The correct option is B.
Explanation
Excretion and osmoregulation in the animal kingdom are performed by specialized structures that vary across different phyla based on their evolutionary complexity and habitat.
- Statement A (Arthropods): Incorrect. Arthropods possess diverse excretory organs depending on their class. Insects typically use Malpighian tubules, while crustaceans (like prawns) utilize antennal glands or green glands.
- Statement B (Platyhelminthes): Correct. Platyhelminthes (flatworms), such as Planaria, possess specialized excretory structures called flame cells or protonephridia. These cells are primarily responsible for osmoregulation (maintenance of ionic and fluid balance) and the removal of nitrogenous wastes.
- Statement C (Chordates): Incorrect. Chordates, particularly vertebrates, have evolved complex tubular organs called kidneys for excretion and osmoregulation. While some primitive chordates like Amphioxus possess protonephridia, the standard diagnostic feature for the group in general comparative anatomy is the kidney.
- Statement D (Annelids): Incorrect. Annelids, such as earthworms, utilize tubular structures called nephridia for the removal of metabolic wastes and to maintain fluid balance.
Flame cells (protonephridia) are the primitive excretory and osmoregulatory structures characteristic of Phylum Platyhelminthes.