–259 |date=March 2011 |pmid=21144906 |doi=10.1016/j.nlm.2010.11.016 }}</ref> Mollusk neurons are able to detect increasing pressures and tissue trauma.<ref name="Illich and Walters, 1997">{{cite journal |vauthors=Illich PA, Walters ET |title=Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization |journal=Journal of Neuroscience |volume=17 |issue=1 |pages=459–469 |date=January 1997 |pmid=8987770 |pmc=6793714 |doi=10.1523/JNEUROSCI.17-01-00459.1997 }}Mechanosensory neurons innervating Aplysia siphon encode noxious stimuli and display nociceptive sensitization. The Journal of Neuroscience, 17: 459–469</ref>

Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, [[nematode]]s and arthropods.<ref name="Eismann et al., 1984">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: 1420–1423</ref><ref name="St John Smith and Lewis, 2009">{{cite journal |vauthors=Smith ES, Lewin GR |title=Nociceptors: a phylogenetic view |journal=Journal of Comparative Physiology A |volume=195 |issue=12 |pages=1089–1106 |date=December 2009 |pmid=19830434 |pmc=2780683 |doi=10.1007/s00359-009-0482-z }}</ref>

====Respiratory system====

[[File:Tracheal system of dissected cockroach.tif|thumb|Tracheal system of dissected [[cockroach]].  The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm.]]
[[File:Cockroach tracheae supplying crop.tiff|thumb|The tracheal system branches into progressively smaller tubes, here supplying the [[Crop (anatomy)|crop]] of the cockroach.  Scale bar, 2.0 mm.]]

One type of invertebrate respiratory system is the open [[respiratory system]] composed of [[Spiracle (arthropods)|spiracles]], tracheae, and [[tracheole]]s that [[terrestrial animal|terrestrial]] arthropods have to transport [[metabolism|metabolic]] gases to and from tissues.<ref>Wasserthal, Lutz T. (1998). Chapter 25: The Open Hemolymph System of Holometabola and Its Relation to the Tracheal Space. In "Microscopic Anatomy of Invertebrates". Wiley-Liss, Inc. {{ISBN|0-471-15955-7}}.</ref>  The distribution of spiracles can vary greatly among the many [[order (biology)|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]] that branch ([[anastomosis|anastomose]]) throughout the body with diameters from only a few micrometres up to 0.8&nbsp;mm. The smallest tubes, tracheoles, penetrate cells and serve as sites of [[diffusion]] for [[water]], [[oxygen]], and [[carbon dioxide]]. Gas may be conducted through the respiratory system by  means of active [[Ventilation (physiology)|ventilation]] or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their [[hemolymph|haemolymph]].<ref>{{cite journal |author1=Westneat, Mark W. |author2=Betz, Oliver |author3=Blob, Richard W. |author4=Fezzaa, Kamel |author5=Cooper, James W. |author6=Lee, Wah-Keat |title=Tracheal Respiration in Insects Visualized with Synchrotron X-ray Imaging |journal=Science |date=January 2003 |volume=299 |pages=558–560 |doi=10.1126/science.1078008 |pmid=12543973 |issue=5606 |bibcode=2003Sci...299..558W |s2cid=43634044 }}{{Dead link|date=December 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

A tracheal tube may contain ridge-like circumferential rings of [[taenidia]] in various [[geometry|geometries]] such as loops or [[helix|helices]]. In the [[Cephalon (arthropod head)|head]], [[Thorax (arthropod anatomy)|thorax]], or [[Abdomen#Arthropoda|abdomen]], tracheae may also be connected to air sacs. Many insects, such as [[grasshopper]]s and [[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]], or function essentially as normal, via a [[Plastron (arthropod)|plastron]]. Despite being internal, the tracheae of arthropods are shed during moulting ([[ecdysis]]).<ref>{{Cite journal|last=Ewer|first=John|date=2005-10-11|title=How the Ecdysozoan Changed Its Coat|journal=PLOS Biology|volume=3|issue=10|pages=e349|doi=10.1371/journal.pbio.0030349|pmid=16207077|pmc=1250302|issn=1545-7885 |doi-access=free }}</ref>

==== Hearing ====

{{excerpt|Ear|Invertebrates}}

=== 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]].<ref>{{cite book|last=Schwartz|first=Jill|title=Master the GED 2011 (w/CD)|year=2010|publisher=Peterson's|isbn=978-0-7689-2885-3|page=[https://archive.org/details/petersonsmasterg0000stew_x3f1/page/371 371]|url=https://archive.org/details/petersonsmasterg0000stew_x3f1/page/371}}</ref> These fuse to form [[zygote]]s, which develop into new individuals.<ref>{{cite book|last=Hamilton|first=Matthew B.|title=Population genetics|url=https://archive.org/details/populationgeneti00hami|url-access=limited|year=2009|publisher=Wiley-Blackwell|isbn=978-1-4051-3277-0|page=[https://archive.org/details/populationgeneti00hami/page/n69 55]}}</ref> Others are capable of asexual reproduction, or sometimes, both methods of reproduction.

