Flatworms (Acoelomate Animals with the Simplest Organ Systems)

• saclike gut (but none in tapeworms),
• bilateral symmetry,
• cephalization and
• acoelomate (no coelom).

Classes of Flatworms

Turbellarians or planarians (Turbellaria) possess a pharynx tube that extends to feed on whole small animals or suck tissues from dead or wounded prey. Planarians have protonephridia, with flame cells, to regulate body fluid volume and composition. Asexual reproduction is by fission of the body.

Flukes (Trematoda) are internal parasites that require a primary host (such as a human) for sexual reproduction and an intermediate host (such as a snail) for development.

This animation (Audio - Important) illustrates the life cycle of a trematode.

Tapeworms (Cestoda) are intestinal parasites of vertebrates, where they absorb predigested nutrients (they have no digestive tract). The body consists of an anterior scolex solely for attachment to the host's gut and a string of proglottids, each of which possesses both male and female organs.

This animation (Audio - Important) illustrates the life cycle of a tapeworm

REVIEW: Bilateral symmetry is characteristic of
Answer flatworms.