ROUNDHEADS

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CHAPTER VI

TRIBE I.- -CYCLOMETOPA

THE name literally means 'those of a circular forehead.' In these Crustacea the carapace is often of a breadth greater than the length, wide and regularly arched in front, more rarely quadrate or suborbicular, but not rostrate. The epistome is short, transverse. The first antennæ are in general transversely folded. The third maxillipeds have the fifth joint articulated at the apex or the inner front angle of the fourth (except in Pirimela). There are nine pairs of branchia, with their efferent channels opening at the sides of the endostome or palate. The verges of the male are inserted at the bases of the last legs of the trunk.

Milne-Edwards states that the different ganglia of the trunk form a sort of circular ring, of which it is often easy to distinguish the constituent elements, and that the two halves of the liver remain distinct without a median lobe.

The tribe has been subdivided into four legionsCancrinea, Cyclinea, Corystinea, Thelphusinea, in defining which I shall follow the safe guidance of Mr. E. J. Miers, as afforded in his report on the Brachyura collected by the Challenger.

Legion 1.-Cancrinea.

The buccal cavity is usually well defined. The flagella of the second antennæ are not greatly elongated. The seventh joint in the walking-legs is generally unarmed. It is either stiliform or in the last pair expanded into an

ovate swimming organ. The species are marine or

littoral.

This legion contains four families-Cancridæ, Trapeziidæ, Portunidæ, Podophthalmida.

Family 1.-Cancrida.

The carapace is commonly transverse and convex, with the antero-lateral margins arcuate, and armed with several lobes, teeth, or spines. The 'front' is of moderate width, in general not projecting over the first antennæ and the bases of the second, the latter being seldom excluded from the inner hiatus of the orbits.

In this family are included about half a hundred genera, some widely and conspicuously distinct, others separated by fine and almost inappreciable differences. Thus Mr. Miers observes of Xantho (Leach, 1813), that 'it is connected by almost insensible gradations on the one hand with Lophoxanthus and Xanthodes, on the other with Panopeus and Eurypanopeus.' Quite recently the genus Panopeus, H. Milne-Edwards, 1834, has been reviewed by James Benedict and Mary Rathbun. They recognise in it thirty-eight species, and re-include within its boundaries Eurytium, Stimpson, 1859, and Eurypanopeus, A. Milne-Edwards, 1880, considering that they have been separated from the parent genus on grounds insufficient or untenable. It will, however, be quite beyond the range of such a manual as this to enter into all the minutiae of generic distinctions. Far less can the characters of innumerable species be discussed. Only the specially typical or the specially anomalous forms may court a passing attention. Here and there a comparison, a description, a comment, may indicate the variety of details upon which classification is founded, or may suggest the endless opportunities for the exercise of keen eyes and acute minds, which the subject provides.

Those whose scientific zeal is limited to the desire of having the specimens in a cabinet rightly arranged and ticketed with their proper names are often puzzled and

HOW GENERA ARE GENERATED

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exasperated to find that there is practically no finality in these matters. It is important to remember that this cannot be helped, so long as knowledge is in the stage of growth, the stage in which it is most acceptable to the human intellect, by continually holding out the invigorating hope of new acquisitions. In the progress of science some animal hitherto unknown or little noticed attracts the attention of a naturalist. Describing some of its salient features, he makes it the typical species of a new genus. In course of time many other animals are found to have characters almost identical, and they constitute the various species of the same genus, till the number of them becomes so large that they are perhaps at first grouped in lettered or numbered sections, to which presently names are given as subgenera, and these in turn are raised to the rank of genera, and sometimes eventually to higher grades in the system. At each successive improvement there comes a displacement of the old names, and for the accurate designation of specimens the unskilled are placed at greater and greater disadvantage. There was a time when all the Crustacea were included among insects, but to call a lobster an insect would now be regarded as a proof of ludicrous ignorance. The existing genus Cancer is an absurd little remnant of that which was originally established by Linnæus, and which has been gradually subdivided into a long array of genera, and families, and legions, and sub-orders, and orders. Bell, in 1853, in his British Stalk-eyed Crustacea,' says, 'There is but one species of this genus, as now restricted, native of the shores of this country, or indeed of Europe, all the others being South American.' He refers, however, to the species discovered by Say, which belong to the East Coast of North America.