Extensive research with model invertebrate species such as ''[[Drosophila melanogaster]]'' and ''[[Caenorhabditis elegans]]'' has contributed much to our understanding of [[meiosis]] and reproduction. However, beyond the few model systems, the modes of reproduction found in invertebrates show incredible diversity.<ref name="Picard2021">{{cite journal |vauthors=Picard MA, Vicoso B, Bertrand S, Escriva H |title=Diversity of Modes of Reproduction and Sex Determination Systems in Invertebrates, and the Putative Contribution of Genetic Conflict |journal=Genes (Basel) |volume=12 |issue=8 |pages=1136 |date=July 2021 |pmid=34440310 |pmc=8391622 |doi=10.3390/genes12081136 |doi-access=free}}</ref>  In one extreme example, it is estimated that 10% of [[Oribatida|orbatid mite]] species have persisted without sexual reproduction and have reproduced asexually for more than 400 million years.<ref name = Picard2021/>

==== Reproductive systems ====
{{excerpt|Reproductive system|Invertebrates}}

===Social interaction===
Social behavior is widespread in invertebrates, including cockroaches, termites, aphids, [[thrips]], ants, bees, [[Passalidae]], [[Acari]], spiders, and more.<ref name=ChoeCrespi1997>{{cite book|title=The Evolution of Social Behavior in Insects and Arachnids|date=1997|publisher=Cambridge University Press|isbn=978-0521589772}}</ref> Social interaction is particularly salient in [[eusocial]] species but applies to other invertebrates as well.

Insects recognize information transmitted by other insects.<ref name="Riley">{{Cite journal| doi = 10.1038/nature03526| pmid = 15889092| year = 2005| last1 = Riley | first1 = J.| last2 = Greggers | first2 = U.| last3 = Smith | first3 = A.| last4 = Reynolds | first4 = D.| last5 = Menzel | first5 = R.| title = The flight paths of honeybees recruited by the waggle dance| volume = 435| issue = 7039| pages = 205–7| journal = Nature |bibcode = 2005Natur.435..205R | s2cid = 4413962}}</ref><ref>{{cite journal |author1=Seeley T.D. |author2=Visscher P.K. |author3=Passino K.M. | year = 2006 | title = Group decision making in honey bee swarms | journal = American Scientist | volume = 94 | issue = 3| pages = 220–9 | doi=10.1511/2006.3.220|doi-broken-date=7 February 2025 }}</ref><ref name="Frisch1967">Frisch, Karl von. (1967) The Dance Language and Orientation of Bees. Cambridge, Massachusetts: The Belknap Press of Harvard University Press.</ref>

=== Phyla ===

[[File:Cladocora.jpg|right|thumb|The fossil coral ''[[Cladocora]]'' from the [[Pliocene]] of [[Cyprus]]]]

