A demitasse is a small cup, usually for coffee.  In contrast to the preceding metaphor "chamber," demitasse emphasizes the small size of the crab (and also, that, like coffee, it is something to be consumed).  Yet where "chamber" begins with a sense of strength and is then undercut, "demitasse" works in a somewhat opposite fashion.  Its sense of smallness is modfied by the grandness of the word.  Demitasse suggests a world of luxury and elegance; it is the kind of cup one might enjoy in the chamber of a grand hotel.  Hence the pairing of "chamber" and "demitasse" beautifully preserves the poem's ambiguous representation of the crab as both great and small.  "Chamber" and "demitasse" are not only opposite words, but each word also carries within it an opposite sense that it is in turn opposite to the opposite word's internally opposite sense.  (A grand and elegant effect through a couple small words!).  The elegance of a demitasse cup--something an artist in a grand hotel, or a cafe, might sip--also associates the crab with art and cultural production.