In the Crab that Played with the Sea what is scuttled off sideways an example of

Alliteration

Explanation:

The line "Scuttled off sideways and stepped into the sea" is an example of the literary device alliteration.

Alliteration is when identical or similar sounds appear consecutively at the beginning of words in a sentence or phrase.

Another example would be something like, "Go and gather the green leaves on the grass."

This is because the letter "g" and the corresponding sound appear at the beginning of most words in the sentence.