Figure 5.22 The life cycle ofSchistosomaspp. includes several species of water snails, which serve as secondary
hosts. The parasite is transmitted to humans through contact with contaminated water and takes up residence in the
veins of the digestive system. Eggs escape the host in the urine or feces and infect a snail to complete the life cycle.
The other medically important group of platyhelminths are commonly known astapeworms(cestodes) and are
segmented flatworms that may have suckers or hooks at thescolex(head region) (Figure 5.21). Tapeworms use
these suckers or hooks to attach to the wall of the small intestine. The body of the worm is made up of segments
calledproglottidsthat contain reproductive structures; these detach when the gametes are fertilized, releasing gravid
proglottids with eggs. Tapeworms often have an intermediate host that consumes the eggs, which then hatch into a
larval form called an oncosphere. The oncosphere migrates to a particular tissue or organ in the intermediate host,
where it forms cysticerci. After being eaten by the definitive host, the cysticerci develop into adult tapeworms in the
host's digestive system (Figure 5.23).Taenia saginata(the beef tapeworm) andT. solium(the pork tapeworm) enter
204 Chapter 5 | The Eukaryotes of Microbiology
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