Editing Sea spider (section)

== Internal anatomy and physiology ==
[[File:20200208 Pycnogonida Pantopoda digestive system.png|thumb|Digestive tract (yellow highlight) of a pantopod pycnogonid]]
[[File:Ammothella longipes (10.3897-zse.90.7520) Figure 3 (cropped).jpg|thumb|Female ''Ammothella longipes'' with pedal gonads full of eggs]]
<gallery mode="packed" heights="160">
File:Pycnogonida anatomy - tagged.png|Sagittal section of an [[Ascorhynchidae|ascorhynchid]] pycnogonid, showing pharynx (F), mid gut (H) and central nervous system (B).
Pycnogonida anatomy tagged.png|Transverse section of a pycnogonid leg, showing gut diverticulum (C, D) and gonad (E)
</gallery>
A striking feature of pycnogonid anatomy is the distribution of their digestive and [[reproductive system]]s. The [[pharynx]] inside the proboscis is lined by dense [[setae]], which is possibly related to their feeding behaviour.<ref name="2017proboscis" /> A pair of [[gonad]]s ([[Ovary|ovaries]] in female, [[Testicle|testes]] in male) is located dorsally in relation to the [[digestive tract]], but the majority of these organs are branched diverticula throughout the legs because the body is too small to accommodate all of them alone. The midgut diverticula are very long, usually reaching beyond the femur (variably down to tibia 2, tarsus or propodus) of each leg, except in [[Rhynchothoracidae]] where they only reach coxa 1. Some species have additional branches (in some ''[[Pycnogonum]]'') or irregular pouches (in ''[[Endeis]]'') on the diverticula. There is also a pair of anterior diverticula which corresponds to the chelifores or is inserted into the proboscis in some chelifores-less species. The palps and ovigers never contain diverticula, although some might possess a pair of small diverticula near the bases of these appendages.<ref name=":24" /> The gonad diverticula (pedal gonad) reach each femur and open via a [[gonopore]] located at coxa 2.<ref>{{Cite journal |last1=Alexeeva |first1=Nina |last2=Tamberg |first2=Yuta |date=2023-09-01 |title=Ultrastructure of the female pedal gonad in Phoxichilidium femoratum (Chelicerata, Pycnogonida) |url=https://linkinghub.elsevier.com/retrieve/pii/S1467803923000622 |journal=Arthropod Structure & Development |volume=76 |pages=101295 |doi=10.1016/j.asd.2023.101295 |pmid=37722770 |bibcode=2023ArtSD..7601295A |issn=1467-8039}}</ref> The structure and number of the gonopores might differ between sexes (e.g. larger in female, variably absent at the anterior legs of some male).<ref name=":4" /> In males, the femur or both femur and tibia possess [[cement]] glands.<ref name=":4" />

Pycnogonids do not require a traditional [[respiratory system]] (e.g. [[gill]]s). Instead, gasses are absorbed by the legs via the non-calcareous, porous exoskeleton and transferred through the body by [[diffusion]].<ref>{{Cite journal |last1=Lane |first1=Steven J. |last2=Moran |first2=Amy L. |last3=Shishido |first3=Caitlin M. |last4=Tobalske |first4=Bret W. |last5=Woods |first5=H. Arthur |date=2018-01-01 |title=Cuticular gas exchange by Antarctic sea spiders |url=https://journals.biologists.com/jeb/article/doi/10.1242/jeb.177568/263002/Cuticular-gas-exchange-by-Antarctic-sea-spiders |journal=Journal of Experimental Biology |volume=221 |issue=8 |pages=jeb177568 |language=en |doi=10.1242/jeb.177568 |issn=1477-9145}}</ref> The morphology of pycnogonid creates an efficient [[surface-area-to-volume ratio]] for respiration to occur through direct diffusion. [[Oxygen]] is absorbed by the legs and is transported via the [[hemolymph]] to the rest of the body with an [[Circulatory system#Open circulatory system|open circulatory system]].<ref name="woods2017"/> The small, long, thin pycnogonid [[heart]] beats vigorously at 90 to 180 beats per minute, creating substantial [[blood pressure]]. The beating of the heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs.<ref name="woods2017">{{Cite journal |last1=Woods |first1=H. Arthur |last2=Lane |first2=Steven J. |last3=Shishido |first3=Caitlin |last4=Tobalske |first4=Bret W. |last5=Arango |first5=Claudia P. |last6=Moran |first6=Amy L. |date=2017-07-10 |title=Respiratory gut peristalsis by sea spiders |url= |journal=Current Biology|language=en |volume=27 |issue=13 |pages=R638–R639 |doi=10.1016/j.cub.2017.05.062 |issn=0960-9822 |pmid=28697358|pmc= |s2cid=35014992 |doi-access=free |bibcode=2017CBio...27.R638W }}</ref><ref name="RTN3PMC" /> Hemolymph circulation in the legs is mostly driven by the [[peristalsis|peristaltic movement]] of the gut diverticula that extend into every leg, a process called gut peristalsis.<ref name="woods2017"/><ref name="RTN3PMC">{{cite journal | pmc=5344685 | year=2017 | last1=Bastide | first1=A. | last2=Peretti | first2=D. | last3=Knight | first3=J. R. | last4=Grosso | first4=S. | last5=Spriggs | first5=R. V. | last6=Pichon | first6=X. | last7=Sbarrato | first7=T. | last8=Roobol | first8=A. | last9=Roobol | first9=J. | last10=Vito | first10=D. | last11=Bushell | first11=M. | last12=von Der Haar | first12=T. | last13=Smales | first13=C. M. | last14=Mallucci | first14=G. R. | last15=Willis | first15=A. E. | title=RTN3 is a Novel Cold-Induced Protein and Mediates Neuroprotective Effects of RBM3 | journal=Current Biology | volume=27 | issue=5 | pages=638–650 | doi=10.1016/j.cub.2017.01.047 | pmid=28238655 | bibcode=2017CBio...27..638B }}</ref> In the case of taxa without a heart (e.g. Pycnogonidae), the whole circulatory system is presumed to be solely maintained by gut peristalsis.<ref name=":24" /> 