The term invertebrates covers several phyla. One of these are the sponges ([[Porifera]]). They were long thought to have diverged from other animals early.<ref>{{cite book|title=An Introduction to Porifera|year=2003|publisher=Anmol Publications PVT. LTD.|isbn=978-81-261-0675-2|author=Bhamrah, H. S.|author2=Kavita Juneja|page=58}}</ref> They lack the complex organization found in most other phyla.<ref>{{cite book|last=Sumich|first=James L.|title=Laboratory and Field Investigations in Marine Life|year=2008|publisher=Jones & Bartlett Learning|isbn=978-0-7637-5730-4|page=67}}</ref> Their cells are differentiated, but in most cases not organized into distinct tissues.<ref>{{cite book|last=Jessop|first=Nancy Meyer|title=Biosphere; a study of life|year=1970|publisher=Prentice-Hall|page=428}}</ref> Sponges typically feed by drawing in water through pores.<ref>{{cite book|last=Sharma|first=N. S.|title=Continuity And Evolution Of Animals|year=2005|publisher=Mittal Publications|isbn=978-81-8293-018-6|page=106}}</ref> Some speculate that sponges are not so primitive, but may instead be secondarily simplified.<ref>{{Cite journal |last1=Dunn |first1=Casey W. |last2=Hejnol |first2=Andreas |last3=Matus |first3=David Q. |last4=Pang |first4=Kevin |last5=Browne |first5=William E. |last6=Smith |first6=Stephen A. |last7=Seaver |first7=Elaine |last8=Rouse |first8=Greg W. |last9=Obst |first9=Matthias |last10=Edgecombe |first10=Gregory D. |last11=Sørensen |first11=Martin V. |last12=Haddock |first12=Steven H. D. |last13=Schmidt-Rhaesa |first13=Andreas |last14=Okusu |first14=Akiko |last15=Kristensen |first15=Reinhardt Møbjerg |date=April 2008 |title=Broad phylogenomic sampling improves resolution of the animal tree of life |url=https://www.nature.com/articles/nature06614 |journal=Nature |language=en |volume=452 |issue=7188 |pages=745–9 |doi=10.1038/nature06614 |pmid=18322464 |bibcode=2008Natur.452..745D |issn=1476-4687}}</ref> The [[Ctenophora]] and the [[Cnidaria]], which includes [[sea anemone]]s, [[coral]]s, and [[jellyfish]], are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.<ref>{{cite book|title=A living bay: the underwater world of Monterey Bay|year=2000|publisher=University of California Press|isbn=978-0-520-22149-9|author=Langstroth, Lovell|author2=Libby Langstroth|author3=Todd Newberry|author4=Monterey Bay Aquarium|page=[https://archive.org/details/livingbayunderwa0000lang/page/244 244]|url=https://archive.org/details/livingbayunderwa0000lang/page/244}}</ref> Both have distinct tissues, but they are not organized into [[organ (anatomy)|organs]].<ref>{{cite book|last=Safra|first=Jacob E.|title=The New Encyclopædia Britannica, Volume 16|year=2003|publisher=Encyclopædia Britannica|isbn=978-0-85229-961-6|page=523}}</ref> There are only two main germ layers, the [[ectoderm]] and [[endoderm]], with only scattered cells between them. As such, they are sometimes called [[diploblastic]].<ref>{{cite book|last=Kotpal|first=R. L.|title=Modern Text Book of Zoology: Invertebrates|publisher=Rastogi Publications|isbn=978-81-7133-903-7|page=184|year=2012}}</ref>

The [[Echinodermata]] are radially symmetric and exclusively marine, including [[starfish]] (Asteroidea), [[sea urchin]]s, (Echinoidea), [[brittle star]]s (Ophiuroidea), [[Holothuroidea|sea cucumbers]] (Holothuroidea) and [[Crinoid|feather stars]] (Crinoidea).<ref>{{cite book|last=Alcamo|first=Edward|title=Biology Coloring Workbook|year=1998|publisher=The Princeton Review|isbn=978-0-679-77884-4|page=220}}</ref>

