Which of the following animals have a single opening in their digestive system that serves both as a mouth and an anus?

The correct answer is Platyhelminthes.

Key Points

• Phylum Platyhelminthes belongs to the kingdom Animalia.
• The organisms are also known as flatworms.
• These are acoelomates and they include many free-living and parasitic life forms.
• They have a single opening in their digestive system that serves both as a mouth and an anus.
• Platyhelminthes have the following important characteristics:
  • They are triploblastic, acoelomate, and bilaterally symmetrical.
  • They may be free-living or parasites.
  • The body has a soft covering with or without cilia.
  • Their body is dorsoventrally flattened without any segments and appears like a leaf.
  • They are devoid of the anus and circulatory system but have a mouth.
  • They respire by simple diffusion through the body surface.
  • They have an organ level of organization.
  • They do not have a digestive tract.
  • The space between the body wall and organs is filled with connective tissue parenchyma which helps in transporting the food material.
  • They are hermaphrodites, i.e., both male and female organs are present in the same body.
  • They reproduce sexually by fusion of gametes and asexually by regeneration by fission and regeneration. Fertilization is internal.
  • The life cycle is complicated with one or more larval stages.
  • They possess the quality of regeneration.
  • The flame cells help in excretion and osmoregulation.
  • The nervous system comprises the brain and two longitudinal nerve cords arranged in a ladder-like fashion.