Which of the following organisms is an acoelomate? . earthworm . insect . flatworm . clam .

The correct answer is option C, Flatworm

Reason -

Acoelomate are group of organisms that do not have body cavity or coelem. They do not have a fluid filled cavity and have a triploblastic body plan.  

Since Flatfworms lacks the body cavity and so they are considered as Acoelomate. They are invertebrate organisms having unsegmented body and also possesses bilateral symmetry. They are parasite who live inside other animals and feed on their digested food. They possess a simple digestive tract, nervous system, and excretory system.

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