 Their eyes are large and covered entirely by a transparent secondary cornea . They are greenish at the base . A pair of prominent ridges ( olfactory crests ) are present on the ventral surface of the head at the rear edge of the eyes . The mouth area is supported by seven triangular flaps ( buccal lappets ) , each with 0 to 7 suckers of less than 0 @.@ 2 mm in diameter and 18 to 25 teeth . The strong , curved , and short beaks ( rostra ) are mostly black to dark brown . The radula has seven rows of teeth . 
