Flatworm

Photograph by Tobias Bernhard, Photolibrary/Getty Images

Here, a pink, leaflike flatworm settles on the seafloor, but these animals typically favor rocky bottom cover and aren’t so easily spotted. Flatworms are simple animals. They have no circulatory systems, and because their bodies are so flat, oxygen simply penetrates directly into tissue without the benefit of a respiratory system. Flatworms' mouths take in nutrients and also expel undigested waste. However, these worms are also accomplished predators. When they catch snails, bivalves, or other prey they simply wrap their bodies around their victims and inject them with digestive enzymes.