Life Science | Resources

Flatworms and Roundworms

Classified into two phylums: Platyhelminthes and Nematodes
Worms are invertebrates with soft bodies and bilateral symmetry.
Three tissue layers (outer, middle and inner) that are organized into organs and organ systems.
Free-living - able to find food and a habitat on their own.
Parasitic - dependent on other organisms for food and a place to live.

Flatworms Flatworms belong to the phylum Platyhelminthes and come in three varieties: planarians, flukes and tapeworms.

Planarians - (free-living) flatworm with triangle-shaped head, two eyespots, mouth, and a digestive tract connected to its mouth by a pharynx.

Feeds on small organisms and dead bodies of larger organisms.
They live under rocks, on plant material or in freshwater
Vary in length from 3 to 30 centimeters (1 to 15 inches)
Covered with fine, hairlike structures called cilia which are used for mobility.
Reproduce asexually by dividing in two (see page 345 figure 18)
Reproduce sexually by producing eggs and sperm (hermaphrodites)
Planarians can also regenerate lost body parts. Each part can actually replace its lost portion resulting in two complete organisms!

Flukes -(parasitic) flatworm that requires more than one host.

Most flukes reproduce sexually.
Males deposit sperm in the female worm and she in turn lays fertilized eggs inside a host. Eggs leave the host in its urine or feces.
Eggs that end up in water usually infect snails until they are grown and able to survive on their own. At this stage they can infect humans while standing in water.
They cause schistosomiasis and can effect blood, lungs, liver, eyes, and other organs of the host.

Tapeworms -(parasitic) flatworm that uses hooks and suckers to attach itself to the intestine of its host. (see page 346 figure 20)

These worms can infect dogs, cats, humans and other animals.
They have no mouth or digestive track but can absorb their food through their entire body while present in the intestine of its host.
Tapeworms grow by producing new segments directly behind it’s head structure.
Each segment has both male and female reproductive organs.
The segments are passed in the wastes of the host and can then be passed on to new hosts.

Limited fossil evidence -evolution of this animal is uncertain.
Could be the first animal to display bilateral symmetry with senses and nerves in the head region.
Possibly the first animal to have a third tissue layer that develops into organs.
Some scientists hypothesize that flatworms and cnidarians could have evolved from a common ancestor.

Roundworms

The most abundant multi-celled animal on Earth.
Estimated that there could be 500,000 different species of this animal.
These animals belong to the phylum Nematode and although some are parasitic, most species are free-living.
They live in soil, plants, animals, fresh water and salt water.
Their bodies are a tube within a tube, containing fluid in between.
Most nematodes have male and female genders that reproduce sexually.
They have two openings - a mouth and an anus (opening at the end of their digestive track through which wastes leave the body).
Many species of roundworms can be beneficial to humans. They can kill insects that do damage to crops and provide nutrients to soils (nitrogen) as they break down organic materials.