Animals in the same phylum may have different nervous system structures. For example, octopuses have large, complex brains while clams have a simple network.

The answer is ring - like

Clams have the ganglia that form a ring around esophagus and that is why this network is called ring - like.

hope this answer correct (^^)..

rain is the best answer

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.