Sentence:  ... [the dogs] sprang up, and, with a grand spraying of the crisp snow as they fleetly clambered up the steep side, they were with us in an incredibly short time, with pink tongues protruding, sides <WSD>bellowsing</WSD>, and sterns wagging. 
Meaning of the word marked with <WSD> tags:  To expand and contract like a bellows.