Doppler ultrasound is a type of ultrasound that depends on the Doppler Effect, a change in the wave's frequency, which can occur from the motion of a reflector, such a red blood cell. We experience the Doppler Effect when, for example, an ambulance approaches us, passes and then drives away.
Definition of PLACODERM. : any of a class (Placodermi) of extinct chiefly Devonian fishes with an armor of bony plates and primitive jaw structures.ADVERTISEMENT. New Latin Placodermi, ultimately from GK plak-, plax + derma skin â more at derm-. any of a class (Placodermi) of extinct chiefly Devonian fishes with an armor of bony plates and primitive jaw structures. ADVERTISEMENT.
Some Placoderms were up to 33 feet (10 meters) in length. Cartilaginous fish such as sharks and rays were common by the late Devonian. Devonian strata also contain the first fossil ammonites. By the mid-Devonian, the fossil record shows evidence that there were two new groups of fish that had true bones, teeth, swim bladders and gills.