Prospero calls caliban (an anagram for the word, cannibal) "earth" (376)

I've always felt this name directly correlates with the way Prospero treats Caliban, and that would be like dirt. He's treated like a slave, Prospero and Miranda revile his race. The reference to his name meaning cannibal is believed by a number of critics. There are also those who see it as a form of Romany..... which would make its meaning black, referring to the color of his skin.

I see Caliban's movements as defensive...... skulking. He would speak with difficulty, as he needed to learn the langusge.

Ariel is also slave to Prospero, but he serves him by choice...... he saved him from his imprisonment in a tree. Because Ariel always understands that freedom is forthcoming, servitude is a small price to pay. He is not a slave in the sense of a captive.