It is doubtful whether this animal ever inhabited a spiral shell or not in its past history; but there is no doubt that a number of peculiar crabs, which caused the older systematists much trouble, are Pagurids, derived from asymmetrical shell-haunting ancestors that have secondarily taken to a different mode of life, and lost, or partially lost those characteristics of ordinary Hermit-crabs which are associated with life in a spiral shell. These are the Lithodidae and the “Robber-crab,” Birgus latro, of tropical coral islands.
Although the Robber-crab and the Lithodidae bear a certain superficial resemblance to one another in that they lead a free existence, and have reacquired to a great extent their symmetry, yet it is clear that they have been independently derived from different groups of asymmetrical Hermit-crabs, and that their resemblance to one another is due to convergence.
Birgus latro (Fig. [119]), a gigantic crab, frequently over a foot in length, lives on land, and inhabits the coasts of coral islands in the Indian and Pacific Oceans where cocoa-nut trees grow. It feeds on the pulp of the cocoa-nut, which it extracts by hammering with its heavy chela on the “eye-hole” until room is made for the small chela to enter and extract the pulp. There is not the slightest doubt that the animal often ascends the cocoa-nut trees for the purpose of picking the nuts, a fact illustrated by a fine photograph by Dr. Andrews, exhibited in the Crustacean Gallery in the Natural History Departments of the British Museum. It uses the husk of the nut to line its burrow, and it is said to have the habit of putting its abdomen into the nut-shell for protection and carrying it about with it. Owing to its terrestrial mode of life, the branchial chamber is highly modified, being divided into two portions—a dorsal space, the lining of which is thrown into vascular ridges and folds for aerial respiration, and a lower portion where the rudimentary branchiae are situated. Although the Robber-crab lives ordinarily on land, it must be supposed that these branchiae are of some service; the young are hatched out as ordinary Zoaeas in the sea, and go through a pelagic existence before seeking the land. At the present time the Robber-crab is confined to the Pacific and the islands of the Indian Ocean, wherever the cocoa-nut grows. It seems, however, that its association with the cocoa-nut is a comparatively modern one. Mr. C. Hedley, of Sydney, who has had great experience of the Pacific Islands, informs me that the cocoa-nut is not, as is usually supposed, a native of these coral islands, but has been introduced, probably from Mexico, by the Polynesian mariners before the discovery of America by Columbus. Before the introduction of the cocoa-nut the Robber-crab must have fed on some other tree, possibly the Screw Pine, Pandanus.
The abdomen is full of oil, and is much prized as a delicacy by the natives, who tell many strange legends about the creature, but the philosopher may well find its structure more strange than fiction, and the consideration of its morphology an intellectual feast.
The appearance of the thorax and of the thoracic limbs is thoroughly Pagurid; the structure of the abdomen is highly peculiar.
From the ventral surface (Fig. [119]) we can see at the tip of the tail three small calcified plates, which represent the fifth and sixth terga and the telson. Attached to the sixth segment are the much reduced and rudimentary pleopods of that segment, and on the left hand side of the body in the female are three well-developed pleopods of the first, second, and third segments, which are used for carrying the eggs. The extraordinary asymmetry of these limbs compared with the complete symmetry of the abdomen itself is only explicable on the hypothesis that these animals are descended from Hermit-crabs which had lost the pleopods on the right side.
Fig. [119].—Birgus latro, ♀, × ⅙, ventral view. Ab, First pleopod; T, last pereiopod.
These appendages are entirely absent in the male. The ventral surface of the abdomen is curiously warty and rugose, and is very soft and pulpy owing to the immense store of oil which it contains.
