Which of the following groups does not have a true nervous system?

Squid do not possess a true nervous system in the same way that vertebrates like fish, turtles, and sharks do. Instead, they have a more decentralized nervous system, which is characteristic of many invertebrate species. Squid are cephalopods and possess a complex nervous system that, while advanced for an invertebrate, lacks the definitive structure of a true nervous system found in vertebrates. Instead of a centralized brain and spinal cord, squid have a large, centralized brain, but it is surrounded by a nervous system that extends throughout their bodies. This provides them with significant control and rapid reflexes, but it does not align with the traditional classification of a "true" nervous system as seen in the vertebrate examples provided.