Here's why:
* Echinoderms are a group of marine animals that includes starfish, sea urchins, and sea cucumbers. They are characterized by their radial symmetry and a unique water vascular system. Seahorses do not have these features.
* Cnidarians are a group of aquatic animals that includes jellyfish, corals, and sea anemones. They are characterized by stinging cells called nematocysts. Seahorses do not have stinging cells.
Seahorses belong to the class Actinopterygii, which is part of the phylum Chordata. This means they are more closely related to fish like tuna and salmon than to echinoderms or cnidarians.