Practice question

Primary body cavity in arthropods is hemocoel, not true coelom, derived mainly from embryonic blastocoel persisting after partial replacement of coelom which is reduced to small cavities around gonads and excretory sacs. Hemocoel is filled with hemolymph, fluid combining blood and interstitial components, conducting nutrients, wastes and hormones, surrounding visceral organs. Coelom sensu stricto is mesoderm-lined peritoneal space fully enclosing organs as in annelids; pseudocoel lacks complete mesodermal lining as in nematodes; sinus is localized widening. Term hemocoel emphasizes dual cavity-circulatory function distinctive of arthropods and molluscs.