The largest animal phylum is also included within invertebrates: the Arthropoda, including insects, [[spider]]s, [[crab]]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.<ref name=forensic>{{cite book|last=Gunn|first=Alan|page=214|title=Essential forensic biology|publisher=John Wiley and Sons|year=2009|isbn=978-0-470-75804-5}}</ref> Two smaller phyla, the [[Onychophora]] and [[Tardigrada]], are close relatives of the arthropods and share some traits with them, excluding the hardened exoskeleton. The [[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.<ref>{{cite book|title=BioInquiry: making connections in biology|year=2003|publisher=John Wiley|isbn=978-0-471-20228-8|author=Prewitt, Nancy L.|author2=Larry S. Underwood|author3=William Surver|page=[https://archive.org/details/bioinquiry00nanc_0/page/289 289]|url=https://archive.org/details/bioinquiry00nanc_0/page/289}}</ref> A number are important parasites.<ref>{{cite book|title=Parasites in social insects|year=1998|publisher=Princeton University Press|isbn=978-0-691-05924-2|last=Schmid-Hempel|first=Paul|page=75}}</ref> 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.<ref name=Gilson>{{cite book|last=Gilson|first=Étienne|title=El espíritu de la filosofía medieval|year=2004|publisher=Ediciones Rialp|isbn=978-84-321-3492-0|page=384}}</ref> These were originally considered primitive, but it now appears they developed from more complex ancestors.<ref>{{cite journal |last1=Ruiz-Trillo |first1=Iñaki |last2=Riutort |first2=Marta |last3=Littlewood |first3=D. Timothy J. |last4=Herniou |first4=Elisabeth A. |last5=Baguñà |first5=Jaume |date=March 1999 |title=Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes |journal=Science |volume=283 |issue=5409 |pages=1919–23 |doi=10.1126/science.283.5409.1919 |pmid=10082465 |issn=0036-8075 |bibcode=1999Sci...283.1919R |s2cid=6079655 }}{{Dead link|date=December 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Flatworms are [[Coelom|acoelomates]], lacking a body cavity, as are their closest relatives, the microscopic [[Gastrotricha]].<ref name="umodena">{{cite web |url=http://www.gastrotricha.unimore.it/overview.htm |title=Gastrotricha: Overview |access-date=2008-01-26 |last=Todaro |first=Antonio |work=Gastrotricha: World Portal |publisher=University of Modena & Reggio Emilia |archive-date=2 October 2006 |archive-url=https://web.archive.org/web/20061002033954/http://www.gastrotricha.unimore.it/overview.htm |url-status=live }}</ref> The [[Rotifera]], or rotifers, are common in aqueous environments. Invertebrates also include the [[Acanthocephala]], or spiny-headed worms, the [[Gnathostomulida]], [[Micrognathozoa]], and the [[Cycliophora]].<ref name="IntroCyclio">{{cite journal |last=Kristensen |first= Reinhardt Møbjerg |date=July 2002 |title=An Introduction to Loricifera, Cycliophora, and Micrognathozoa |journal=Integrative and Comparative Biology |volume=42 |issue=3 |pages=641–651 |doi =10.1093/icb/42.3.641 |pmid=21708760|doi-access=free }}</ref>

Also included are two of the most successful animal phyla, the Mollusca and Annelida.<ref>{{cite web |url=http://www.lophelia.org/lophelia/biodiv_6.htm |title=Biodiversity: Mollusca|access-date=2007-11-19 |publisher=The Scottish Association for Marine Science |archive-url = https://web.archive.org/web/20060708083128/http://www.lophelia.org/lophelia/biodiv_6.htm |archive-date = 2006-07-08}}</ref><ref>{{cite video | people = Russell, Bruce J. (Writer), Denning, David (Writer) | title = Branches on the Tree of Life: Annelids| medium = [[VHS]] | publisher = BioMEDIA ASSOCIATES |date = 2000 }}</ref> The former, which is the second-largest animal phylum by number of described species, includes animals such as [[snail]]s, [[clam]]s, and [[squid]]s, and the latter comprises the segmented worms, such as [[earthworm]]s and [[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.<ref>{{cite journal|last1=Eernisse|first1=Douglas J. |last2=Albert|first2=James S. |last3=Anderson|first3=Frank E. | title=Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralean metazoan morphology | journal=Systematic Biology | volume=41 | issue=3 | pages = 305–330 | date=1 September 1992 | doi=10.2307/2992569 | issn=1063-5157| jstor=2992569 }}</ref> Now, this is generally considered [[convergent evolution]], owing to many morphological and genetic differences between the two phyla.<ref>{{cite journal|author=Eernisse, Douglas J. |author2=Kim, Chang Bae |author3=Moon, Seung Yeo |author4=Gelder, Stuart R. |author5=Kim, Won |title=Phylogenetic Relationships of Annelids, Molluscs, and Arthropods Evidenced from Molecules and Morphology |journal=Journal of Molecular Evolution |volume=43 |issue=3 |pages=207–215 |date=September 1996 |doi=10.1007/PL00006079 |pmid=8703086 |bibcode=1996JMolE..43..207K }}{{dead link|date=June 2016|bot=medic}}{{cbignore|bot=medic}}</ref>

Among lesser phyla of invertebrates are the [[Hemichordata]], or acorn worms,<ref>{{cite book|title=Asking about life|year=2005|publisher=Cengage Learning|isbn=978-0-534-40653-0|author=Tobin, Allan J.|author2=Jennie Dusheck|page=497}}</ref> and the Chaetognatha, or arrow worms. Other phyla include [[Acoelomorpha]], [[Brachiopoda]], [[Bryozoa]], [[Entoprocta]], [[Phoronida]], and [[Xenoturbellida]].