The great eatable crab of our own shores is well known. Dr. Leach remarks that 'at low tide they are often found in holes of rocks, in pairs, male and female, and if the male be taken away, another will be found in the hole at the next recess of the tide. By knowing this fact, an experienced fisherman may twice a day take with little trouble a vast number of specimens, after having once

discovered their haunts.' Mr. Couch found that this referred to the mating time, which occurs just after the female has cast her coat, her new shell being still soft. It is easy to understand why the exuviation of the male takes place at a different period, as otherwise the pair would be defenceless together. Of this Cancer pagurus, Linn., small specimens are often sheltered in considerable numbers in cavities wrought in the vast masses of the sand-tubes of the marine worm Sabellaria alveolata.

Cancer irroratus, Say, is the commonest species of the genus in America. It is exceedingly like the European species, but smaller, with the chelipeds less bulky, and distinguished by a strong tooth on the fifth joint. It is said to be common under the large rocks near low-water mark, often lying nearly buried in the sand and gravel beneath them. It is also frequent on sandy shores, and occurs in the tidal pools, where, according to Professor Verrill, 'the comical combats of the males may sometimes be witnessed.' Miss J. M. Arms founds upon it the following description of a crab's method of walking:

'The legs of one side are used to push with, and those of the other to pull with, when the crab is in motion. Those of the same side do not, however, all move together, but alternately, so that there is no halting in their gait; some of the legs are always in the act of taking new steps, and by shoving and pulling in unison a continuous motion is kept up. This crawling by means of jointed appendages can be imitated after having once seen a live crab. Cross the two wrists side by side, placing the fingers down on a level table; bind the wrists by an elastic band, hold them well up from the table, so as to show the fingers. Then let one set crawl while the other pushes, so as to keep up a continuous motion sidewise without assistance from the arms. The terminal sections of the legs show wear only on the points where these are inserted in the ground.'

It will subsequently be seen that there are some crabs which are by no means limited to the slow progression denoted by the word crawling.

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Say established the species Cancer irroratus in 1817, but in 1859 Stimpson discovered that under one name Say had combined two species, having been misled into matching the male of Cancer irroratus with the female of Cancer borealis. The differences between the sexes which often exist in Crustacea have more commonly led naturalists into the opposite mistake of instituting a separate species for each sex. Cancer borealis, Stimpson, occurs in the same localities as Cancer irroratus, only being a heavier and more massive species it does not equally court shelter and retirement, but will rest entirely exposed on bare rocks and ledges, or clinging to weeds amid the onset of the waves. Yet the strength of its shell does not save it from the gulls and crows which take advantage of its venturesome position to carry it off for their own consumption.

It is not only the sexes of the adult crustaceans that often differ considerably in appearance, but in many instances between the egg and maturity there are stages to be passed through in which the forms of the young are so startlingly different from those of their parents that they have been placed in different genera, until the relationship was eventually proved or made probable. To these larval stages various names have been given, some of them borrowed from the names of the supposed genera to which the young animals had been at first assigned.

The Dutch naturalist, Martin Slabber, in 1769, was the first to publish an account of a crustacean metamorphosis so striking that, as he says, had he not himself witnessed it, he should have placed the two forms in different genera. Yet this singular observation was left barren, until in 1823 Mr. Vaughan Thompson was induced to follow it up, with results that have since been far-reaching. One very curious circumstance in this history is that the two forms which Slabber figures evidently do belong to perfectly distinct groups, the first or Zoëa form to the Brachyura, and the second to the Macrura. Bell, in the Introduction to his 'British Stalk-eyed Crustacea,' reproaches Thompson for coming to the conclusion that Slabber lost his Zoëa, in

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