Editing Sponge (section)

=== Coordination of activities ===

Adult sponges lack [[neuron]]s or any other kind of [[nervous tissue]]. However, most species have the ability to perform movements that are coordinated all over their bodies, mainly contractions of the [[pinacocyte]]s, squeezing the water channels and thus expelling excess sediment and other substances that may cause blockages. Some species can contract the [[osculum]] independently of the rest of the body. Sponges may also contract in order to reduce the area that is vulnerable to attack by predators. In cases where two sponges are fused, for example if there is a large but still unseparated bud, these contraction waves slowly become coordinated in both of the "[[Siamese twins]]". The coordinating mechanism is unknown, but may involve chemicals similar to [[neurotransmitter]]s.<ref>{{cite journal |last=Nickel |first=M. |title=Kinetics and rhythm of body contractions in the sponge Tethya wilhelma (Porifera: Demospongiae) |journal=The Journal of Experimental Biology |volume=207 |issue=Pt 26 |pages=4515–24 |date=December 2004 |pmid=15579547 |doi=10.1242/jeb.01289 |doi-access=free |bibcode=2004JExpB.207.4515N }}</ref> However, [[glass sponge]]s rapidly transmit electrical impulses through all parts of the [[syncytium]], and use this to halt the motion of their [[flagella]] if the incoming water contains toxins or excessive sediment.<ref name="Ruppert_2004"/> [[Myocyte]]s are thought to be responsible for closing the osculum and for transmitting signals between different parts of the body.<ref name="Bergquist_1998"/>

Sponges contain [[gene]]s very similar to those that contain the "recipe" for the post-[[synapse|synaptic]] density, an important signal-receiving structure in the neurons of all other animals. However, in sponges these genes are only activated in "flask cells" that appear only in larvae and may provide some sensory capability while the larvae are swimming. This raises questions about whether flask cells represent the predecessors of true neurons or are evidence that sponges' ancestors had true neurons but lost them as they adapted to a sessile lifestyle.<ref>{{cite journal |last1=Sakarya |first1=Onur |last2=Armstrong |first2=Kathryn A. |last3=Adamska |first3=Maja |last4=Adamski |first4=Marcin |last5=Wang |first5=I-Fan |last6=Tidor |first6=Bruce |last7=Degnan |first7=Bernard M. |last8=Oakley |first8=Todd H. |last9=Kosik |first9=Kenneth S. |title=A Post-Synaptic Scaffold at the Origin of the Animal Kingdom |journal=PLOS ONE |volume=2 |issue=6 |date=6 June 2007 |issn=1932-6203 |pmid=17551586 |pmc=1876816 |doi=10.1371/journal.pone.0000506 |doi-access=free |page=e506|bibcode=2007PLoSO...2..506S }}</ref>
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Sponges have several cell types:
* [[Archaeocytes]] (or [[amoebocytes]]) have many functions; they are [[totipotent]] cells which can transform into [[sclerocytes]], [[spongocytes]], or [[collencytes]]. They also have a role in nutrient transport and sexual reproduction.
* Cells are arranged in a gelatinous non-cellular [[Matrix (biology)|matrix]] called mesohyll
Sponges have three body types: [[asconoid]], [[syconoid]], and [[leuconoid]].
* [[Choanocyte]]s (also known as "collar cells") function as the sponge's [[digestive system]], and are remarkably similar to the [[protist]]an [[choanoflagellate]]s. The collars are composed of many [[Microvillus|microvilli]] and are used to filter particles out of the water. The beating of the choanocytes' [[flagellum|flagella]] creates the sponge's water current.
* [[Collencytes]] secrete [[collagen]].
* [[Myocytes (sponge)|Myocytes]] are modified pinacocytes which control the size of the osculum and pore openings and thus the water flow.
* [[Pinacocytes]] which form the [[pinacoderm]], the outer epidermal layer of cells. This is the closest approach to true tissue in sponges
* [[Porocytes]] are tubular cells that make up the sponge's pores.
* [[Sclerocytes]] secrete [[calcareous]] siliceous spicules which reside in the mesohyl.
* [[Spongocytes]] secrete [[spongin]], [[collagen]]-like fibers which make up the mesohyl.
* [[Sponge spicule|Spicules]] are stiffened rods or spikes made of [[calcium carbonate]] or [[Silicon dioxide|silica]] which are used for structure and [[antipredator adaptation|defense]].
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