Category: Invertebrate Unit

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