Animals with dorso-ventrally flattened body belong to ?PlatyhelminthAschelminthAnnelidaArthropoda

Step 1: Break Down the Problem

The question asks which group of animals is characterized by a dorso-ventrally flattened body. We need to analyze each of the provided options to identify the correct one.

Step 2: Relevant Concepts

1. Platyhelminthes: This phylum includes flatworms that are indeed dorso-ventrally flattened.
2. Aschelminthes: This is an outdated term that previously encompassed several phyla of invertebrates; it does not specifically describe flattened bodies.
3. Annelida: This phylum includes segmented worms, which typically have cylindrical bodies, not flattened ones.
4. Arthropoda: This is a large group that includes insects, arachnids, and crustaceans, characterized by having segmented bodies and exoskeletons, generally not dorso-ventrally flattened.

Step 3: Analysis and Detail

• Platyhelminthes are known for their flat shape and include species like planarians and tapeworms. Their flattened bodies provide a large surface area relative to their volume, which is advantageous for respiration and nutrient absorption.
• The other options, including annelids and arthropods, showcase coelomic cavities with either segmented or jointed body structures without a significant dorso-ventral flattening.

Step 4: Verify and Summarize

Given the characteristics of each group under consideration, Platyhelminthes is the only phylum that specifically denotes dorso-ventral flattening in their body structure.

Final Answer

Animals with dorso-ventrally flattened bodies belong to Platyhelminthes.