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

Platyhelminthes

The correct answer is Option (3) – Platyhelminthes

The correct answer is:

Platyhelminthes, commonly known as flatworms, have a digestive system with a single opening called a "gastrovascular cavity." This opening serves both as a mouth for ingesting food and as an anus for expelling waste. This type of digestive system is known as incomplete because it lacks a separate mouth and anus.

On the other hand:

• Arachnids: Arachnids, such as spiders and scorpions, have a complete digestive system with separate mouth and anus.
• Echinoderms: Echinoderms, like starfish and sea urchins, have a complete digestive system with separate openings for the mouth and anus.
• Arthropods: Arthropods, including insects and crustaceans, also have a complete digestive system with separate mouth and anus.