The most advanced organism in an evolutionary trend among the following is

Evolutionary advancement in animals is assessed by the complexity of body organisation: number of tissue layers, body symmetry, presence of a body cavity (coelom), organ systems, and segmentation.

Consider the organisms listed in order of increasing complexity:

• Paramecium is a unicellular protozoan. It has no tissues or organs, and all life functions occur within a single cell. This places it at the lowest level of organisation among the options.
• Hydra is a cnidarian with two cell layers (diploblastic) and radial symmetry. It has a simple body plan with a gastrovascular cavity but no true organs or body cavity.
• Planaria is a flatworm (platyhelminth) with three cell layers (triploblastic) and bilateral symmetry. It has simple organ systems, including a rudimentary nervous system with a brain-like ganglion, but it lacks a true body cavity (acoelomate).
• Earthworm is an annelid that is triploblastic, bilaterally symmetrical, and possesses a true coelom. It has well-developed organ systems, including a closed circulatory system, a segmented nervous system, nephridia for excretion, and metameric segmentation. This level of structural and functional complexity makes it the most advanced organism among the four.

Exam tip: When ranking organisms by evolutionary advancement, look for key milestones: unicellular to multicellular, diploblastic to triploblastic, radial to bilateral symmetry, acoelomate to coelomate, and the development of organ systems and segmentation.