The metastoma is a ventral single plate located in the opisthosoma of non-arachniddekatriatanchelicerates such as eurypterids,[1]chasmataspidids[2] and the genusHouia.[3] The metastoma located between the base of 6th prosomal appendage pair and may had functioned as part of the animal's feeding structures. It most likely represented a fused appendage pair originated from somite 7 (first opisthosomal segment), thus homologous to the chilaria of horseshoe crab and 4th walking leg pair of sea spider.[2] In eurypterids, the plate was typically cordate (heart-shaped) in shape, though differed in shape in some genera, such as Megalograptus.[4]
^Tollerton, Victor P. (1989). "Morphology, taxonomy, and classification of the order Eurypterida Burmeister, 1843". Journal of Paleontology. 63 (5): 642–657. doi:10.1017/S0022336000041275. ISSN0022-3360.