There are three complete tissue layers. b. Flatworms may be either free living or parasitic. c. Flatworms have a sac body plan, with only a mouth and no anus. d. Parasitic flatworms have a well-developed head with eyespots and nerves concentrated into a brain. e. Most planaria are found in marine environments, but some dwell in freshwater and moist terrestrial environments.

Complete question:

Which of the following statements is NOT correct about the flatworms?

A. There are three complete tissue layers.

B. Flatworms may be either free-living or parasitic.

C. Flatworms have a sac body plan, with only a mouth and no anus.

D. Parasitic flatworms have a well-developed head with eyespots and nerves concentrated into a brain.

E. Most planaria are found in marine environments, but some dwell in freshwater and moist terrestrial environments.

D. Parasitic flatworms have a well-developed head with eyespots and nerves concentrated into a brain.

Explanation:

Flatworms are inferior organisms, meaning that they do not have nerves concentrated in a brain. Their nervous system is, in general, composed of nerve cords forming a net. The arrangement depends on each of the groups. They have one or more pairs of primitive eyes, which function is to find the light. These eyes cup-shaped and are composed of pigmentary cells.

Flatworms have bilateral symmetry. Their body is plane dorsoventrally and can be divided into two specular halves by the sagittal plane. This plane crosses through the body from the anterior extreme to the posterior extreme. The frontal plane divides the body into two different halves, the ventral one with the mouth and the dorsal one.

These organisms only have one opening in their bodies directed to the digestive system, which is the mouth.

Flatworms are aquatic and, most of them are marine animals, although there can be a few groups that inhabit freshwater. Some species might even be terrestrial. Most of them are free-living organisms, but there are also some parasitic forms.