Page:EB1911 - Volume 17.djvu/473

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treatise by C. F. Menêtrier (Paris, 1689); Marquis of Bute in Dublin Review (1885); A. Harnack in Zeitschrift für Kirchengeschichte, Bd. III.

MALACOSTRACA. Under this zoological title are included several groups of Crustacea (q.v.), united by characters which attest their common origin, though some, and probably all of them, were already separated in distant geological ages, and some have now attained a peculiar isolation. Throughout the whole, the researches made since 1860 have not only added a great throng of new species, genera and families, but have thrown a flood of light upon questions of their phylogeny, systematic arrangement, horizontal and bathymetric distribution, organization, habits of life and economic importance. There are at least seven orders: the stalk-eyed Brachyura, Macrura, Schizopoda, Stomatopoda, and the sessile-eyed Sympoda, Isopoda, Amphipoda. An ocular segment claimed by the former division is not present or in no case demonstrable in the latter. In neither does the terminal segment or telson, whether large or obsolescent, whether articulated or coalescent, carry appendages, unless occasionally in fusion with itself. Between the eyes and the tail-piece in all the orders nineteen segments are counted, the proof of a segment’s existence depending on its separateness, complete or partial, or on a sutural indication, or else on the pair of appendages known to belong to it. All these marks may fail, and then the species must be proved to be Malacostracan by other evidence than the number of its segments; but if some exceptions exhibit fewer, none of the Malacostraca exhibits more than 19 (+1 or + 2) segments, unless the Nebaliidae be included. Of the corresponding pairs of appendages thirteen belong to the head and trunk, two pairs of antennae, one pair of mandibles, two pairs of maxillae, followed by three which may be all maxillipeds or may help to swell the number of trunk-legs to which the next five pairs belong. The abdomen or pleon carries the remaining six pairs, of which from three to five are called pleopods and the remainder uropods. Underlying the diversity of names and functions and countless varieties of shape, there is a common standard to which the appendages in general can be referred. In the maxillipeds and the trunk-legs it is common to find or otherwise easy to trace a seven-jointed stem, the endopod, from which may spring two branches, the epipod from the first joint, the exopod from the second.[1] The first antennae are exceptional in branching, if at all, at the third joint. In the mandibles and maxillae some of the terminal joints of the stem are invariably wanting. In the rest of the appendages they may either be wanting or indistinguishable. The latter obscurity results either from coalescence, to which all joints and segments are liable, or from subdivision, which occasionally affects joints even in the trunk-legs. The carapace, formerly referred only to the antennar-mandibular segments, may perhaps in fact contain elements from any number of other segments of head and trunk, Huxley, Alcock, Bouvier giving support to this opinion by the sutural or other divisional lines in Potamobius, Nephrops, Thalassina, and various fossil genera. Not all questions of classification internal to this division are yet finally settled. Between the Brachyura and Macrura some authors uphold an order Anomura, though in a much restricted sense, the labours of Huxley, Boas, Alcock and conjointly Alphonse Milne-Edwards and Bouvier, having resulted in restoring the Dromiidea and Raninidae to the Brachyura, among which de Haan long ago placed them. The French authors argue that from the macruran lobsters (Nephropsidae) anciently diverged two lines: one leading through the Dromiidea to the genuine Brachyura; or crabs, the other independently to the Anomura proper, which may conveniently be named and classed as Macrura anomala. Spence Bate maintained that the Schizopoda ought not to form a separate order, but to be ranged as a macruran tribe, “more nearly allied to the degraded forms of the Penaeidea than to those of any other group” (“ChallengerReports, “Macrura,” p. 472, 1888). According to Sars, the Sympoda (or Cumaceans), in spite of their sessile eyes, have closer affinities with the stalk-eyed orders. H. J. Hansen and others form a distinct order Tanaidea for the decidedly anomalous group called by Sars Isopoda chelifera.

