Which structure is found in both arthropods and mollusks but not in sponges?

The presence of a nerve cord in both arthropods and mollusks but not in sponges is a significant feature that highlights the evolutionary complexity of these two groups compared to sponges. Arthropods and mollusks are part of the more advanced animal phyla characterized by their more developed nervous systems.

Arthropods possess a segmented body plan with specialized ganglia that form a central nervous system, including dorsal and ventral nerve cords. Mollusks also possess a nervous system that includes a nerve cord, often forming a ring around the esophagus. In contrast, sponges are part of the Porifera phylum, which lack true tissues and organs, including a nervous system. They rely on cellular processes rather than a nervous system to respond to their environment.

This distinction underscores the evolutionary advancements in the organization of body systems in bilaterally symmetric animals like arthropods and mollusks compared to the more primitive structure found in sponges.