Consider the semicolon in bold, which is used to connect two sentences. If it is not the best choice, replace it with a different punctuation mark. Bootlace worms are less than an inch thick but can reach an astonishing length; they have been measured at one hundred feet, but they can grow to nearly twice that length.

The semicolon in the sentence is used correctly to connect two closely related independent clauses. However, if you want to replace it with a different punctuation mark, you could use a period to separate the sentences more distinctly.<br /><br />Here is the revised sentence with a period:<br /><br />Bootlace worms are less than an inch thick but can reach an astonishing length. They have been measured at one hundred feet, but they can grow to nearly twice that length.