At first sight, most sponges take on the appearance and character of plants. Therefore, It is easy to forget that sponges are animals.
Sponges are multi-cellular animals that belong to the phylum Porifera ( meaning pore bearer). Sponges are amazing organisms that don't conform to traits that what we most often associate with animals such as the ability to move, and having centralized nervous, digestive or circulatory systems. However, they do have similar traits to other animals in that they are multicellular, heterotrophic (feed on other organisms), they have no cell walls and produce sperm cells for reproduction.
While most of the approximately 5,000–10,000 known species feed on bacteria and other food particles in the water, some host photosynthesizing micro-organisms as endosymbionts and these alliances often produce more food and oxygen than they consume. A few species of sponge that live in food-poor environments have become carnivores that prey mainly on small crustaceans.