This page on the history of invertebrates suggests that our common ancestors had bilateral symmetry, triploblastic and with hox genes.

Hox genes suggest that they both had a modular body plan of some sort. Triploblasty implies some complexity (the least complex triploblastic organism today is a flatworm).

I'd be very surprised if most recent common ancestor didn't have neurons similar to most neurons today, as I've had a hard time finding out the differences between the two. A basic introduction to nervous systems suggests they are very similar.