1. Brachyura.—For the present, as of old, the true Brachyura are divided into four tribes: Cyclometopa, with arched front as in the common eatable crab; Catometopa, with front bent down as in the land-crabs and the little oyster-crab; Oxyrhyncha, with sharpened beak-like front as in the various spider-crabs; Oxystomata, including the Raninidae, and named not from the character of the front but from that of the buccal frame which is usually narrowed forwards. In these tribes the bold and active habits, the striking colours, or the fantastic diversities of structure, have so long attracted remark that recent investigations, while adding a multitude of new species and supplying the specialist with an infinity of new details, have not materially altered the scientific standpoint. New light, however, has been thrown upon the “intellectual” capacity of Crustacea by the proof that the spider-crabs deliberately use changes of raiment to harmonize with their surroundings, donning and doffing various natural objects as we do our manufactured clothes. Others have the power of producing sounds, one use to which they put this faculty being apparently to signal from their burrow in the sand that they are “not at home” to an inopportune visitor. Deep-sea exploration has shown that some species have an immensely extended range, and still more, that species of the same genus, and genera of the same family, though separated by great intervals of space, may be closely allied in character. A curious effect of parasitism, well illustrated in crabs, though not confined to them, has been expounded by Professor Giard, namely, that it tends to obliterate the secondary sexual characters. Modern research has discovered no crab to surpass Macrocheira kämpferi, De Haan, that can span between three and four yards with the tips of its toes, but at the other end of the scale it has yielded Collodes malabaricus, Alcock, “of which the carapace, in an adult and egg-laden female, is less than one-sixth of an inch in its greatest diameter.” The most abyssal of all crabs yet known is Ethusina abyssicola, Smith, or what is perhaps only a variety of it, E. challengeri, Miers. Of the latter the “Albatross” obtained a specimen from a depth of 2232 fathoms (Faxon, 1895), of the former from 2221 fathoms, and of this S. I. Smith remarks that it has “distinctly faceted black eyes,” although in them “there are only a very few visual elements at the tips of the immobile eye-stalks.”

The Brachyura anomala, or Dromiidea, “have preserved the external characters and probably also the organization of the Brachyura of the Secondary epoch” (Milne-Edwards and Bouvier, 1901). They agree with the true crabs in not having appendages (uropods) to the sixth segment of the pleon, the atrophy being complete in the Homolidae and Homolodromiidae, whereas in the Dromiidae and Dynomenidae a pair of small plates appear to be vestiges of these organs. In the family Homolidae stands the strange genus Latreillia, Roux, with long slender limbs and triangular carapace after the fashion of oxyrhynch spider-crabs. In Homola the carapace is quadrilateral. Between these two a very interesting link was discovered by the “Challenger” in the species Latreillopsis bispinosa. Henderson. Bouvier (1896) has shown that Palaeinachus longipes, Woodward, from the Forest Marble of Wiltshire, is in close relationship, not to the oxyrhynch Inachidae, but to the genera Homolodromia and Dicranodromia of the Homolodromiidae, and that the Jurassic crabs in general, of the family Prosoponidae (Meyer), are Dromiidea.

2. Macrura.—The Macrura anomala, or Anomura in restricted sense, are popularly known through the hermit-crabs alone. These only partially represent one of the three main divisions, Paguridea, Galatheidea, Hippidea. The first of these is subdivided into Pagurinea, Lithodinea, Lomisinea, each with a literature of its own. Among the Pagurinea is the Birgus latro, or robber-crab, whose expertness in climbing the coco-nut palm need no longer be doubted, since in recent years it has been noted and photographed by trustworthy naturalists in the very act. Alcock “observed one of these crabs drinking from a runnel of rain-water, by dipping the fingers of one of its chelipeds into the water and then carrying the wet fingers to its mouth.” Hermits of the genus Coenobita he found feeding voraciously on nestling sea-terns. That pagurids must have the usually soft pleon or abdomen protected by the shell of a mollusc is now known to be subject to a multitude of exceptions. Birgus dispenses with a covering; Coenobita can make shift with half the shell of a coco-nut; Chlaenopagurus wraps itself up in a blanket of colonial polyps; Cancellus tanneri, Faxon, was found in a piece of dead coral rock; Xylopagurus rectus, A. Milne-Edwards, lodges in tubes of timber or bits of hollow reed. The last-named species has a straight symmetrical abdomen, with the penultimate segment expanded and strongly calcified to form a back-door to the very unconventional habitation. This it enters head-foremost from the rear, while “hermits” in general are forced to go backwards into their spiral or tapering shelters by the front. Some of the species can live in the ocean at a depth of two or three miles. Some can range inland up to a considerable height on mountains. The advantage that this group has derived from the adoption of mollusc

  1. In Huxley’s terminology the first two or three joints of the stem constitute a “protopodite,” from which spring the “endopodite” and “exopodite.”