placozoans tissues muscles nerve tissues digestive sponges flatworms
With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues. These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
