Here are the correct word forms needed for the gaps in your passage:

Penguins are (0) flightless birds which live south of the Equator. As their legs are short, they (56) **usually** stand upright and walk when they are on land. When they find it (57) **necessary** to travel at greater speed, they often drop onto their stomachs and slide along. But it is at sea, (58) **especially** when diving, that penguins really move fast, the (59) **weight** of their streamlined bodies allowing them to reach a (60) **depth** of up to 265 metres in some cases. The sixteen species of penguin tend to look rather (61) **similar** with black or dark blue backs and white fronts. But (62) **variations** in size and head patterns allow them to be (63) **identified**. The fact that a number of species spend their whole lives in Antarctica where there is little (64) **protection** from the world's least (65) **welcoming** weather conditions, makes their continued survival one of the wonders of nature.