What if that worm-like ancestor instead used its other body opening, the mouth, to expel gametes? Reproductive function would be moved forward, away from other messy, but necessary functions. Of course, it would mean that a kiss would be fraught with major new connotations and consequences, but I think we could cope.
Those same pre-Cambrian ancestors are thought to have been literally brainless, and only later would they evolve a central nervous system. Initially, they had only a simple strip of nervous tissue, and later still, part of that strip grew larger and more complicated to form a proper brain. There's no particular necessity that the brain would form in the head--that's again a product of convenience, since more sensory organs were located in the front of the animal, and induced an enlargement of the local part of the nervous system to cope with their input.
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