Response

phylum Platyhelminthes
Nervous systems of free-living flatworms allow them to gather information from their environment. Free-living flatworms have well-developed nervous system than cnidarians and sponges. Flatworms are capable of simple learning and can sense taste, smell, touch, and light. Free-living flatworms have a brain, a control center of a simple nervous system, located in the anterior end. Long nerve cords stretches throughout the body from the anterior end to posterior end. Many flatworms have light sensitive organs called ocelli, or eyespots. Flatworms also have cells that are sensitive to chemicals found in food, and other cells that tell the worm which way the water around them is flowing.
Parasitic flatworms do not have much of nervous systems because they live in their host and absorb food from intestinal walls.

★The eyespots do not help flatworms to see objects; however, they detect whether the animal is in light or in darkness.

phylum Nematoda
Roundworms have simple nervous systems. They contain ganglia, or groups of nerve cells, in the head region. Roundworms have sensitive organs that detect chemicals given off by hosts. Nerves stretch from the ganglia at the head region to the posterior part of the body. These nerves transmit sensory information and control movement.

Reproduction

phylum Platyhelminthes
Most flatworms are hermaphrodites, which mean that they have both male and female organs.
Asexual
Free-living flatworms reproduce asexually by fission. Flatworms have great ability to regenerate their damaged or lost parts of bodies. Sometimes, worms simply separate into pieces and each piece will grow into new worms. Parasitic flatworms do not reproduce asexually because their life cycle is more complicated than free-living flatworms.

Sexual
During sexual reproduction, two worms join and one worm deliver sperm to the other worm while receiving sperm from its partner at the same time. With in a few weeks, eggs hatch in small clusters.

Some roundworms are hermaphrodites as well.

phylum Nematoda
Sexual
Unlike flatworms, roundworms have separate males and females. Fertilization occurs inside the body of the female.

Movement

phylum Platyhelminthes
Free-living flatworms use cilia on their epidermal cells to help them glide through the water and over the bottom. Muscle cells are controlled by the nervous system which can help flatworms to twist and turn in their environment.

phylum Nematoda
The muscles help roundworms to move like a snake and their bodies can contract and push their way through the soil by thrashing around.

Respiration/Circulation/Excretion

phylum Platyhelminthes
Flatworms lack circulatory or respiratory systems because they are so flat. Therefore, Oxygen and nutrients are transported to other tissue cells by diffusion. And flatworms also get rid of carbon dioxide and metabolic wastes by diffusion.

Freshwater flatworms such as planarians have structures called flame cells that help them get rid of water as they form a network that empties through tiny pores in the animals’ skin.

phylum Nematoda
Roundworms also lack respiratory and circulatory system. They transport nutrients, collect oxygen and get rid of metabolic waste by diffusion through their bodies.

Digestion/Feeding

phylum Platyhelminthes
Flat worms are aquatic and free-living organisms. Some flatworms may be carnivores that feed on tiny aquatic animals and others maybe scavengers that feed on dead animals. Free-living flatworms have a gastro vascular cavity with one opening at the end of a muscular tube called a pharynx. Once pharynx sucks food into the gastro vascular cavity, intestine is formed with many branches along the body of the flatworm. In the intestines, enzymes break down food into small particles and these particles are taken inside the cells of the intestinal wall. Because intestine branches are spread along the body of the flatworm, tiny food particles diffuse to other body tissues. Unnecessary materials that cannot be digested exit through the mouth like cnidarians.

Parasitic flatworms feed on blood, tissue fluids, or pieces of cells inside the body of their host. Some of parasitic flatworms have pharynx to suck food into intestinal sacs where food is digested.

★ Tapeworms, which lives within the intestines of their host, do not have digestive tracts. They have suckers with absorb the food that has already been broken down by the host’s digestive enzymes.

phylum Nematoda
Roundworms have a long tube-shaped digestive tract with opening at the both ends. Food enters through the mouth and leaves through an opening called anus.
Free-living roundworms are carnivores that eat tiny animals. Some digest bacteria and fungi that break down dead organisms.
Some parasitic roundworms attach to the roots of green plants and absorb juices which damage many croups around the world and others live in plant tissues to collect nutrients.

What are unsegmented worms?

Unsegmented worms are the simplest animals with bilateral symmetry to have a digestive system. Most of the members exhibit enough cephalization, or development of the anterior end. Unsegmented worms can be separated into two major phyla: phylum Platyhelminthes and phylum Nematoda. Free-living and parastic flat worms belong to phylum Platyhelminthes and round worm belong to phylum Nematoda. (Also phylum Rotifera)

Characteristics
Phylum Platyhelminthes is also known as flatworms because bodies are flatten dorsal-ventrally. Flatworm is acoelomate and contains three body layers: outer Ectoderm, middle Mesoderm, and inner Endoderm