Corystes cassivelaunus (Brachyura, Corystidae), a crab which burrows in clean sublittoral sand, was investigated at several inshore locations around the Isle of Man. It usually buries itself so as to leave little or no external sign of its presence. Immature crabs remain buried by day throughout the year, as also do mature crabs except during the breeding season from April to June. It is probable that Corystes emerges nocturnally to forage over the surface of the sand. The morphology of the respiratory system, and the respiratory behavior patterns, are both modified on account of the burrowing habit. The food consists almost entirely of burrowing invertebrates, predominantly lamellibranchs, polychaetes and amphipods. There is a puberty-moult in each sex, which is marked by morphological changes, and following which the crab becomes sexually mature. There is no postpuberty moulting. The terminal anecdysis is maintained by production of moult inhibiting hormone from the eyestalk, and consequently eyestalk removal induces proecdysis and attempted ecdysis in mature crabs. Ovulation occurs in May to July, and the larvae hatch in March and April: thus incubation takes about ten months. Females breed for several successive years. Of the various organs studied all exhibited simple allometric growth except for the chelae of post-puberty males. It is postulated that this is due to their inability to achieve the very high size increments required at the puberty-moult in order to do this.