Figure 11.14: Tapeworms attach to the intestinal wall with a head region that has hooks
and suckers. ( 12 )
organs. A roundworm’scomplete digestive tract, meaning the digestive tract includes
both a mouth and anus, includes a large digestive organ known as the gut. Roundworms
also have a simple nervous system with a primitive brain. Both their anterior and posterior
ends have specialized sensory nerves. These nerves are connected with a ventral and dorsal
nerve cord that run the length of the body.
Roundworms can be free-living organisms, but they are probably best known for their role
as significant plant and animal parasites. The heartworms, which cause serious disease in
dogs while living in the heart and blood vessels, are a type of roundworm. Round worms can
also cause disease in humans. Elephantiasis, a disease characterized by the extreme swelling
of the limbs, is caused by infection with a type of roundworm (Figure11.16).
Segmented Worms
The phylum Annelida includes the segmented worms such as the common earthworm,
some marine worms, and leeches (Figures11.17and11.18). These worms are known as the
segmented worms because their bodies aresegmented, or separated into repeating units.
Most segmented worms feed on dead organic matter, while leeches can live in freshwater
and suck blood from host organisms. Leeches can also be used medicinally to remove excess
blood.
Segmented worms have a well-developed body cavity filled with fluid, which serves as a
hydroskeleton,a supportive structure that aids in muscle contraction. Segmented worms
also tend to have organ systems that are more developed than the roundworms or flat-
worms. Earthworms, for example, have a complete digestive tract including an esophagus