The best way to find food is to go out and hunt for it. But to hunt, you need to be able to move forward. And to move forward, you usually need a head with paired sense organs to know where you are going attached to a symmetrical body to get you there.
Scientists believe that a flatworm-like animal was the first creature to develop a head, brain, paired senses and a tail, the first to move forward and thus the first to hunt for food and mates. This breakthrough in bilateral design was enormous. It granted these creatures huge advantages over animals that could only sit and wait for food to float to them, e.g. sponges and polyps, or simply pulse aimlessly through the water in search of a meal, e.g. jellyfish.
Modern representatives of this pioneering ancient design are the flatworms -- a group of animals in the phylum Platyhelminthes (pronounced "pla-tee-HEL-minthes"). This unsung group includes such animals as freshwater planaria, psychedelic marine polyclads and parasitic tapeworms and flukes. Platyhelminthes may not be famous, but they certainly aren't rare. About 20,000 kinds of flatworms are alive and well today, in fresh and salt-water environments and other nice damp niches, like the insides of other animals.
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