Page:VCH Cornwall 1.djvu/308

 A HISTORY OF CORNWALL taken no particular heed. Such vernacular names as some of them enjoy are local, divergent, no easier to understand than their scientific equivalents. The singular bond which unites these diversified groups is a limitation in the number of body segments, and this limitation needs a little explaining. There is a controversial first segment which sometimes carries a pair of eyes on movable stalks. There is a controversial twenty-first segment called the telson, which carries no independent appendages. But between or apart from these two terminals there are nineteen, and never more than nineteen, appendage-bearing segments traceable throughout all the Malacostraca. 1 It must be well understood that though the number is never exceeded, neither is it in practice ever distinctly attained. Always some of these pieces of the body's framework are soldered together, so as not to be separately movable. Occasionally a segment is altogether lost and its appendages have vanished. Hence this numerical relation, systematically so important, cannot be regarded as a first aid to the uninstructed. It binds the malacostracan sub-class into a united whole. Some of the features which distinguish the subordinate groups will be brought into view in the following discussion. The Brachyura, or ' short-tails,' form an order entirely consisting of crabs. Intellect or its equivalent places them at the head of the class. Fourteen segments, solidified together and covered dorsally and laterally by a shield or carapace, carry in successive pairs the pedunculate eyes, the first and second antennae, the jaws in six highly diversified patterns, and the trunk-legs, of which the first are chelipeds, grasping organs often unequal in size, while the other four couples are symmetrical, adapted for walking or swimming. The seven remaining segments, sometimes all separate, some- times in various combinations of coalescence, form an insignificant tail. This is commonly much broader in the female than in the male. In both sexes it folds closely against the animal's breast, except when the mother is using it to clasp the mass of her extruded eggs, or when some parasite occupies the position of those eggs and apparently finds safety by appealing to a deluded instinct. It is a distinctive mark of the genuine Brachyura that the sixth segment of this thinly flattened tail is always without appendages, though these are obscurely indicated in an anomalous group. The creatures spread about the seas and lands of the world, and having in common the curious negative characteristic just mentioned, are surprisingly numerous. They have long been, and for the present still are, arranged in four tribes, the Cyclometopa, or crabs which are arched in front ; the Catometopa, in which the proper front is more or less depressed ; the Oxyrrhyncha, having this front or interorbital part produced into a pointed, bifid or trifid rostrum ; and lastly the Oxystomata, which have the mouth-cavity triangularly shaped forwards, in contrast with its more or less quadrangular form in the other three tribes. In all four the females have the first segment of the pleon or tail devoid of appendages. For discriminating the crabs of Cornwall, few as they are compared with those of all the globe, these main divisions will be found of service. They are themselves split up into many families and sub-families. Neglecting the latter, for the former it will be convenient for us to follow the admirable arrangement supplied by Alcock in his recent ' Carcinological Fauna of India.' 2 He divides the Cyclometopa into five families, the Telphusidae, Xanthidae, Portunidae, Cancridae, and Corystidae. With the Telphusidae, or river crabs, which should rather be called Potamonidae, the Cornish fauna is not concerned, since they are not represented either here or in any other part of Great Britain. All the families agree in having nine gills in the branchial chamber at each side of the carapace. ' The gills are phyllobranchiae ; that is to say, the gill-elements are broad thin leaves arranged in two series along a central stem, like the barbs of a feather-vane ' (Alcock). To carry off the water which has bathed these breathing-organs, there are channels opening at each side of the so-called palate or mouth-cavity. The genital ducts of the male open on the bases of the last pair of legs. In the second and third families the first antennae fold transversely or in an obliquely trans- verse direction, whereas they fold longitudinally in the fourth and fifth families. The Xanthidae have the last legs, like the three preceding pairs, ambulatory, but the Portunidae for the most part have the last pair natatory, the terminal joints being widened into swimming paddles. A hard and fast line, however, cannot well be drawn between the two families by any single character. Still less easy is it to distinguish Cancridae and Corystidae by any mark at once steadfast and easy to observe. In the former the epistome is usually of fair length ; in the latter it is evanescent. The epistome, or part between the antennulary septum and the mouth-cavity, is regarded as the sternum or ventral portion of the segment which carries the second antennae, the sides and back of this segment taking an important but not easily distinguishable share in the formation of the carapace. About a score of Cornish crabs are distributed over these four families. In the Xanthidae stand the genera Xantho, Xanthodts, Pilumnus y and Pilumnoides. (For some unexplained reason or by accident, Jonathan Couch wrote ' Zantbo ' instead of ' Xantbo.') * The species now known as Xantho incisus, Leach, but perhaps equivalent to the earlier X. poressa (Olivi), he calls in English the ' furrowed crab,' and its congener X. hydrophilus (Herbst) the ' less furrowed crab,' 1 The malacostracan position of NebaKa is not established. 256
 * Journ. Asiatic Soe. of Bengal (1899), vol. Ixviii, pt. ii.
 * Cornish Fauna (1838), p. 67. This work will subsequently be cited as Fauna.