Editing Sponge (section)

=== Cell types ===

{{anchor|Ostium (sponges)}}
{{Annotated image/Porifera cell types}}
A sponge's body is hollow and is held in shape by the [[mesohyl]], a jelly-like substance made mainly of [[collagen]] and reinforced by a dense network of fibers also made of collagen. 18 distinct cell types have been identified.<ref>{{cite journal |last1=Musser |first1=Jacob M. |last2=Schippers |first2=Klaske J. |last3=Nickel |first3=Michael |last4=Mizzon |first4=Giulia |last5=Kohn |first5=Andrea B. |last6=Pape |first6=Constantin |last7=Ronchi |first7=Paolo |last8=Papadopoulos |first8=Nikolaos |last9=Tarashansky |first9=Alexander J. |last10=Hammel |first10=Jörg U. |last11=Wolf |first11=Florian |last12=Liang |first12=Cong |last13=Hernández-Plaza |first13=Ana |last14=Cantalapiedra |first14=Carlos P. |last15=Achim |first15=Kaia |last16=Schieber |first16=Nicole L. |last17=Pan |first17=Leslie |last18=Ruperti |first18=Fabian |last19=Francis |first19=Warren R. |last20=Vargas |first20=Sergio |last21=Kling |first21=Svenja |last22=Renkert |first22=Maike |last23=Polikarpov |first23=Maxim |last24=Bourenkov |first24=Gleb |last25=Feuda |first25=Roberto |last26=Gaspar |first26=Imre |last27=Burkhardt |first27=Pawel |last28=Wang |first28=Bo |last29=Bork |first29=Peer |last30=Beck |first30=Martin |last31=Schneider |first31=Thomas R. |last32=Kreshuk |first32=Anna |last33=Wörheide |first33=Gert |last34=Huerta-Cepas |first34=Jaime |last35=Schwab |first35=Yannick |last36=Moroz |first36=Leonid L. |last37=Arendt |first37=Detlev |display-authors=6 |title=Profiling cellular diversity in sponges informs animal cell type and nervous system evolution |journal=Science |volume=374 |issue=6568 |pages=717–723 |date=November 2021 |pmid=34735222 |pmc=9233960 |doi=10.1126/science.abj2949 |bibcode=2021Sci...374..717M }}</ref> The inner surface is covered with [[choanocyte]]s, cells with cylindrical or conical collars surrounding one [[flagellum]] per choanocyte. The wave-like motion of the whip-like flagella drives water through the sponge's body. All sponges have '''ostia''', channels leading to the interior through the mesohyl, and in most sponges these are controlled by tube-like [[porocytes]] that form closable inlet valves. [[Pinacocyte]]s, plate-like cells, form a single-layered external skin over all other parts of the mesohyl that are not covered by choanocytes, and the pinacocytes also digest food particles that are too large to enter the ostia,<ref name="Ruppert_2004"/><ref name="Bergquist_1998"/> while those at the base of the animal are responsible for anchoring it.<ref name="Bergquist_1998"/>

Other types of cells live and move within the mesohyl:<ref name="Ruppert_2004"/><ref name="Bergquist_1998"/>
* [[Lophocyte]]s are [[amoeba]]-like cells that move slowly through the mesohyl and secrete collagen fibres.
* [[Collencyte]]s are another type of collagen-producing cell.
* [[Rhabdiferous]] cells secrete [[polysaccharide]]s that also form part of the mesohyl.
* [[Oocyte]]s and [[spermatocyte]]s are reproductive cells.
* [[Sclerocyte]]s secrete the mineralized [[sponge spicule|spicules]] ("little spines") that form the [[skeleton]]s of many sponges and in some species provide some defense against predators.
* In addition to or instead of sclerocytes, [[demosponge]]s have [[spongocyte]]s that secrete a form of collagen that [[polymer]]izes into [[spongin]], a thick fibrous material that stiffens the mesohyl.
* [[Myocyte]]s ("muscle cells") conduct signals and cause parts of the animal to contract.
* "Grey cells" act as sponges' equivalent of an [[immune system]].
* [[Archaeocytes]] (or [[amoebocytes]]) are [[amoeba]]-like cells that are [[totipotent]], in other words, each is capable of transformation into any other type of cell. They also have important roles in feeding and in clearing debris that block the ostia.

Many larval sponges possess neuron-less [[eye]]s that are based on [[cryptochrome]]s. They mediate phototaxic behavior.<ref>{{cite journal |last1=Rivera |first1=A.S. |last2=Ozturk |first2=N. |last3=Fahey |first3=B. |last4=Plachetzki |first4=D.C. |last5=Degnan |first5=B.M. |last6=Sancar |first6=A. |last7=Oakley |first7=T.H. |title=Blue-light-receptive cryptochrome is expressed in a sponge eye lacking neurons and opsin |journal=The Journal of Experimental Biology |volume=215 |issue=Pt 8 |pages=1278–86 |date=April 2012 |pmid=22442365 |pmc=3309880 |doi=10.1242/jeb.067140 |bibcode=2012JExpB.215.1278R }}</ref>

[[Glass sponge]]s present a distinctive variation on this basic plan. Their spicules, which are made of [[silica]], form a [[scaffolding]]-like framework between whose rods the living tissue is suspended like a [[Spider web|cobweb]] that contains most of the cell types.<ref name="Ruppert_2004"/> This tissue is a [[Hexactinellid#Syncytia|syncytium]] that in some ways behaves like many cells that share a single external [[Cell membrane|membrane]], and in others like a single cell with multiple [[cell nucleus|nuclei]].