Question

Which of these is the most suitable description about phylum Platyhelminthes?
A. A group of triploblastic, radially symmetrical, pseudocoelomate animals.
B. A group of triploblastic, bilaterally symmetrical, acoelomated animals.
C. A group of triploblastic, radially symmetrical, tricoelomate animals.
D. A group of triploblastic, bilaterally symmetrical, schizocoelomate animals.

Answer 1

Hint: Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented and soft-bodied invertebrates. Unlike other bilaterians, they do not have a body cavity. They also do not have any specialized circulatory and respiratory organs. This restricts them to having flattened shapes, allowing oxygen and nutrients to pass through their bodies by the method of diffusion.

Complete answer:
Option A. A group of triploblastic, radially symmetrical, pseudocoelomate animals: Platyhelminthes are triploblastic (they have three embryonic cell layers - ectoderm, mesoderm, and endoderm). But Platyhelminthes are not radially symmetrical, instead they are bilaterally symmetrical. Platyhelminthes are not pseudocoelomates (body cavity is not derived from mesoderm).
Hence option A is not correct.
Option B. A group of triploblastic, bilaterally symmetrical, acoelomate animals: Platyhelminthes are triploblastic (three embryonic cell layers – ectoderm, endoderm and mesoderm). Platyhelminthes are bilaterally symmetrical (opposite sides are similar). They are acoelomated animals which means they do not have a body cavity.
Hence option B is the correct answer.
Option C. A group of triploblastic, radially symmetrical, tricoelomate animals: Although Platyhelminthes are triploblastic, they are not radially symmetrical and tricoelomate animals.
Hence option C is not correct.
Option D. A group of triploblastic, bilaterally symmetrical, schizocoelomate animals: Platyhelminthes are triploblastic and bilaterally symmetrical. But they are not schizocoelomate animals.
Hence option D is not correct.

Hence, Option B: A group of triploblastic, bilaterally symmetrical, acoelomate animals is the correct answer.

Note:
Platyhelminthes are dorsoventrally flattened which means they have a well-developed ventral surface with mouth and gonopore. The body of Platyhelminthes are generally shaped as a worm but can vary from moderately elongated flattened to long ribbon-like and leaf-like shapes. Majority of Platyhelminthes are white and colorless. Ingested food gives some of the colors, while free-living forms are grey, brown-black, or brilliantly colored.