A B737-700 First Officer reported the Captain's windshield cracked and delaminated climbing out of FL180. An emergency was declared a safe return to the departure airport was accomplished.

Narrative

I was pilot flying on departure. Passing 18;000 FT; the Captain's side of the windshield cracked suddenly accompanied by a snapping noise. The Captain took control of the aircraft; stopped the climb; directed me to request an immediate descent from Center with a turn back to [departure airport]; and to declare an emergency. We received an immediate descent and vector from the Center direct [departure airport]. The spider cracks and delamination continued to expand as we made the turn and there was significant electrical arcing on the windshield. We shut off the windshield heat which was effective in stopping the arcing. Appropriate emergency and normal checklists were accomplished. Flight attendants and passengers were notified of the event and that we were returning to [departure airport]. Operations planning; FMC reprogramming; and avionics setup for the visual approach were accomplished. The controlling agencies were helpful in making room for the descent since we were so close to the field and had slowed the aircraft down. Eventually we passed souls on board and fuel once the emergency procedures were accomplished. Stable approach and landing accomplished by the Captain followed with uneventful taxi to the gate. A walk-around was accomplished and nothing was out of the ordinary; other than the windshield.