From the evolutionary point of view which one among the following is the most primitive animal in comparison to the other three ?

From an evolutionary perspective, the Roundworm (Phylum Nematoda) is the most primitive among the options provided. In the phylogenetic tree of invertebrates, phyla such as Nematoda occupy a lower position and are classified as 'lower invertebrates' due to their simpler body organization [t2]. In contrast, the Snail (Phylum Mollusca), Prawn (Phylum Arthropoda), and Spider (Phylum Arthropoda) are categorized as 'higher invertebrates' [t2]. These higher groups possess more complex organ systems and body plans, such as the segmented bodies and jointed appendages seen in arthropods [c2][c5]. While all four belong to the Protostomia clade, Nematodes lack the advanced coelomic structures and specialized circulatory systems found in Mollusks and Arthropods [t4][t9]. Therefore, based on the complexity of body organization and their relative positions on the phylogenetic tree, the roundworm represents a more ancestral or primitive lineage [t2][t6].

Sources

1. [1] https://www.uou.ac.in/sites/default/files/slm/MSCZO-501.pdf
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Annelide > p. 155
3. [3] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arachnids > p. 156