2015م - 1444هـ
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Class Digenea (Trematoda) - The Flukes

A. The information on the Platyhelminthes provided in the previous section should be

reviewed, as it still applies.

B. Adult trematodes are parasites of vertebrates. All have complex life cycles requiring one

or more intermediate hosts. Most are hermaphroditic, many capable of self-fertilization.

C. Eggs shed by the adult worm within the vertebrate host pass outside to the environment,

and a larva (called a miracidium) may hatch and swim away or (depending on species)

the egg may have to be ingested by the next host.

D. Every species of trematode requires a certain species of molluscan (snail, clam, etc) as

an intermediate host. A complex series of generations occurs in the mollusk, resulting

ultimately in the liberation of large numbers of larvae known as cercariae.

E. To reach the vertebrate host, cercariae (depending on species):

1. Penetrate directly through skin and develop into adults.

2. Enter a second intermediate host, and wait to be ingested (they are now called

metacercariae).

3. Attach to vegetation, secrete a resistant cyst wall, and wait to be eaten (now

called metacercariae)