The following classification is based largely on that given in Synopsis and Classification of Living Organisms (1982) but has been modified to take account of advances made since that date. Groups marked with a dagger (†) are extinct and known only from fossils. Subphylum Crustacea Two pairs of sensory appendages in front of mouth, and 3 pairs of jaws behind mouth; some parasitic and lack all appendages when adult; mostly aquatic; about 45,000 species known. Class Cephalocarida (horseshoe shrimps) Holocene; primitive; blind; head shield without carapace; maxilla and all trunk limbs alike, with jointed inner branch and leaflike outer branches; ...(100 of 7175 words)