LIMULI.-GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 281 the central line.-The Limuli are confined to the shores of tropical Asia, the Asiatic Archipelago, and tropical America. As the best-known species comes from the Mollucca Islands, they are sometimes termed Molucca Crabs. Of their habits very little has been ascertained. They appear to prefer the neighbourhood of sandy shores, and it is said that when kept from the water, they bury themselves in the sand, in order to avoid the violent heat of the sun, which causes them speedily to perish. They feed upon animal flesh. The long horny process of the posterior shield is not developed in the young animal; which also wants the posterior or branchial members. This process is used by some of the Malays as a point for their arrows; the wounds it makes being dangerous, like those made by the spines of many Fishes, on account of their jagged character. America, where the Limulus is known as the Casserole Fish, the shell is employed as a ladle for water.-Fossil remains of this genus are not uncommon in the Secondary and Tertiary strata; and it may be further remarked that, in several particulars, the Trilobites may be regarded as having been probably analogous to it. On the coast of Geographical Distribution of Crustacea. The extensive study of this large and important class, by M. Milne-Edwards, has enabled him to arrive at some very interesting conclusions in regard to its Geographical Distribution. An outline of these will be here introduced, because they would probably serve, with some modifications, to represent the general facts relating to the distribution of other Classes. 818. It has been pointed out, in the preceding sketch of the principal forms of the Crustacea, that different species have different localities, or residences assigned to them (as it were), on the surface of the globe. We have seen that some are exclusively confined to fresh water,-that others are inhabitants of the brackish water of estuaries,-that others take up their abode on the shore, where they are periodically covered and left dry by the tide, that others frequent the shallow waters in the neighbourhood of the shore, that others are found near the bottom of the deeper waters, at no great distance from land,—that others, 282 GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. again, roam freely through the open sea,―and finally, that others are only to be met with on the dry land, at a considerable distance from the shore. Thus we see that each species has an appropriate kind of residence, for which it is peculiarly adapted by its organisation and habits; but it may be further stated, that each species has an appropriate place of residence, which is very much determined by the temperature of the region. It is true that there are many species very extensively distributed; but this results from their being adapted, by some peculiarities of structure and habit, which we cannot detect, to sustain life under a considerable variety of external conditions (§ 13). The extent to which even these species are distributed, however, will depend, in great part, upon the locomotive powers with which they are endowed, either in their adult or their young states; and also (in regard, at least, to all but the freely-swimming marine species) to the continuity of a line of coast, from one point to any other, along which their migrations may be effected. The existence of constant or periodical currents, too, such as the Gulf Stream of Mexico,-will often affect the distribution of species; thus it is probably to this cause, that we are to attribute the presence of some American Crustacea on the shores of the Canary Islands. 819. The following are the general principles arrived at by M. Milne-Edwards in regard to the influence of Temperature on the Geographical Distribution of Crustacea. I. The different forms and modes of organisation of these animals manifest themselves more, in proportion as we pass from the Polar Seas towards the Equator. Thus, on the coasts of Norway, where there is frequently a vast multiplication of individuals of the same species, the number of species is very small; but the latter increase rapidly as we go southwards. Thus the number of species of Crustacea of the first two Orders, known to exist on the coast of Norway and the neighbouring seas, is only sixteen; but eighty-two are known to be inhabitants of the western shores of Britain, France, Spain, and Portugal; one hundred and fourteen are known in the Mediterranean Sea; and two hundred and two in the Indian Ocean. A similar increase may be observed in folowing the coast of the New World, from Greenland to the GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 283 Caribbean Sea; the number of species of Decapods in the former region being only twelve, whilst in the latter it is seventy-one. II. The differences of form and organisation are not only more numerous and more characteristic in the warm than in the cold regions of the globe; they are also more important.