THE OYSTER-CRAB

was no longer especially necessary.

101

That the crab may

be at times useful to the mollusc seems after all not so very improbable, for at the approach of an enemy so nervous a creature as a crab would no doubt begin to scuttle about and in this way communicate its terror to its more apathetic companion, which would then naturally close its doors against the danger. Dr. H. Woodward has recently recorded a remarkable instance of a Pinnotheres found encysted in a pearl-like formation of the pearloyster, Meleagrina margaritifera.

Pinnotheres veterum, Bosc, and Pinnotheres pisum (Linn.) are common European and British species. Giard and Bonnier suppose that under the latter name several distinct species have been confounded. Its Zoëa, long ago studied and drawn by Mr. Vaughan Thompson, is a singular-looking microscopic object. Among the names of other species some which indicate the animal's residence may be mentioned, as Pinnotheres ascidicola, Hesse, from the coast of France, the Japanese Pinnotheres pholudis, de Haan, and Pinnotheres lithodomi, Smith, from the Pearl Islands and Lower California. A similar indication is given in the generic name, Holothuriaphilus, Nauck.

In The Civil and Natural History of Jamaica,' when speaking of a Pinnotheres, which he calls the Oyster-Crab, Patrick Browne says: "This little species is generally found with the Mangrove oysters, in their shells, where they always live in plenty, and spawn at the regular seasons; and such as eat the oysters, do not think them a bit the worse for being accompanied with some of these crabs, which they swallow with the fish. They are very small and tender, and nearly of the same length and breadth, seldom exceeding a quarter of an inch either way.'

Hymenosoma, Desmarest, 1823, was established under a name invented by Leach, and signifying a membranaceous body. This is a character in which many members of the family partake. Hymenosoma orbiculare, Latreille and Desmarest, is a South African species. Halicarcinus, White, 1846, is closely allied to Hymenosoma, but courts

attention for the sake of the species Halicarcinus planatus (Fabricius), which is widely distributed over the Antarctic or Austral region, being the only Brachyurous Decapod, it is said, proper to that wide area of distribution. Mr. Haswell considers that the Elamene Mathæi of MilneEdwards is probably the young male of this species, and that it is quite distinct from the original Hymenosoma Mathæi of Desmarest.

Scopimera, de Haan, 1833, was established for the single species, Scopimera globosa, in which the arm of the chelipeds and the corresponding fourth joint (the socalled merus) of the hinder legs has the outer margin cartilaginous instead of crustaceous, with a transparent membrane in the flat part. This peculiarity explains de Haan's choice of a generic name, which means 'thighs with windows in them.' From the resemblance to the head of a drum these membranous pieces have been called ' tympana.'

Dotilla, Stimpson, was substituted for Doto, de Haan, 1833, a pre-occupied name. In this genus Dotilla fenestrata, Hilgendorf, from the East Coast of Africa, has the windows or tympana also in the sternum. Dotilla brevitarsis, de Man, is from the Mergui archipelago. Dr. de Man makes Scopimera a synonym of Dotilla, but, if the two genera are united, Scopimera as the older name must take precedence.

Hexupus, de Haan, 1835, is entirely devoid of the last pair of walking legs, so that instead of decapods these crabs have become octopods, and if the chelipeds are excluded and only the walking legs counted they may be regarded as hexapods, or six-legged crabs, and to this view the name of the genus refers.

Thaumastoplax, Miers, 1881, it is said, 'is closely allied in all its characters and particularly in wanting the fifth pair of thoracic legs, to the genera Hexapus, de Haan, and Amorphopus, Bell, but is distinguished from the former by the much greater development of the second ambulatory legs and the structure of the outer [third] maxillipedes, whose merus [fourth] joint is elongated and

TRANSITIONAL FORMS

103

narrowed at its summit, where it is articulated with the next joint, and from the latter by the well-formed orbits and the entire absence of rudimentary fifth legs.' Those who are always sceptically inquiring for links in the chain of evolution and for the fine gradations which the transmutation of species postulates, may be invited to observe in this family the genus Pinnixa, White, in which the fifth legs are often short, the genus Amorphopus, in which they are rudimentary, and lastly Thaumastoplax and Hexapus, from which they have vanished altogether.

