CHAPTER 14 Flatworms Phylum Platyhelminthes, Mesozoa, Nemertea 14-1
14-2 General Features n Two major evolutionary advances n Cephalization n Concentrating sense organs in the head region n Primary bilateral symmetry n Body can be divided along only 1 plane of symmetry to yield 2 mirror images of each other n Active, directed movement most efficient with an elongated body form with anterior (head), posterior (tail), dorsal, and ventral sides
General Features n n Mesoderm well-defined Triploblastic (3 germ layers ectoderm, endoderm, mesoderm) n Typical acoelomates have only one internal space, the digestive cavity n Region between the epidermis and digestive cavity is filled with parenchyma 14-3
Characteristics n Commonly called flatworms n Vary from a millimeter to many meters in length n Some free-living; others parasitic 14-4
Form and Function n Epidermis and Muscles n Most have cellular, ciliated epidermis n Rod-shaped rhabdites n Swell and form a protective mucous sheath n Most turbellarians have dual-gland adhesive organs n Viscid gland cells fasten microvilli of anchor cells to substrate n Secretions of releasing gland cells provide a quick chemical detachment 14-5
Nutrition and Digestion n Cestodes have no digestive system n Others have a mouth, pharynx, and intestine n In planarians n Pharynx may extend through the ventral mouth n Intestine has three branches n One anterior and two posterior n Gastrovascular cavity lined with columnar epithelium n Mouth of trematodes opens near the anterior end n Pharynx is not extensible n Intestine ends blindly, varies in degree of branching 14-6
14-7
n Planaria n Carnivorous and detect food by chemoreceptors n Food trapped in mucous secretions from glands and rhabdites n Wrap themselves around prey n Extend the proboscis to suck up bits of food 14-8
n Trematodes n Feed on host cells, cellular debris, and body fluids n Enzymes from the intestine are secreted for extracellular digestion n Phagocytic cells in gastrodermis complete digestion at intracellular level n Undigested food passed out the pharynx n Cestodes n Rely on the host s digestive tract n Absorb digested nutrients 14-9
Excretion and Osmoregulation n Flatworms have protonephridia n Used for osmoregulation (fluid/ion control) n Beating flagella drive fluids down collecting ducts n Wall of the duct beyond the flame cell bears folds or microvilli to resorb ions and molecules n Collecting ducts join and empty at nephridiopores n Majority of metabolic wastes n Removed by diffusion across the body wall 14-10 Phylum Platyhelminthes
Nervous System n Subepidermal nerve plexus resembles nerve net of cnidarians n One to five pairs of longitudinal nerve cords lie under the muscle layer n Freshwater planarians n One ventral pair of nerve cords forming a laddertype pattern n Brain is a bilobed ganglion anterior to the ventral nerve cords 14-11
Sense Organs n n n n Phylum Platyhelminthes Ocelli (light-sensitive eyespots) Tactile and chemoreceptive cells n Abundant, especially in the ear-shaped auricles Statocysts (equilibrium) and rheoreceptors (sense direction of water currents) Sensory nerve endings found in n Oral suckers and genital pores of parasitic groups 14-12
Reproduction and Regeneration Class Turbellaria n Fission n Many turbellarians constrict behind the pharynx and separate into two animals n Each half regenerates the missing parts n Provides for rapid population growth n Regeneration n If the head and tail are cut off n Each end grows the missing part 14-13
n Nearly all are monecious but cross fertilize n Turbellarians develop male and female organs opening at a common pore n After copulation, eggs and yolk cells enclosed in small cocoon n Attach by a stalk to plants n Embryos emerge and resemble little adults 14-14
Class Trematoda Clonorchis: Human Liver Fluke n Adults live in bile passageways of humans and other fish-eating mammals n Eggs containing a complete miracidium are shed into water with feces n The eggs hatch only when ingested by snails of specific genera n Miracidium enters snail tissue and transforms into a sporocyst n Sporocyst produces one generation of rediae, which begin differentiation 14-15
n Rediae pass into the snail liver n Continue embryonation into tadpole-like cercariae n Cercariae escape into water n Bore into fish muscles or under scales n Shed tail and encyst as metacercariae n A mammal eats raw fish n Cyst dissolves and flukes migrate up bile duct 14-16
Schistosoma: Blood Fluke n Eggs discharged in human feces or urine n In water, eggs hatch as ciliated miracidia n Must contact a particular species of snail to survive n In the snail, they transform to sporocysts n Sporocysts produce cercaria directly n Cercariae escape the snail and swim until they contact bare human skin n Cercariae pierce the skin and shed their tails 14-17
n Enter blood vessels and migrate to the hepatic portal blood vessels n Develop in the liver and they migrate target sites n Eggs released by females are extruded through gut or bladder lining and exit with feces or urine n Eggs that remain behind become centers of inflammation 14-18
Class Cestoda Taenia saginata: Beef Tapeworm n Lives as an adult in the alimentary canal of humans n Juvenile form found in inter-muscular tissue of cattle n Mature adults can reach over 10 meters in length with over 2000 proglottids n Scolex has four suckers but no hooks n Gravid proglottids (with shelled, infective larvae) pass in feces 14-19
n Each mature proglottid has muscles and parenchyma plus male and female organs n Gravid proglottids usually crawl out of feces n Proglottids rupture as they dry n Embryos are viable for five months and are picked up by grazing 14-20
Life Cycle n Cattle swallow shelled larvae that hatch as oncospheres n Oncospheres use hooks to burrow through the intestinal wall into blood or lymph vessels n When they reach voluntary muscle, they encyst to become bladder worms (cysticerci) n When the infected meat is eaten, the cyst wall dissolves and the scolex evaginates to attach to intestinal mucosa 14-21
n New proglottids develop in 2 3 weeks n Infected individuals expel numerous proglottids daily n Infection can be avoided by eating only thoroughly cooked beef 14-22