Sponges are extremely primitive invertebrate life so are classed under the Kingdom Animalia, they do not photosynthesise so are not plants. They are also very different to fungi and have an entirely different composition and life cycle. A sponge if passed through a sieve gradually reforms its original shape.ponges are extremely primitive invertebrate life so are classed under the Kingdom Animalia, they do not photosynthesise so are not plants. They are also very different to fungi and have an entirely different composition and life cycle. A sponge if passed through a sieve gradually reforms its original shape.