Unsegmented Worms

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Phylum Platyhelminthes
Characteristics

• Called flatworms because bodies are flattened dorso-ventrally

• Acoelomate - solid bodies without a lined body cavity

• Have 3 body layers --- outer ectoderm, middle mesoderm, & inner endoderm

• Bilaterally symmetrical

• Show cephalization (concentration of sensory organs at anterior or head end)

• Body cells exchange oxygen & carbon dioxide directly with environment by diffusion

• Single opening into gastrovascular cavity; two-way digestive tract

• Some are parasites & others are free-living

• Parasitic worms have thick cell layer called tegument covered with a nonliving cuticle covering their bodies as protection inside hosts

• Includes 3 classes --- Turbellaria (planarians), Trematoda (parasitic flukes), & Cestoda (parasitic tapeworms

Class Turbellaria

• Most are marine but includes freshwater planarian (Dugesia)

Planarians

• Spade-shaped at the anterior end & have two, light-sensitive eyespots

• Can sense light, touch, taste, & small

• Have 2 clusters of nerve cells or ganglia to form a simple brain

• Nervous system composed of a nerve net

• Capable of simple learning

• Move by tiny hairs or cilia over a mucus layer that they secrete

• Feed by scavenging or protozoans

• Have a single opening or mouth located at the end of a muscular tube called the pharynx which can be extended when feeding

• Flame cells help remove wastes to excretory pores

• Hermaphrodites that cross-fertilize eggs that are then deposited into a capsule until hatching in 2-3 weeks

• Reproduce asexually by fragmentation

Class Trematoda

• Includes parasitic flukes

• About 1 cm long & oval shaped

• Require a host to live

• Have both oral & ventral suckers to cling to host & suck blood, cells, & body fluids

• Oral sucker around mouth at anterior end sucks blood

• May be endoparasites (live inside a host) or ectoparasites (live on the outside of host

• Covered in tough, unciliated tegument

• Nervous & excretory systems like turbellarians

• Hermaphrodites

• Have a long, coiled uterus that stores & releases 10,000+ eggs

• Eggs released through genital pore & develop into larva

• Show complex life cycles

• Life cycle of sheep liver fluke:
  * Adult liver flukes live in sheep liver & gall bladder where they mate & form eggs
  * Eggs enter intestines, pass out with feces, & hatch in water
  * Larva enter snails, asexually multiply, then leave snail & form cysts
  * Cysts (dormant larva with hard, protective covering) clings to grass
  * Sheep ingest cysts when they eat grass
  * Cysts hatch in digestive tract & bore through intestines into bloodstream
  * Mature & reproduce in the liver

• Schistosomiasis (disease caused by parasitic blood flukes) infects people in Asia, Africa, & South America causing intestinal bleeding & tissue decay that can result in death

Class Cestoda

• Includes tapeworms
• Adapted for parasitic life
• Tough outer tegument prevents being digested by host
• Anterior end called scolex contains hooks & suckers for attachment to intestine of host

• Long, ribbon-like bodies up to 12 m in length

• Nervous system extends length of body but lacks sense organs

• Lacks mouth & digestive tract but absorbs digested nutrients from host

• Grows by making body segments called proglottids

• Each proglottid produces eggs & sperm that cross-fertilize with other segments & also self-fertilize (hermaphrodites)

• Oldest, mature proglottids containing eggs at posterior end break off & pass out with feces

• Life cycle of beef tapeworm:
  * Cattle eat grass with proglottids containing fertilized eggs
  * Eggs hatch into larva & bore through cow's intestine into bloodstream
  * Larva burrow into cow's muscle & form cysts
  * Humans eat beef (muscle) & cysts travels to intestines
  * Cyst breaks open & adult beef tapeworm forms

BEEF TAPEWORM LIFE CYCLE

Phylum Nematoda
Characteristics

• Called roundworms

• Includes Ascaris, hookworms, Trichinella, & pinworms

• Pseudocoelomates have fluid-filled body cavity partially lined with mesoderm

• Pseudocoelom contains the body organs & provides hydrostatic skeletal support for muscles

• Have long slender bodies that taper at both ends

• Covered with flexible cuticle

• Digestive tract with anterior mouth & posterior anus; called one-way digestive tract

• Separate sexes in most species

• Most are free living

• Some are parasites on plants & animals

• Ascaris is a parasitic roundworm living in the intestines of pigs, horses, & humans

• Ascaris life cycle:
  * Enter body in contaminated food or water & hatch in intestines
  * Larva bore into bloodstream & carried to lungs & throat
  * Larva coughed up, swallowed, & return to intestines to mature & mate
  * Block the intestine causing death

• Hookworm eggs hatch in moist soil & larva bore through bare feet of new host 

• Trichinella are human parasites caused by eating undercooked pork containing the cysts
  * Cause disease called trichinosis
  * Cysts cause muscle pain & stiffness

 CYSTS IN CONTAMINATED PORK

Phylum Rotifera
Characteristics

• Known as rotifers or wheel animals

• Transparent, free-swimming microscopic animal

• Freshwater & marine

• Have a ring of cilia around mouth that rotates like a wheel to bring in food

• Feed on unicellular algae, bacteria, & protozoa

• Have a muscular organ called the mastax behind the pharynx to chop food

• Nervous system composed of anterior ganglia & 2 long nerve cords

• Show cephalization (head end)

• Have 2 anterior, light-sensitive eyespots