Why are sponges are not in the plant kingdom?

Compare the two:

ARGUMENT 1: SPONGES ARE ANIMALS

an animal is: any of a kingdom (Animalia) of living things including many-celled organisms and often many of the single-celled ones (as protozoans) that typically differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials (as proteins), in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation

a sponge is: an elastic porous mass of interlacing horny fibers that forms the internal skeleton of various marine animals (phylum Porifera) and is able when wetted to absorb water (2) : a piece of sponge (as for scrubbing and cleaning) (3) : a porous rubber or cellulose product used similarly to a sponge b : any of a phylum (Porifera) of aquatic chiefly marine lower invertebrate animals that are essentially double-walled cell colonies that are permanently attached as adults.

According to this information... you can decide whether you think a sponge is an animal or not. I personally think that sponges are animals, not plants!