Editing Invertebrate (section)

=== 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}}