Editing Eurypterid (section)

==Description==
[[File:Eurypterus_anatomy.png|left|thumb|Restoration of ''[[Eurypterus]]'' with body parts labelled|alt=]]
Like all other [[arthropod]]s, eurypterids possessed [[Segmentation (biology)|segmented]] bodies and jointed appendages (limbs) covered in a [[cuticle]] composed of [[protein]]s and [[chitin]]. As in other [[chelicerates]], the body was divided into two [[Tagmata (arthropod anatomy)|tagmata]] (sections); the frontal [[prosoma]] (head) and posterior [[opisthosoma]] (abdomen).{{sfn|Størmer|1955|p=23}} The prosoma was covered by a [[carapace]] (sometimes called the "prosomal shield") on which both [[compound eyes]] and the [[ocelli]] (simple eye-like sensory organs) were located.{{sfn|Braddy|Dunlop|1997|pp=437–439}}

The prosoma also bore six pairs of appendages which are usually referred to as appendage pairs I to VI. The first pair of appendages, the only pair placed before the mouth, is called the [[chelicerae]] ([[Homology (biology)|homologous]] to the fangs of spiders). They were equipped with small pincers used to manipulate food fragments and push them into the mouth.{{sfn|Braddy|Dunlop|1997|pp=437–439}} In one lineage, the [[Pterygotidae]], the chelicerae were large and long, with strong, well-developed teeth on specialised [[Chela (organ)|chelae]] (claws).{{sfn|Tetlie|Briggs|2009|p=1141}} The subsequent pairs of appendages, numbers II to VI, possessed gnathobases (or "tooth-plates") on the [[Arthropod coxa|coxae]] (limb segments) used for feeding. These appendages were generally walking legs that were cylindrical in shape and were covered in spines in some species. In most lineages, the limbs tended to get larger the farther back they were. In the [[Eurypterina]] [[suborder]], the larger of the two eurypterid suborders, the sixth pair of appendages was also modified into a swimming paddle to aid in traversing aquatic environments.{{sfn|Braddy|Dunlop|1997|pp=437–439}}

The opisthosoma comprised 12 segments and the [[telson]], the posteriormost division of the body, which in most species took the form of a blade-like shape.{{sfn|Braddy|Dunlop|1997|pp=437–439}} In some lineages, notably the [[Pterygotioidea]], the [[Hibbertopteridae]] and the [[Mycteroptidae]], the telson was flattened and may have been used as a rudder while swimming. Some genera within the superfamily [[Carcinosomatoidea]], notably ''[[Eusarcana]]'', had a telson similar to that of true [[scorpion]]s and may even have been capable of using it to inject [[venom]] like them.{{sfn|Plotnick|Baumiller|1988|p=22|pp=}}{{sfn|Clarke|Ruedemann|1912|p=244|pp=}} The coxae of the sixth pair of appendages were overlaid by a plate that is referred to as the metastoma, originally derived from a complete exoskeleton segment. The opisthosoma itself can be divided either into a "[[mesosoma]]" (comprising segments 1 to 6) and "[[metasoma]]" (comprising segments 7 to 12) or into a "preabdomen" (generally comprising segments 1 to 7) and "postabdomen" (generally comprising segments 8 to 12).{{sfn|Braddy|Dunlop|1997|pp=437–439}}

The underside of the opisthosoma was covered in structures evolved from modified opisthosomal appendages. Throughout the opisthosoma, these structures formed plate-like structures termed {{lang|de|Blattfüsse}} ({{lit|leaf-feet}} in German).<ref>{{cite web | url=https://core.ac.uk/reader/213406250 | title=Selectivity in the evolution of Palaeozoic arthropod groups, with focus on mass extinctions and radiations: A phylogenetic approach }}</ref> These created a branchial chamber (gill tract) between preceding {{lang|de|Blattfüsse|italics=unset}} and the [[ventral]] surface of the opisthosoma itself, which contained the respiratory organs. The second to sixth opisthosomal segments also contained oval or triangular organs that have been interpreted as organs that aid in respiration. These organs, termed {{lang|de|Kiemenplatten}} or "gill tracts", would potentially have aided eurypterids to breathe air above water, while {{lang|de|Blattfüssen|italics=unset}}, similar to organs in modern [[horseshoe crab]]s, would cover the parts that serve for [[underwater respiration]].{{sfn|Braddy|Dunlop|1997|pp=437–439}}

The appendages of opisthosomal segments 1 and 2 (the seventh and eighth segments overall) were fused into a structure termed the genital operculum, occupying most of the underside of the opisthosomal segment 2. Near the [[anterior]] margin of this structure, the genital appendage (also called the {{lang|de|Zipfel}} or the median abdominal appendage) protruded. This appendage, often preserved very prominently, has consistently been interpreted as part of the reproductive system and occurs in two recognized types, assumed to correspond to male and female.{{sfn|Braddy|Dunlop|1997|pp=437–439}}