( This ( ( is ( ( the ( coloured circle ) ) ( which ( ( surrounds ( the pupil ) ) ( at ( ( the front ) ( of ( the eye ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (NP (NP (DT the) (VBN coloured) (NN circle)) (SBAR (WHNP (WDT which)) (S (VP (VBZ surrounds) (NP (DT the) (NN pupil)) (PP (IN at) (NP (NP (DT the) (NN front)) (PP (IN of) (NP (DT the) (NN eye)))))))))) (. .)))
This is the coloured circle which surrounds the pupil at the front of the eye.
(ROOT (S (NP (NP (DT The) (NN opening)) (PP (IN in) (NP (NP (DT the) (NN front)) (PP (IN of) (NP (DT the) (NN eye)))))) (VP (VBZ is) (VP (VBN called) (NP (NN pupil)))) (. .)))