MARINE CRUSTACEANS. 199 
Il. PORTUNIDAE (SWIMMING CRABS). 
1. General. 
The family Portunidae are distinguished from the rest of the round-fronted (cyclometope) 
crabs by the adaptation of some of their legs for swimming, to which end these limbs are 
transformed into flattened paddles. The result is often to confer upon the crabs a power 
of darting at high speed through the water, which would hardly be credited by those who 
have not watched them. Corresponding to this mobility they have a thin flattened form 
of body, enabling them to pass sideways through the water, and a lightness gained at the 
expense of the protective cuticle. These peculiarities give the swimming crabs a strikingly 
different bodily form from the heavily-built, slow-moving Xanthids, which is moreover 
accompanied by an equally marked difference of habitat. The Xanthids are usually to be 
found on the reef or shore exposed to the full force of the breakers. In this position the 
lightly-built swimming crabs would be dashed to pieces against the rocks. Their proper 
haunt is a space of quiet waters, such as the lagoon of a coral atoll, and as these places 
are, in the tropics, generally bottomed with white or greyish coral sand, on which the 
crabs lie, and in which they often hide their bodies, they frequently mimic it by their 
pale greyish colour!, often in a manner as striking as that in which flat-fish resemble the 
shingly bottom they live on. 
At the same time the swimming crabs are by no means entirely confined to a bottom 
of coral sand even in the tropics. In deep water, where rocks are not associated with danger, 
they are found on every kind of bottom in about equal numbers, and here, if they hide, 
it must be under stones. They even occur, though not so very often, on the reef. But 
the individuals, found in this position, may possibly have strayed from the lagoon with the 
outgoing tide. Probably, when more is known about the lives of the species, it will be 
found that certain of them maintain their existence on the reef by sheltering under stones 
or in blocks of coral, where if anywhere they are always found, and that others—certainly 
the bulk of individuals—prefer the lagoon. In their habits these crabs are active and 
intelligent, escaping capture with cleverness. The lagoon forms usually keep close to the 
sand and do not rise more than a few feet into the water, but others swim as boldly and 
strongly as fish. 
The bodies of most Portunidae are adorned or protected with sharp thorns or teeth, 
and it is on such characters as the number and size of these and the shape of the lobed 
front that the species are generally distinguished, though in most cases enough is not known 
of their habits to make it possible to say whether, and if so how, these be of use to the 
animals. 
The family is highly variable and varietal and is probably undergoing rapid evolution 
in many directions. 
1 This colouring would also resemble, though not so closely, that of coral blocks or rubble. 
