A further primitive characteristic is that the skeleton consists of cartilage and not bone. Lampreys are similar in shape to eels, and have a series of uncovered round gill openings (known as gill pores) on the sides of the head. The river lamprey can be distinguished from other lampreys as it has two separate dorsal fins, and just a few teeth around it's mouth. It is bluish grey or green on the back and sides and the underside is white. The larval stages of different lampreys are very similar in appearance.