Flatworms
Worms in the phylumPlatyhelminthesare called flatworms because they have flattened
bodies. Some species of flatworms are free live-living organisms that feed on small organisms
and decaying matter. These types of flatworms include marine flatworms and fresh-water
flatworms such asDugesia(Figures11.11and11.12). Other types of flatworms are parasitic
and rely on a host organism for energy. For example,tapewormshave a modified head
region with tiny hooks that help the worm attach to the intestines of a animal host (Figures
11.13and11.14).
Figure 11.11:Dugesiais a type of flatworm with a head region and eyespots. ( 14 )Flatworms have no true body cavity and anincomplete digestive system,meaning that
the digestive tract has only one opening. Flatworms do not have a respiratory system, so gas
exchange occurs at surface of their bodies. Furthermore, there are no blood vessels or true
circulatory system in the flatworms. Theirgastrovascular cavityserves for both digestion
and to distribute nutrients. The flatworms do have a ladder-like nervous system with a
distinct head region with a concentration of nerve cells and sensory organs such as eyespots
(Figure11.11). The development of a head region, calledcephalization,arose with the
development of bilateral symmetry in animals.
Roundworms
ThephylumNematodaincludesnon-segmentedwormsknownasnematodesorroundworms
(Figure11.15). Unlike the flatworms, the roundworms have a body cavity with internal