A B737 Captain noted an airspeed discrepancy at 80 KT but continued the takeoff. Once airborne he gave the aircraft to the First Officer because his airspeed and altimeter failed. An emergency was declared and the flight returned to land. The Captain's pitot tube was not torqued.

Narrative

We started the takeoff roll and the First Officer made the 80 knot callout. I was looking down the runway for takeoff and at the aircraft instruments. I noticed there was no movement on my airspeed indicator. I noticed the First Officer's airspeed was accelerating through 100 knots. I decided to proceed with the takeoff. I rotated; and at positive rate asked for the gear up. The Master Caution illuminated HYD and Flight Control. I looked at the hydraulic pressure and quantity; which were both normal. I recall the Master Caution lights were all normal. At approximately 500'; I asked the First Officer if his flight instruments were working properly and if he would feel comfortable taking control of the aircraft. He assumed aircraft control. At that point; we decided to return for landing and requested a return to the field from ATC. We declared an emergency; made an announcement to the passengers; and spoke to the Flight Attendants via the intercom to inform them of the situation. We landed safely and proceeded to the gate. We communicated the situation to Maintenance Control; Dispatch; and Operations. We made a logbook entry. [The problem was an] inoperative captain airspeed and altimeter. After our return; Maintenance said that the pitot tube had not been torqued. Maintenance should test the pitot tube after taking it off the aircraft.