Animals lacking excretory, respiratory and circulatory structures are

The correct option is Sponges.

To understand why this is the correct answer, let us break down the biological characteristics of the options provided:
1. Sponges (Phylum Porifera): These are primitive multicellular organisms that exhibit a cellular level of organization. They do not possess true tissues, organs, or organ systems. Instead, they rely on a unique water canal system for all their vital processes. Water flows through their body wall into a central cavity (spongocoel) and exits through the osculum. This water current facilitates:
- Excretion: Cellular waste (primarily ammonia) is removed directly from cells into the passing water current by simple diffusion.
- Respiration: Oxygen is absorbed directly by cells from the passing water, and carbon dioxide is diffused out.
- Circulation: There is no bloodstream or heart. The movement of water through the canal system distributes nutrients and disposes of wastes.
Therefore, sponges completely lack specialized excretory, respiratory, and circulatory organs or structures.

2. Threadworms (Phylum Nematoda): These organisms possess a pseudocoelom (false body cavity) and have a simple excretory system (using renette cells or excretory canals) to remove wastes. While they lack specialized circulatory and respiratory systems, they do have a primitive excretory structure, making this option incorrect.

3. Tapeworms and Liver flukes (Phylum Platyhelminthes): These flatworms are triploblastic and possess specialized excretory structures called flame cells (protonephridia) for osmoregulation and excretion. Thus, they do not lack excretory structures, making these options incorrect.

Consequently, Sponges are the only organisms among the choices that entirely lack all three types of structures: excretory, respiratory, and circulatory systems.