new latin french greek horn constitutes major subdivisions contains crabs sea spiders including spiders originated marine animals possibly period confirmed fossils date million years ago late period surviving marine species include species crabs possibly species sea spiders hand identified species air breathing unidentified species like bodies limbs covered cuticle proteins consists lost visible division sections group appear mouth sub groups modest feed spiders form species use venom prey group open system typical tube like heart pumps blood major body cavity marine gills air breathing forms generally book lungs general living central nervous systems fuse large masses wide variations fusion limited regarded oldest primitive group spiders rely modified touch information vibrations air currents chemical changes environment active hunting spiders acute originally predators group use major feeding strategies predation eating decaying organic matter digest solid food modern narrow generally food grinding flooding digestive conserve water air breathing waste removed blood structures evolved independently insects marine crabs rely external air breathing use internal usually indirect predatory species generally use elaborate courtship rituals prevent males eaten mate lay eggs hatch look like miniature adults species eggs inside bodies young emerge species young fend species spider females protect feed young evolutionary origins early debated decades considerable agreement relationships sub groups inclusion taxon recently questioned exact position controversial long considered primitive basal venom spider species dangerous humans medical researchers investigating use treatment disorders ranging cancer medical industry uses blood crabs test presence bacteria cause humans transmit diseases humans livestock agricultural description cuticle bodies limbs covered cuticle proteins heads composed segments fuse development embryo reduced blood driven tube like heart bodies consist sets segments serve similar functions foremost called rear called abdomen visible division sections formed embryo fusion carries eyes segments seven paired segment lost embryo development segment pair small form segment pair sub groups perform sensory functions remaining segments pairs legs primitive forms pair compound eyes sides pigment cup little eyes middle mouth segments consists fewer segments originally formed groups seven segments terminating spike abdominal modern missing heavily modified example spiders remaining form silk crabs form gills like bodies covered tough cuticle mainly hardened proteins stretch animals grow words grow new soft cast old wait new new cuticle animals vary widely form function consistent difference position arise segment ahead mouth segment mouth horns sub normally consist sections formed section rigid extension second spiders sections second forms away use relative sizes vary widely fossil modern form large claws extended ahead body tiny feeding project slightly head relatively small sensors male spiders tips act sperm females reproductive openings mating form large claws capturing prey body systems body small restricted small areas round reproductive systems main body cavity runs length body blood flows driven tubular heart collects blood rear pumps forward direct blood specific parts body open ends joining directly veins open systems typical respiratory systems depend individual sub groups environments modern terrestrial generally book lungs deliver oxygen remove waste gases blood blood transport system living crabs aquatic book gills lie horizontal plane long time assumed extinct gills fossil evidence ambiguous fossil millimetres long late period appear pairs vertically oriented book gills internal structure similar book lungs feeding modern narrow solid food spiders predators pre process food formed bases predominantly vegetarian spider species known supplement blood parasites predatory vegetarian sub groups retractable feeding assembly consists parts forms cavity pre processing food minority living solid food group includes predators crabs capable processing solid food use distinctive feeding system claws tips legs grab small invertebrates pass food groove runs legs mouth underside head faces slightly backwards bases legs form toothed grind food push mouth earliest thought fed crabs convert dump gills little extract toxic rapidly large quantities water terrestrial afford use water convert chemicals dry matter extraction combinations filter blood dump system evolved independently insects groups nervous system nervous systems based standard model pair nerve segment brain formed fusion mouth ahead assume lose segment bears antennae brains include pair pre oral instead segments available bears notable variable trend fusion brain brains crabs include plus segments segments retain separate pairs living true including normally fused single mass regarded primitive living spiders rear remain fused abdomen retains separate pairs senses block information outside world penetrated sensors connections sensors nervous system fact spiders modified elaborate sensors touch vibration sensors called respond different levels force strong contact weak air currents chemical sensors provide taste smell means living compound eyes crabs compound eye reduced cluster pairs mounted sides head plus pigment cup little eyes mounted middle eyes crabs detect movement form images extreme jumping spiders wide field vision main eyes times acute able colors light reproduction crabs aquatic use external words sperm meet outside parents bodies like larvae look like miniature adults sets eyes initially pairs book gills gain pairs air breathing animals living use internal direct species words males contact females species indirect male spiders use sperm females reproductive openings produce packages sperm females bodies courtship rituals common especially powerful predators males risk eaten mating lay eggs eggs inside bodies hatch offspring like miniature adults emerge levels parental care young range zero prolonged carry young backs semi social species young remain mother spiders care young example wolf spider brood rough mother females species respond begging behavior young giving prey provided longer struggling food evolutionary history fossil record large gaps fossil record like organic fossils rare conditions exceptionally suited preserving fairly soft tissues animals million years ago classified pattern segments grouped especially head resemble crabs analyses consider wider ranges characteristics place debate earlier period million years ago fossil originally classified closely related gap million years record true nearest relatives recently earliest known fossil dated late stage million years ago older specimen reported million years ago late left good fossils earliest confirmed appear late period little million years ago oldest known million years ago period triangular abdomen legs pair million years ago period bears earliest known silk producing hailed spider lacked true spider likely sister group spiders clade named spiders members primitive group represented late classified differed significantly modern appears wholly aquatic gills book lungs mouth completely head pair legs extinct living crabs fossils terrestrial book lungs found early rocks million years ago relationships traditional view family tree shows closely related major living groups crustaceans includes insects includes groups recent research molecular application analysis especially organisms dna detailed examination nervous systems develop embryos suggests closely related crustaceans closest relatives results derived analyzing living including extinct ones causes swing traditional view placing sister group plus closely related groups major sub groups generally agreed contain classes spiders etc crabs sea extinct extinct sub group sea spiders traditionally classified features suggest representatives earliest known groups evolved structure family tree relationships controversial late 19th century attempt combine analysis features modern features modern fossil ones produced credible results lower level groups results high level relationships major sub groups unstable words minor changes caused significant changes computer program analysis features produced right noted remain position particularly controversial early fossils late classified described wholly aquatic gills book lungs mouths completely heads pair legs extinct living crabs presents difficult choice classify aquatic fossils despite similarities accept consist separate aquatic terrestrial groups treat closely related possibly crabs spiders analyses recovered based species breathing apparatus reflected terrestrial animal diversity insects diverse groups animals living species described scientific publications estimates suggest species spider nearly species earliest living marine animals breathe dissolved oxygen vast majority living species air spider species build diving bell enable live water like ancestors living mainly small invertebrates species feed parasites interaction humans past native americans ate flesh crabs tail spines spear tips shells bail water recent attempts use crabs food livestock abandoned found gave meat bad taste crab blood contains agent test kidney machines ensure free dangerous bacteria detect spinal cooked spiders considered southern venezuela spider alternative conventional deadly insects great majority harmless possible medical uses spider investigated treatment cardiac disease strokes spider silk light strong attempts produce goats milk leaves plants means genetic engineering reliably reported deaths spider bites 20th century compared thought significant danger developed countries example cause deaths year mexico years usa incidents caused accidental human invasions nests medical uses venom investigated treatment brain bone diseases parasitic transmit micro organisms parasites cause diseases humans species directly cause removed day closely related humans causing intense bites skin species normally animals humans normal hosts eliminated species threat honey bees largest single problem faced worldwide cause forms diseases including hay fever significant crop predatory useful controlling
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