Definition of "openmouthed"

The edges of these bones are fringed with hairy fibres, through which the Right Whale strains the water, and in whose intricacies he retains the small fish, when openmouthed he goes through the seas of brit in feeding time.

1851 November 14, Herman Melville, Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley