flat worms greek meaning flat root meaning worm relatively simple soft bodied invertebrates unlike having body cavity specialized respiratory organs having flattened shapes allow oxygen nutrients pass bodies digestive cavity opening intake nutrients removal result food processed continuously traditional medicinal texts divided non parasitic animals entirely parasitic groups proven classification free living predators live water shaded humid terrestrial environments leaf litter complex life cycles mature stages live parasites digestive systems fish land intermediate stages secondary hosts eggs main hosts adult generate vast numbers segment like mature release eggs unlike parasitic groups external parasites aquatic animals larvae adult form suitable host internal body regarded primitive stage evolution animals symmetry distinct rear ends analyses mid-1980s separated basal closer original modern groups remaining form group contains descendants common ancestor member group main groups complex analyses concluded excluding consists forming branch traditional regarded wholly parasitic groups descended group half known species parasitic enormous harm humans livestock caused genus second devastating human diseases caused parasites surpassed malaria arises larvae pork penetrate central nervous system major cause acquired worldwide threat parasites humans developed countries rising popularity raw lightly cooked foods imports food high risk areas developed countries people afford fuel required cook food thoroughly poorly designed water supply irrigation projects increase dangers presented poor farming species successfully philippines indonesia hawaii new_guinea guam control populations imported giant african displacing native concern threat native northwest europe concerns spread new_zealand description distinguishing features symmetrical animals left right sides mirror images implies distinct surfaces distinct head tail ends like main cell layers symmetrical cell layers defined particular series unlike internal body cavity described lack specialized respiratory organs facts defining features classifying anatomy bodies soft features common lack respiratory organs limits sizes shapes enable oxygen reach carbon dioxide leave parts bodies simple microscopic large species flat ribbon like leaf like shapes large species branches nutrients parts body surface body makes vulnerable fluid loss environments unlikely sea freshwater moist terrestrial environments leaf litter grains soil parasites animals space skin gut filled tissue cells reinforced fibers act type skeleton providing attachment points muscles contains internal organs allows passage oxygen nutrients waste products consists main types cell fixed cells fluid filled stem cells transform type cell tissues injury reproduction material mouth long species complex branched mouth difficult gut lined single layer cells absorb digest food species break food gut throat animals need concentration dissolved substances body fairly constant level internal parasites free living marine animals live environments high concentrations dissolved material generally let tissues level concentration environment freshwater animals need prevent body despite difference environments use system control concentration body flame cells called beating looks like flame extract water contains material drive networks tube cells lined tube cells drive water exits called materials water needed body right concentration combinations flame cells tube cells called nervous system concentrated head end marked nerve nets like head rings head main nerve running bodies major early classification divided groups classification long recognized artificial proposed correct classification massively split dozen orders joined new order classification presented early traditional classification scientific articles species free living range length predators terrestrial species nocturnal live shaded humid locations leaf litter rotting wood animals crustaceans parasites free living black brown gray larger ones brightly colored traditionally regarded regarded members separate separate genus simple animals reclassified separate simple lined generally feed sweep food particles small prey mouths usually middle extended turned inside mouths different species underside freshwater species open mouth wide body long swallow prey large pigment cup little eyes pair species pairs large species eyes clusters brain mounted spaced uniformly edge body distinguish direction light coming enable animals avoid groups fluid filled chambers containing small solid particle groups thought balance acceleration sensors function perform sensory sense movements positions solid particles unknown hand touch cells scattered bodies especially edges specialized cells pits grooves head probably smell sensors famous ability divided cuts bodies experiments fragments head new head grows quickly closest original head suggests growth head controlled chemical concentration head tail clone transverse longitudinal division especially reproduce vast majority female male reproductive cells eggs internally larger aquatic species mate fencing duel tries loser adopts female role developing eggs species miniature adults emerge eggs hatch large species produce like larvae parasites refers greek hole resemble anchor hosts skin species layer cells shares single external membrane divided groups known called flat shapes like old english species combined second parasites adults usually ring mouth larger midway underside free living means generations complex life cycles seven stages depending combinations environments early stages encounter importantly eggs deposited land water intermediate stages transfer parasites host definitive host adults develop land earliest host juvenile stages usually live land water cases fish second host example adjoining illustration shows life cycle moves fish body flesh moves small land animal eats fish raw generates eggs completing cycle cause devastating tropical disease belong group adults range length individual adult single sex species slender females live enclosed grooves run bodies males partially emerge lay eggs species adults complex reproductive systems produce times eggs free living addition intermediate stages live reproduce adults different species different parts definitive host example lungs large blood vessels liver adults use relatively large muscular cells cell fragments body blood adults stages live external dissolved nutrients host adult live oxygen long periods members small group single divided row cover underside bony fish turtles body marine freshwater eggs produce swimming larvae life cycle hosts parasites attach hosts means bear crescent shaped hooks divided species external parasites require particular host species mainly fish cases amphibians aquatic reptiles internal parasites adult large attachment organs rear greek means catch hooks flattened bodies species digest host skin allowing parasite feed blood cellular debris externally hosts skins based fact parasites generation called flat slender long bodies derived latin word means tape adults species internal parasites mouths skin nutrients mainly host avoid attacks host immune system shortage host diet stunts growth parasites kills generally use simple inefficient chemical processes compensate consuming large amounts food relative size majority species known true neck produces chain segments called process known mature adults humans form chains metres long metres typical male female reproductive organs host gut contains adults species generally worm eggs fully developed separate host life cycle complex varies depending species example adults fish juveniles use crustaceans intermediate hosts release eggs water eggs hatch swimming larvae sheds skin makes way internal cavity main system attached small hooks eaten fish small drills gut adult species humans cats dogs juveniles use example pigs cattle rabbits intermediate hosts release eggs stick grass leaves hatch makes way muscles oval worm millimetres long kept inside definitive host eats infested raw meat intermediate host worm pops gut adult develops members smaller group known produce body shapes like fish turtles classification evolutionary relationships oldest identified parasitic fossils eggs found shark hooks attached represent parasitic simple life cycles oldest known free living specimen fossil preserved amber placed species oldest specimens eggs discovered ancient egyptian distinguishing features animals makes difficult work relationships groups animals relationships different groups described members traditional view 1990s formed sister group include example molecular aims work evolutionary family trees comparing different organisms dna proteins radically changed scientists view evolutionary relationships animals detailed morphological analyses features mid-1980s molecular analyses different sections dna agree consisting traditionally regarded simple small group previously classified sister group including rest study concluded distinct groups agreed closely related etc defined organ originally classified primitive recently reclassified excluding contain main groups generally agreed contains descendants ancestor member group early molecular analyses left combined single group study concluded excluding molecular analyses agree closely related grouping known generally agreed closely related includes worms majority view significant minority researchers regard sister group agreed wholly parasitic groups form larger grouping adults members skins debate combined intermediate group generally agreed sub group levels family tree traditional sub include descendants sub group evolution outline origins parasitic life style proposed feeding fish hosts basal shift free living ancestors evolutionary step change blood common ancestor likely earliest known fossils dated million years ago found interaction humans cause important diseases humans livestock cause losses stocks fish farms known fever second devastating parasitic disease tropical countries malaria carter center estimated million people countries infected disease half victims live africa condition low mortality rate chronic illness damage internal organs growth development children increase risk cancer adults disease caused genus bore human skin people risk use infected bodies water recreation laundry estimated million people infected beef million pork infection digestive system adult causes abdominal symptoms unpleasant life threatening resulting penetration larvae central nervous system major cause acquired worldwide million people infected naturally fish crustaceans pass humans eat raw lightly cooked infection humans broad fish occasionally causes severe cases threat humans developed countries rising result social trends increase organic farming uses sewage artificial spreads parasites directly feed increasing popularity raw lightly cooked foods imports meat vegetables high risk areas underlying cause reduced awareness parasites compared public health issues pollution developed countries inadequate use human night soil fish farm ponds continues spread parasitic poorly designed water supply irrigation projects provided additional channels spread people countries afford cost fuel required cook food thoroughly kill parasites controlling parasites humans livestock difficult species resistant drugs effective mainly killing juveniles meat poorer countries struggle infection cases reported intentional infection desperate rapid weight loss concern northwest europe including british isles new_zealand australian prey thought reached europe containers plants imported botanical gardens benefits hawaii control imported giant african displacing native purpose philippines indonesia new_guinea guam declined sharply hawaii doubts contributed given credit severely reducing places achieving greater success biological pest control programs generally aim low stable population pest species ability different kinds prey resist starvation account ability abilities raised concerns threat native study argentina shows potential reduce populations species experiment showed particular prey species maintain steady predation rate time ability live artificial containers showed potential placing species popular breeding sites reduce borne diseases
Flatworm