Page:EB1911 - Volume 18.djvu/238

 course of the historical development of those various groups of metamerized animals, which have undoubtedly sprung from ancestors with more numerous somites than they themselves possess, it appears that we may formulate the following laws as the tenth, eleventh, twelfth and thirteenth laws of metamerism.

The is that individual somites tend to atrophy and finally disappear as distinct structures, most readily at the anterior and the posterior ends of the series constituting an animal body. This is very generally exhibited in the head of Arthropoda, where, however, the operation of the law is largely modified by fusion (see below). With regard to the posterior end of the body, the atrophy of segments does not, as a rule, affect the telson itself so much as the somites in front of it and its power of producing new somites. Sometimes, however, the telson is very minute and nonchitinized (Hexapoda).

The may be stated thus: any somite in the series which is the anterior or posterior somite of a tagma may become atrophied, reduced in size or partially aborted by the suppression of some of its meromes; and finally, such a somite may disappear and leave no obvious trace in the adult structure of its presence. in ancestral forms. This is called the excalation of a somite. Frequently, however, such “excalated” somites are obvious in the embryo or leave some merome (e.g. neuromere, muscle or chitin-plate) which can be detected by minute observation (microscopic) as evidence of their former existence. The somite of the maxillipede (third post-oral appendage) of Apus cancriformis is a good example of a somite on its way to excalation. The third prae-oral and the praemaxillary somites of Hexapod insects are instances where the only traces of the vanished somite are furnished by the microscopic study of early embryos. The praegenital somite of the Arachnida is an example of a somite which is preserved in some members of the group and partially or entirely excalated in other cases, sometimes with fusion of its remnants to neighbouring somites.

The of metamerism might very well be placed in logical order as the first. It is the law of lipomerism, and asserts that just as the metameric condition is produced by a change in the bodies of the descendants of unisegmental ancestors, so highly metamerized forms—i.e. strongly segmented forms with specialized regions of differentiated metameres—may gradually lose their metamerized structure and become apparently and practically unisegmental animals. The change here contemplated is not the atrophy of terminal segments one by one so as to reduce the size of the animal and leave it finally as a single somite. On the contrary, no loss of size or of high organization is necessary. But one by one, and gradually, the metameric grouping of the bodily structures disappears. The cuticle ceases to be thickened in rings—the muscles of the body-wall overrun their somite boundaries. Internal septa disappear. The nerve-ganglia concentrate or else become diffused equally along the cords; one pair of renal coelomoducts and one pair of genital coelomoducts grow to large size and remain—the rest disappear. The appendages atrophy or become limited to one or two pairs which are widely dislocated from their ancestral position. The animal ceases to present any indication of metameric repetition of parts in its entire structure. Degrees in this process are frequently to be recognized. We certainly can observe such a change in the posterior region of some Arthropods, such as the hermit-crabs and the spiders. Admitting that the Echiurids are descended from Chaetopoda, such a change has taken place in them, amounting to little short of complete lipomerism, though not absolutely complete.

Recent suggestions as to the origin of the Mollusca involve the supposition that such an effacement of once well-marked metamerism has occurred in them, leaving its traces only in a few structures such as the multiple gill-plumes and shell-shields of the Chitons and the duplicated renal sacs of Nautilus.

A further matter of importance in this connexion is that when the old metameres have been effaced a new secondary segmentation may arise, as in the jointed worm-like body of the degenerate Acarid, Demodex folliculorum.

Such secondary annulation of the soft body calls to mind the secondary annulation of the metameres of leeches and some earthworms. Space does not permit of more than an allusion to this subject; but it is worth while noting that the secondary annuli marking the somites of leeches and Lumbricidae in definite number and character are perhaps comparable to the redundant pairs of appendages on the hinder somites of Apus, and are in both cases examples of independent repetition of tegumentary meromes—a sort of ineffectual attempt to subdivide the somite which only prevails on the more-readily susceptible meromes of the integument.

