circle will bring him ridicule.  He fears more time "pinned and wriggling on the wall" where he will be scrutinized and critcized for his differing opinons.  So rather than face the contempt of his peers, he backs away from the question, much like a crab walks backwards, and remains silent.  Then he thinks to himself that no matter how grand his thoughts or intellect, he might as well be a crab (and note the synecdoche that makes him even less than a crab) since he has no courage to speak his mind. It is really very sad.

A professor of mine in grad school insisted that much of this poem should be read with humor—lines such “shall I wear my trousers rolled” and “do I dare eat a peach” provide good examples of this.  The humor would be based on irony, the speaker in the poem not as aware as his creator or we the audience of what a pathetic figure he is.  This line can be read in the same spirit:  on the one hand it is, as other responders have explained, very poetic, using devices such as synecdoche to create the image of the crab, which is the speaker, while the sibilants in “scuttling,” “floors,” “silent” and “seas” invite us to slide over, if not linger on, these words, all interrupted with the harsh “k” and “t” sounds of  “scuttling,” almost onomatopoetic in their effect.  But then the humor:  this guy wants to be a crab; he thinks he’s as low as a crab—his self image is really that bad?  Do we laugh at him or pity him?

You are not alone being confused by these lines.  Scholars have long debated the lines.  Some believe the reference is to a lobster, but most to a crab because the emphasis in the line in on "scuttling." The technique Eliot uses is synedoche, substituting the claws for the crab.  Eliot is known for his literary allusions. The poem draws heavily on a connection to Shakespeare's Hamlet. The crab imagery is most likely an allusion to a comment Hamlet makes to Polonius in Act II: "For you yourself, sir, should be as old as I am--if, like a crab, you could go backward."  Hamlet is contrasting his age and virility with Polonius old age.  Prufrock similarly seems to wish that he could have moved backward to relive the opportunities he missed. As with any Eliot line, one reading is not enough to understand the layers of meaning he includes.  The claws are also a likely continuation of the sexual imagery throughout the poem connected to fingers, hands, and arms.

The speaker of Eliot's poem is expressing his sense of isolation and loneliness, and his inability to fit into a world that he perceives as hostile.  Better for him, he thinks, to be a creature like a crab who is protected by a thick outer armor and better still, that he does not have to truly interact with anyone or care about what they think of him, or he of them.

This complex poem has had many interpretations, but this one line  is key to what Eliot is trying to express -- the facticity of human existence, the present phenomenon of being a human being -- how complex, how convoluted it is, how wrapped up in our awareness of time:

  Time for you and time for me, 
And time yet for a hundred indecisions,
And for a hundred visions and revisions,
Before the taking of a toast and tea.

The antithesis of human existence, of human consciousness, would be to be alive, but without self-consciousness, without the need or the "desire" to be aware, would be to be a sea creature of purely utilitarian powers, functions, needs, and capabilities.  His metaphorical equivalent of that condition is a creature with ragged claws, neither knowing or caring about its own existence or its "meaning" in the "ocean" of physicality.  This line, then counterbalances all the lines articulating human actions and decisions and complexities.  It is important here and in all analysis of a poem's "meaning" to remember Eliot's admonishment:  "A poem must not mean but be."