These animals have bilateral symmetry. There are acoelomate with three tissue layers which give rise to specialized organs. There are three classes of flatworm.
1. Class turbellaria: Planaria
a. Digestive system
They have a mouth, pharynx, and a branched gastrovacular cavity. There is one opening, but two-way traffic. Planarians are carnivorous. Through muscular contractions, the planaria sucks in small pieces of meat.
b. Respiration
Gas exchange is accomplished by diffusion through the skin.
a. Excretion
Water balance is regulated by flame cells that sweep excess water and nitrogenous wastes (ammonia) into tubules and out of the body through pores.
b. Nervous System
There are two main nerves with many side branches. The nerve centers (ganglia) are located in the head (encephalization), and support sensory structures such as eye spots and touch sensitive cells.
c. Locomotion
Epithelial cells are ciliated. The ventral epithelial cells secrete mucous so that planarians provide their own slime trail.
d. Sexual Reproduction
All are hermaphroditic. They have both ovaries and testes. The mating pair exchange sperm, and fertilized eggs are released through the genital pore.
e. Miscellaneous
They can reproduce asexually through regeneration, in which half of an organism regenerates the missing half.
2. Class Cestoda: Tapeworm
The head (scolex) is equipped with a ring of hooks and suckers for attachment to intestine. Te body is divided into proglottids that contain ovaries, testes, and excretory tubules. The ‘ripe’ proglottids (filled with eggs) break off and come out with the feces. The life cycle begins when the primary host eats the eggs and becomes infected. The eggs hatch into larvae, which burrow out and travel to muscle tissue and encyst (bladderworm). The secondary host may become infected by eating infected meat (muscle) and larvae grow to an adult form in the intestine. Adult tapeworms can be up to 20 meters in length.
3. Class Trematoda: Flukes
These are parasitic, but have a digestive system. The mouth is surrounded by a sucker, which pumps in nutrients from the host’s intestine or liver. Flukes are hermaphroditic and the fertilized eggs pass out of the host with feces. Life cycle: larva is eaten by a snail (primary host). The larva develops into a sporocyst. The sporocyst leaves the snail and changes into a third larval form in the water. The larva burrows into fish muscle (secondary host). The infected fish is eaten by the final host, which the larva grows to the adult stage.