The sponges (phylum Porifera) are rough masses of flesh. They're not built on skeletons, though they grow hard skeletal bits inside themselves. They are simple animals, their cells only a step or two removed from their evolutionary predecessors—microscopic protists that formed colonies. Their individual cells take on specialized jobs like digestion, but they change jobs when necessary. The cells do not form discrete tissues, which are masses meant for very specific tasks. The lack of tissues sets them apart from most other animals.
1. Some sponges excrete toxins to discourage predators. People who keep them in aquariums sometimes get a rash simply by putting a hand into the water. One such species is the fire sponge (pictured).
2. Some sponges poison the adventurous diner. They can paralyze, and possibly kill, a human.
More information on sponges.