Zoology 10th Edition by Stephen Miller,John Harley

Exercise 6

Mesodermally derived parenchyma may provide support, nutrient storage, motility, reserves of regenerative cells, transport, and oxygen storage. Turbellarians are free-living flatworms. Trematodes are the flukes. As adults, they are endoparasites of vertebrates. The different stages in the life cycle of a typical fluke include eggs, miracidia, sporocysts, cercariae, and adults. Monogenetic flukes are so named because they have only one (mono) generation in their life cycle. They are ectoparasites of fish. The Cestoidea are the tapeworms. As adults, they are also endoparasites of vertebrates. The absence of a mouth and digestive tract and, the presence of repeating proglottids, characterize members of this class. A similar body wall in Trematoda and Cestoidea represents an evolutionary adaptation to the parasitic way of life.
Why is it important for an adult trematode or cestode to produce thousands of eggs?