The [[central nervous system]] of pycnogonid largely retains a segmented ladder-like structure. It consists of a dorsal [[brain]] ([[supraesophageal ganglion]]) and a pair of [[ventral nerve cord]]s, intercepted by the [[esophagus]]. The former is a fusion of the first and second brain segments (cerebral [[ganglia]])—protocererum and deutocerebrum—corresponding to the eyes/ocular somite and chelifores/somite 1 respectively. The whole section was rotated during pycnogonid evolution, as the protocerebrum went upward and the deutocerebrum shifted forward.<ref name=":6" />  The third commissure is established inferior to the esophagus.<ref>{{cite journal|title=Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment|journal=Nature|year=2005|volume=437|pages=1144-1148|doi=10.1038/nature03984|last1=Maxmen|first1=Amy|last2=Browne|first2=William E.|last3=Martindale|first3=Mark Q.|last4=Giribet|first4=Gonzalo}}</ref>  This third brain segment, or tritocerebrum (corresponding to the palps/somite 2), is fused to the oviger/somite 3 ganglia instead, which is followed up by the final ovigeral somata in the protonymphon larva of ''[[Pycnogonum litorale]]''.<ref name=":6" />  A series of leg ganglia (somite 4 and so on) develop as molts progress,<ref>{{cite journal|title=Postembryonic development of pycnogonids: A deeper look inside|last1=Alexeevna|first1=Nina|last2=Tamberg|first2=Yuta|last3=Shunatova|first3=Natalia|journal=Arthropod Structure & Development|volume=47|issue=3|pages=299-317|year=2018|doi=10.1016/j.asd.2018.03.002}}</ref> with incorporation of the first leg ganglia into the subesophageal ganglia in certain taxa.<ref name=":24">{{Cite journal |last1=Frankowski |first1=Karina |last2=Miyazaki |first2=Katsumi |last3=Brenneis |first3=Georg |date=2022-03-31 |title=A microCT-based atlas of the central nervous system and midgut in sea spiders (Pycnogonida) sheds first light on evolutionary trends at the family level |journal=Frontiers in Zoology |volume=19 |issue=1 |pages=14 |doi=10.1186/s12983-022-00459-8 |doi-access=free |issn=1742-9994 |pmc=8973786 |pmid=35361245}}</ref>  The leg ganglia might shift anteriorly or even cluster together, but are never highly fused into the ring-like synganglion of other chelicerates.<ref name=":24"/> The abdominal ganglia are vestigal, absorbed by the preceding leg ganglia during juvenile development.<ref name=":12">{{Cite journal |last1=Brenneis |first1=Georg |last2=Scholtz |first2=Gerhard |date=2014-04-15 |title=The 'Ventral Organs' of Pycnogonida (Arthropoda) Are Neurogenic Niches of Late Embryonic and Post-Embryonic Nervous System Development |journal=PLOS ONE |language=en |volume=9 |issue=4 |pages=e95435 |doi=10.1371/journal.pone.0095435 |doi-access=free |issn=1932-6203 |pmc=3988247 |pmid=24736377|bibcode=2014PLoSO...995435B }}</ref>