Keeping your anus immediately behind your head makes it advantageous to grow larger at the front so that you can fit in a larger gut between your mouth and your anus. Conversely though, your ‘tail’ can become pure muscle, enabling you to propel yourself along more quickly.

This means you can actually become quite large, muscular and strong, and specialise in hunting high value prey, or stay relatively small, capable of eating lots and lots of low value food.