Why are sponges considered animals?
The classification of living things has only two sub groups, animals and plants. Animals are distinguished from plants in several ways. They have cells without cellulose walls. They lack chlorophyll and the capacity for photosynthesis. They require more complex food materials like proteins. They are often more complex organisms (but not necessarily so). They usually have the capacity for spontaneous movement and can respond rapidly to stimulation. By these definitions, protozoa and sponges are classified as animals – even though they lack some of the basic things (eg organs) we associate with higher life forms. So the short answer is because they are not plants, they must be animals.
