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Flatworms
I. What Is a Flatworm?
A) soft
B) flattened
C) has tissues and internal organ
systems
D) simplest animal to have three
embryonic germ layers (endoderm,
mesoderm, ectoderm)
E) bilateral symmetry
F) cephalization
G) acoelomates, which means—no
coelom.
1) coelom is fluid-filled body
cavity, lined with tissue derived
from mesoderm
2) digestive cavity is the only
body cavity in a flatworm
II. Form and Function in Flatworms
A) thin and most of cells are close
to external environment
1) rely on diffusion for
respiration, excretion, and
circulation
B) Free-living flatworms have organ
systems for digestion, excretion,
response, and reproduction.
BUT
C) Parasitic species are simpler in
structure—usually no organs for
digestion, excretion & response
III. Feeding
A) single opening through which
both food and wastes pass (both in
and out—no digestive tract)
1) Near mouth is muscular tube
called a pharynx
2) extend pharynx out of the
mouth then pumps food into the
digestive cavity
BUT
B) parasitic worms do not need a
complex digestive system
C) obtain nutrients from digested
foods from host