Quite logically, animals without a backbone or any bone structure to speak of are called invertebrates ('in-' meaning 'not'). A whole host of sea creatures, such as jellyfish and squid, are in this category. For economy of energy (and perhaps for camouflage), their tissues are often translucent or even transparent. If you are lucky enough to observe one in its natural element(water), you can see vital organs in action. At other times some of these creatures are phosphorescent, giving off pulsing light like some underwater UFO. Moreover, fragile and often vividly-coloured coral which makes up reefs, an important ocean biotope, is also classified as "animal" (and not "plant" as often supposed.)
Hard-shell beetles, ladybugs and other such insects have their skeleton (the shell itself) on the outside instead. The same goes for crabs, oysters, mollusks, sea urchins and other types of seafood. The sea turtle and his "cousin" the tortoise (land turtle) also have shells to protect their internal organs instead of backbones.
Remember,too, all those invisible critters - microscopic organisms - which are either plants or animals, the amoeba being the smallest animal of all - a single cell.