The last law of metamerism which we shall attempt to formulate here, as the, relates to the fusion or blending of neighbouring somites. Fusion of adjacent somites has often been erroneously interpreted in the study of Arthropoda. There are, in fact, very varying degrees of fusion which need to be carefully distinguished. The following generalization may be formulated. “The homologous meromes of two or more adjacent somites tend to fuse with one another by a blending of their substance. Very generally, but not invariably, the fused meromes are found as distinct separated structures in the embryo of the animal, in which they unite at a later stage of growth.” The fusion of neighbouring meromes is often preceded by more or less extensive atrophy of the somites concerned, and by arrest of development in the individual ontogeny. Thus, a case of fusion of partially atrophied somites may simulate the appearance of incipient merogenesis or formation of new somites, and, vice versa, incipient merogenesis may be misinterpreted as a case of fusion of once separate and fully-formed somites.

A very complete fusion of somites is that seen on the head of Arthropoda. The head or prosoma of Arthropoda is a tagma consisting of one, two, or three prosthomeres or somites in front of the mouth and of one, two, three, up to five or six opisthomeres. The cephalic tagma or prosoma may thus be more or less sharply divided into two subtagmata, the prae-oral and the post-oral.

 METAMORPHISM (Gr. , change of, and  , shape), in petrology, the alteration of rocks in their structural or mineral characters by which they are transformed into new types. In the history of rock masses changes of many kinds are inevitable. Loose sands, clays and heaps of shells are gradually converted into sandstones, shales and limestones by the action of percolating water and the pressure of over-lying accumulations. All rocks exposed at the earth’s surface or traversed by waters circulating through the earth’s crust, undergo changes in their component minerals due to weathering and the chemical action of the atmosphere and of rain. These processes of cementation and decomposition, though not unlike those of metamorphism, are not regarded as essentially the same. They are considered, so to speak, normal episodes in the history of rocks to which all are subject. When rocks, however, are exposed to the heat of intrusive masses (granite, &c.) or have been compressed, folded, crushed, and more or less completely recrystallized, they assume new characters so different from their original ones that they are ascribed to a quite distinct class, namely, the metamorphic rocks.

The transformation is always gradual, so that in suitable districts every stage can be followed from an unaltered or nearly unaltered sedimentary or igneous rock to a perfectly metamorphic one. The transition may be slow or rapid, and the abundance of intermediate forms renders it impossible to lay down any hard and fast lines of distinction. A black shale with fossils may in two or three feet pass into a splintery hornfels; a sandstone or grit becomes a sheared grit, a granulitic gneiss, and a completely recrystallized gneiss sometimes within a few hundred yards; in a thoroughly metamorphic hornblende-schist or chlorite-schist small kernels sometimes occur which can easily be recognized as little modified dolerites or diabases. Still, the metamorphic rocks as a class have many well-defined characteristics, and in perfectly typical development cover enormous areas of the earth’s surface and must be, in the aggregate, of vast thickness. A great number of them are recognizably of igneous origin; others are equally certainly sedimentary. Hence some writers have suggested that they are not entitled to rank as a separate class, but only as states or conditions of other rocks. It is generally agreed, however, that when the primitive structures and the original minerals of sedimentary or igneous rocks are so transformed as to be no longer easily recognizable the rock should be included in the metamorphic class.

Only rarely, however, does metamorphism produce much difference in the chemical composition of the rocks affected. Sandstones become quartzite’s and quartz schists, limestones are converted into marbles, granite passes into gneiss, and so on, without their bulk composition being greatly modified. From all that we know it seems established that however great the heat and pressure to which metamorphic rocks have been exposed they have very rarely been melted or reduced to the liquid state. Hence there has been no opportunity for intermixture by solution or diffusion; the changes, including the growth of crystals of new-formed minerals, have gone on in the solid rocks. The chemical molecules already present have aggregated into new combinations and have built up new minerals without travelling for more than infinitesimal distances from the places they occupied in the original rock. Exceptions to this occur, but they are so few that they do not