Final answer:
Fish are vertebrates with a backbone and advanced systems, while insects and jellyfish are invertebrates without backbones. Invertebrates can thrive in diverse habitats and constitute the majority of animal species on Earth. The distinction is based on anatomical features and evolutionary complexity.
Step-by-step explanation:
Fish are considered vertebrates because they possess a backbone, a characteristic of the subphylum Vertebrata, which falls under the phylum Chordata.
Fish evolved the vertebral column and became a dominant form in aquatic environments. They have complex organs and systems, such as a centralized nervous system with a brain, and highly developed sense organs. In contrast, insects are categorized as invertebrates because they lack a vertebral column.
Like jellyfish, insects belong to the myriad species that make up the invertebrates group, which comprises at least 95 percent of all animal species. Invertebrates can be found in diverse environments ranging from the depths of the ocean to the canopies of rainforests.
Every animal is identified as either an invertebrate or a vertebrate. This classification is based on the presence ('vertebrate') or absence ('invertebrate') of a vertebral column or backbone.
Vertebrates include classes such as fish (Class Agnatha, Class Chondrichthyes, and Class Osteichthyes), amphibians, reptiles, birds, and mammals.
All these groups have advanced anatomical and physiological features, including immune systems that comprise both innate and adaptive components. Invertebrates, while they have innate immune systems, lack the adaptive immune systems found in vertebrates, suggesting an evolutionary advancement in vertebrates.