Possible Clues:

• It's not right to have had fifty cut off the top of it
• Might not be taken after fifty in water
• Creature abandoned with head chopped off
• After half a century it would not be right to be so amphibious
• A new T might get left to the last
• So new a T gets left to the last
• Left three-quarters in water
• That gets left three-quarters in the water
• Small creature that undergoes metamorphosis
• A new T gets left at the end