Some curious facts relating to the organs of vision in certain members of this tribe are worthy of mention. In the family Ocypodidæ the genus Bathyplax, A. MilneEdwards, 1880, contains but a single species. Specimens taken by the U.S.S. Blake from depths between four and five hundred fathoms were found to have the eye-stalks very short, almost immovable, and with the corneæ not developed. Accordingly the species was named typhlus, 'the blind.' But specimens taken from smaller depths by the Challenger agreed with the others in all respects except just this one, that they possessed small, distinct, terminal corneæ. As these specimens were obviously not blind, Mr. Miers named them 'var. oculiferus.' In regard to another genus, also but more doubtfully included in the Ocypodidæ, Professor Perrier cites the observation of A. Milne-Edwards, that in Geryon tridens, Kröyer, a species which descends to great depths, the eyes are brilliantly luminous.

CHAPTER VIII

TRIBE III.-OXYRRHYNCHA

THE carapace is more or less narrowed anteriorly and usually rostrate, with the hepatic regions small, the branchial large. The epistome is generally large. The buccal frame is quadrate, with the anterior margin straight. There are nine pairs of branchia, with the efferent channels opening at the sides of the endostome. In this, as in the two preceding tribes, the afferent channels open behind the pterygostomian regions, in front of the base of the chelipeds. The first antennæ are longitudinally folded. The third maxillipeds have the fifth joint articulated at the apex or at the front inner angle of the fourth. The verges of the male are exserted through the bases of the last pair of walking legs.

This tribe of the 'sharp-snouted' crabs is divided into two legions, the Maiinea and Parthenopinea. It has been observed in many cases that the two halves of the large liver are not separate, but united by a median lobe. The nervous system is said to attain a higher degree of centralisation in this group of Crustacea than in any other, the ganglia of the trunk forming a single solid disk-like mass. Legion 1.-Maiinea.

The basal joint of the second antennæ is well developed, inserted beneath the eyes, and usually occupies a great part of the infra-ocular space.

This legion contains three families, the Inachida, Maiidæ, and Pericerida.

Family 1.-Inachido.

The eyes are non-retractile, or retractile against the sides of the carapace. In general the orbits are not de

fined, but there is often a well-developed præocular and postocular spine. The basal joint of the second antennæ is generally slender, sometimes moderately enlarged. The carapace varies in shape, being subtriangular, or truncately triangular or subpyriform, rarely suborbicular. The rostrum is simple or bifid, sometimes very short. The chelipeds never have the fingers excavate at the tips. The walking legs are sometimes very long. In both sexes the number of distinct segments of the pleon varies from four to seven.

To this family belong nearly forty genera, three of which are included in the British fauna.

Macropodia, Leach, 1814, meaning 'long-foot,' had been already called Macropus by Latreille, but that form of the name was preoccupied. By Lamarck in 1818 it was named Stenorynchus, narrow-snout,' a very appropriate name but without any title to supersede the earlier Macropodia of Leach. Just as the title of the tribe Oxyrhyncha ought in accordance with its Greek original to be spelt Oxyrrhyncha, so should Stenorynchus have been spelt Stenorrhynchus. Part of this correction has been adopted in the commonly used form Stenorhynchus, and naturalists have been so much tickled with the pleasing sound that, instead of leaving the monopoly of it to the Crustacea, they have employed it also among beetles, reptiles, birds, and mammals. The British species Macropodia rostratus (Linn.) is described in Bell's History under the name Stenorynchus Phalangium (Pennant). The specific name given by Pennant alludes to the resemblance which these crabs with long thin legs bear to the Pycnogonids or Sea-spiders, and which has won for them the designation of spider-crabs. In spite of their long limbs they are a sluggish and slow-moving race, and in consequence are devoured in great numbers by other inhabitants of the sea. The fact that there are great numbers of them to be devoured shows that nature has not left them entirely without means of defence, of which some account will presently be given. Macropodia longirostris (Fabricius), which Bell calls Stenorynchus tenuirostris