Brachyura (i.e. short-tailed), the highest sub-order of the Decapoda, an order of Crustacea: the crab is the best known example. The main character of the group is that the tail (or strictly the abdomen) is very short and tucked up closely beneath the body, so that it is useless for swimming and cannot be seen from above; moreover, the body is wide instead of long, so that the nervous "ganglia" or centres are connected more closely together than in such long-tailed, elongated forms as the lobster. The majority of the group are marine, living on the shore; they rarely swim, but a few are enabled to do so by means of their flattened limbs; they can live for some time out of water, and some families live on land and only go to the sea at breeding time. Such e.g. are the Land Crab of the genus Gecarcinus, or the West Indian Gelasimus. The Brachyura are world-wide in distribution, and are first certainly known from the Cretaceous; but doubtful forms occur much earlier.