The heart is a vital organ found in all animals (except for a few very simple ones). It's a muscular pump that circulates blood throughout the body, delivering oxygen and nutrients to cells and removing waste products.

Here are some interesting facts about the heart as an animal structure:

* Size and shape: Hearts vary greatly in size and shape depending on the animal. For example, a hummingbird's heart beats over 1,200 times per minute, while a whale's heart might weigh over 400 pounds!

* Chambers: The number of chambers in a heart also varies depending on the species. Humans have a four-chambered heart, while fish only have two.

* Evolution: The evolution of the heart is a fascinating story, showcasing how complex organ systems have developed over millions of years.

Let me know if you'd like to learn more about the heart or any other animal structure!