The animal kingdom is comprised of millions of profoundly diverse creatures. Though animals have the same types of organs and body parts, they appear very different from animal to animal. Consider animal ears, for example. The ears of animals can be very large or very small and come in many different shapes.
More Information on Ears
In animals, the ear not only includes the fleshy structure on the outside of the animal (the outer ear), but also the structures inside of the animal (the middle and inner ear). Mammals are the only animals that have visible outer ears, which are made of cartilage. However, other animals, such as birds, reptiles, amphibians and fish, do not have outer ear structures; they only have inner ears. Invertebrate (spineless) animals, such as insects and worms, do not have outer or inner ears, but rather have other organs for hearing. Animal ears are not scientifically classified into "types" because, regardless of size and shape, all animal ears have the same parts.
Mammals have the greatest diversity, more than any other type of animal, in ear shape and size. Animal ears can be incredibly large (as in the case of the African elephant) or very tiny (such as gerbil ears). Animal ears can be fairly uniform throughout the genus, as with horses, or they can be exhaustively diverse, as with dog ears. The shape of the outer ear can enhance its functionality; for example, hares have long ears to increase their range of hearing to avoid predators. For some animals, ears are important for more than hearing, as is the case with elephants; their large ears have many blood vessels that regulate body temperature.
Birds, Reptiles and Fish
Many birds, reptiles and fish do not have visible outer ears, but that does not mean that these animals cannot hear. They have well-developed middle and inner ears, just like mammals. Birds are known for their melodic calls which they use to communicate with other birds, so, of course, they must have great hearing. Reptiles and amphibians will possess at most a visible, outer tympanic membrane, which covers the middle ear. Fish have inner ears, but generally have poor hearing. Also, though whales and dolphins are mammals, they do not have outer ears, but rather have structures similar to birds and reptiles.
Invertebrate animals, such as insects and sea creatures, do not have ears at all. Some, like sponges, are primitive, simple animals that do not require hearing, smell, or taste and therefore lack those sense organs. Insects hear with other organs, called tympanal organs, that are located in their abdomens and exoskeletons. Many insects have very sensitive hearing. Worms do not have ears and cannot hear at all. They have other organs that can sense vibrations in the ground.