—The number of natural groups, which we find represented in the Polar and Temperate Regions, is much smaller than that of which we find types or examples in Tropical Seas. In fact, nearly all the principal forms, which are met with in colder regions, also present themselves in warm; but a very large proportion of the latter have no representatives among the former. Thus, of the three primary groups, into which the Class is at first subdivided, the Xyphosura are altogether wanting beyond the forty-fourth degree of latitude. Again, the Brachyourous and Anamourous Decapods appear to be altogether excluded from some of the most northern regions that have been explored. Of the family of Squillidæ (§ 791), so highly characteristic of the Order Stomapoda, it is rare to meet with any members, north of the forty-fifth degree of latitude. And the curious group of Phyllopoda is restricted within a still nearer neighbourhood of the Tropical Region. III. Not only are those Crustacea, which are most elevated in the scale, deficient in the Polar Regions; but their relative number increases rapidly as we pass from the Pole towards the Equator. Thus the Brachyoura, which must be considered as the most elevated of the whole series, are totally absent, as we have just seen, in some parts of the arctic region; and we find their place taken by the far less complete Edriophthalma, with a small number of Anomourous and Macrourous Decapods. In the Mediterranean, however, the Decapods surpass the Edriophthalma in regard to the number of species; and the Brachyourous division predominates over the Macrourous, in the proportion of two to one. And in the East and West Indies, the short-tailed are to the long-tailed Decapods, as three, four, or even five, to one.-Again, the Land-Crabs, which are probably to be regarded as taking the highest rank among the Brachyoura, are only to be met with between the tropics. And of the fluviatile Decapods (those which inhabit rivers, brooks, and lakes), a large proportion belong in tropical regions to the elevated type of the Brachy 284 GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. oura; whilst all those found in the temperate and arctic zones belong to the Macrourous division. IV. When we compare together the Crustacea of different parts of the world, we observe that the average size of these animals is considerably greater in tropical regions, than in the temperate or frigid climes. The largest species of the arctic and antarctic seas, are far smaller than those of the tropical ocean; and they bear a much smaller proportion to the whole number. Further, in almost every group, we find that the largest species belong to the equatorial regions; and that those which represent them (or take their place, as it were) in temperate regions, are of smaller dimensions. V. It is where the species are most numerous and varied, and where they attain the greatest size,-in other words, where the temperature is most elevated,—that the peculiarities of structure, which characterize the several groups, are most strongly manifested. Thus the transverse development of the cephalo-thorax, which is so remarkable in the Brachyourous Decapods (the breadth of the carapace of the typical Crabs being much greater than its length from back to front), is carried to its greatest extent in certain Crustacea of the Equatorial region; and the same might be stated of the characteristic peculiarities of most other natural groups. Further, it is in this region that we find the greater part of those anomalous forms, which depart most widely from the general structure of the Class. VI. Lastly, there is a remarkable coincidence between the temperature of different regions, and the prevalence of certain forms of Crustaceous animals. Thus there are few genera to be met with in the West Indian seas, which are not represented in the East Indian,-the species, however, being usually different. The same may be said of the genera inhabiting the temperate regions of the globe;—similar generic forms being usually met with in the corresponding parts of the Old and New World, and of the Northern and Southern Hemispheres, although the species are almost invariably different. CHAPTER XI. OF THE CLASS OF MYRIAPODA. 820. The Class Myriapoda is the lowest in which we meet with articulated members, or distinct jointed legs, as well as with an articulated body. These legs are intermediate in conformation between the more highly-organized legs of Insects, and the simple bristle-like appendages possessed by some of the Annelida; and this is exactly the place to which we should refer the animals of this class, from a regard to their general structure. For, on looking at the form of their bodies, we see that they are distinguished by a uniformity in the character of their segments, nearly as great as that which prevails in the Annelida; so that an Iulus (Fig. 511) might almost be likened to an Earthworm, provided with a stiffer integument and with slender legs:whilst, on the other hand, the adaptation of the respiratory organs to breathe air with regularity and energy, the complexity of the masticating apparatus, the possession of distinct eyes, and many other characters, indicate their affinity with Insects; in which class, indeed, some Naturalists comprehend them. They differ from insects, however, not merely in the absence of wings, but in the great multiplication of the segments, which are nearly always twenty-four at the least, each provided with a pair of legs; and also in the absence of any line of division between the thorax and abdomen. As we have already seen (§ 797), there are certain Crustacea which bear a considerable resemblance to them in regard to the equality of the segments, and general organisation; but these are characterized by their branchial respiration, and the number of their segments is usually much inferior. 821. The covering of the body of these animals is firm, and of a somewhat horny character, resembling that of many Insects. The division into segments is very distinct; a flexible membrane |