largest classes crustaceans containing living species divided orders members display great diversity body forms include crabs shrimp shrimp familiar animals abundant marine environments freshwater terrestrial habitats animals united common body plan comprising body segments rarely divided head abdomen etymology coined french zoologist curator collection paris comes greek roots meaning soft meaning shell misleading shell soft immediately usually hard contrasted applied crustaceans outside named obsolete taxon description class includes species arguably contains greater diversity body forms class animal kingdom members characterised presence specialized multiple segments head abdomen segments retain ancestral condition seven abdominal segments abdominal fact major taxa body segment bears pair lost head bears pairs antennae branching parts second pair bear outer branches flattened scales known consist pairs second pair usually pair compound eyes present taxa eyes reduced lost segments fused head form associated turn forward modified absent present lost cover head abdomen variable form fused segments occasionally parts typically better developed branches crawling consist seven segments basis formed taxa lost clear seven abdomen taxa abdominal segment carries pair swimming gas exchange creating current brooding eggs second abdominal modified male form segment typically flattened terminal tail fan sudden tail fan provides thrust rapid escape response crustaceans tail fan steering instead form caudal spine like internal anatomy digestive tract straight consists short stomach contains like mill grinding food walls ridges teeth fine particles material moved chemical processing absorption takes place pairs large digestive concerned water formation situated base like crustaceans open system heart pumps blood body cavity supplies needs organs oxygen nutrients heart typical respiratory pigment structures function located near base antennae brain exists form close antennae segment collection major sensory organs include compound eyes simple eyes sensory eye characteristic consists cup shaped facing different directions able distinguish light darkness ecology live wide range marine freshwater habitats orders terrestrial members terrestrial crabs crabs families terrestrial abundant marine ecosystems species crabs filter life cycle species distinct sexes phenomenon known species exhibit female openings located sixth segment male eighth segment small number species seventh stages reduced place hatching occur usually occurs adult forms primitive free swimming stage mating mating behavior studied freshwater shrimp multiple common occurs reproductive success found genetic mother finding suggests sperm competition post female choice occurs female choice increase fitness avoiding lead expression widely accepted supported common morphological traits present group confirmed molecular studies number problems difficult determine relationships orders include differences rates different different patterns evolution apparent different sources data including evolution long branch attraction agreement status single extant order depending leaf like limbs single multiple origin authors advocate placing group classification systems include molecular study american concluded phylogenetic evidence support grouping regarded diverged main lineage early date extant order orders extinct thought primitive date period range length suspension head abdomen seven abdominal segments families known genera species found worldwide zone deep ocean species benthic living extant order orders extinct commonly known range length predators flattened body shield like armed powerful claws normally carried folded position species living tropical subtropical seas live temperate areas benthic hiding cracks living emerging forage ambush predators contains vast majority approximately living species consists small found freshwater subterranean habitats characterised having brood young found marine freshwater terrestrial habitats include includes crabs fossil record appeared animals belonging appeared classification following classification living based american marine curator crustaceans extinct orders added indicated class brooks
Malacostraca