Why is a starfish not fish?

Starfish are not fish because they belong to a different phylum. Fish belong to the phylum Chordata, which is characterised by the presence of a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail. Starfish, on the other hand, belong to the phylum Echinodermata, which is characterised by the presence of a water vascular system, a unique digestive system, and a lack of a vertebral column.