Question

Which options are in contrast to Annelids the Platyhelminthes show.
A) Absence of body cavity
B) Presence of pseudocoel
C) Radial symmetry
D) Bilateral Symmetry

Answer 1

Platyhelminthes are worms that are bilaterally symmetrical, triploblastic and acoelomate. Whereas, Annelids are bilaterally symmetrical, triploblastic and coelomate organisms.

Complete answer: Flatworms are mostly endoparasites found in animals including human beings. They possess specialized structures called hooks and suckers. Some of them absorb nutrients from the host directly through their body surface. They have specialized cells called flame cells that help in osmoregulation and excretion. Sexes are not separate. The mode of fertilization is internal and development in these organisms occur through larval stages. In Annelida, a closed circulatory system is present. Nephridia helps in osmoregulation and excretion. Their neural system consists of a paired ganglia connected by lateral nerves to a double ventral nerve cord. Platyhelminthes have an acoelomate body cavity, i.e., they have no body cavity. The pseudocelom is a body cavity and Annelida has a true coelomate body cavity, whereas Platyhelminthes have an acoelomate body cavity. The true coelomate is derived from the pseudocoel, but acoelomate is not derived from it. Organisms belonging to both the Phyla have bilateral symmetry.
Hence, A is the correct answer.

Note: Flatworms or Platyhelminthes have no body cavity other than the gut and they lack an anus, while Annelids have a full filled cavity between the body wall and the gut. Their body surface is distinctly marked into segments or metameres.