In what ways arc planarians more advanced than a jellyfish? How are they more primitive than a clam? Solution Planarians are the advanced organisms than jellyfishes: Planarians also known as Platyhelminthes or Flatforms. Many characteristic features of planarians are considered to be more advanced than Jellyfishes. The features include: Presence of Bilateral symmetry, Distinct brain, Specialized digestive & excretory system etc. Planarians contain chemoreceptors that are used to find food which is again a most advanced feature than Jellywish. Presence of specialised nervous system which contains inter connected nerve cords not nerve cells (present incase of Jellyfishes). Planarians are the primitive organisms than clams: Absence of Body cavity containing internal organs & Specialized circulatory and respiratory organs shows that these flatworms are more primitive than clam.The lack of circulatory and respiratory organs limits platyhelminths to sizes and shapes that enable oxygen to reach and carbon dioxide to leave all parts of their bodies by simple diffusion. Well organised nervous system with 3 pairs of ganglia is the most advanced feature of clams when compared to planarians..