Arthropoda

“ The Articulate phylum embraced the Infusoria and intestinal worms, as well as the Annelids, along with the Arthropoda as restricted above. The Arthropoda further formed two groups: — Carides, the branchiate Arthropods or Crustaceans; and Tracheata, the Arachnids, Myriapods, and Insects, which breathe by tracheæ. ”

“ In this group the organs of special sense reach for the first time a high stage of development. Compound eyes are peculiar to Arthropoda. The heart lies above the alimentary canal. Respiration is carried on by gills in the aquatic forms, and by a remarkable system of air-tubes or tracheæ in the land forms (insects) . The sexes are usually distinct, and reproduction is almost universally sexual. Most of the species lay eggs.
The Arthropods are animals of a high degree of organization. ”

“ This consideration points directly to the origin of vertebrates from the most highly organized invertebrate group—the Arthropoda—for among all the groups of animals living on the earth in the present day they alone possess a central nervous system closely comparable with that of vertebrates. Not only has an upward progress taken place in animals as a whole, but also in the tissues themselves a similar evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely those of the arthropod than of any other invertebrate group. ”

“ The great branch Arthropoda includes a host of familiar animals. It contains more species than any other branch of the animal kingdom. To it belong the crayfishes, shrimps, crabs, lobsters, water-fleas, and other animals which compose the class Crustacea; the centipeds and thousand-legged worms which compose the class Myriapoda; the true or six-footed insects forming the class Insecta, which includes nearly two-thirds of all the known species of animals; and the scorpions, mites, ticks, and spiders which constitute the class Arachnida. ”

“ After the determination of a number of cosmopolitan insects that may well have been artificially introduced, there remains a large proportion of endemic species—sometimes referable to distinct genera—which suggest a high antiquity for the truly insular faunas.
Relationships and Phylogeny
The Hexapoda form a very clearly defined class of the Arthropoda, and many recent writers have suggested that they must have arisen independently of other Arthropods from annelid worms, and that the Arthropoda must, therefore, be regarded as an “unnatural,” polyphyletic assemblage. ”

“ The Arthropoda, to make the simplest possible definition, are invertebrate animals with segmented body and appendages. The most primitive arthropod would appear to be one composed of exactly similar segments bearing exactly similar appendages, the segments of the appendages themselves all similar to one another. It is highly improbable that this most primitive arthropod imaginable will ever be found, but after a survey of the whole phylum, it appears that the simpler trilobites approximate it most closely. ”

“ A brief notice must suffice of the structure and history of the Eyes, the Tracheae and the so-called Malpighian tubes of Arthropoda, though special importance attaches to each in regard to the determination of the affinities of the various animals included in this great sub-phylum.
The Eyes.—The Arthropod eye appears to be an organ of special character developed in the common ancestor of the Euarthropoda, and distinct from the Chaetopod eye, which is found only in the Onychophora where the true Arthropod eye is absent. ”

“ Truly, at the time when vertebrates first appeared, the direction and progress of variation in the Arthropoda was leading, owing to the manner in which the brain was pierced by the œsophagus, to a terrible dilemma—either the capacity for taking in food without sufficient intelligence to capture it, or intelligence sufficient to capture food and no power to consume it. ”

“ Natural as may seem the assemblage included under Arthropoda, there is no group in which adaptive modifications have introduced so much diversity of anatomical and physiological relations. Metamorphoses, the changes of form which changes of external conditions have promoted, are met with of very various amount. The progress of the embryo from the first appearance of the blastoderm up to sexual maturity of the adult may be direct, without metamorphosis, or may be retarded by changes of form and habit, rendering the young animal capable of sustaining life under very various conditions. ”

“ The following synoptic tables are presented in the hope that they may be of service in giving the reader a perspective of the relationships of the Arthropoda in general and enabling him to identify the more important species which have been found noxious to man. Though applicable chiefly to the arthropods found in the United States, exotic genera and species which are concerned in the transmission of disease are also included. For this reason the keys to the genera of the Muscids of the world are given. As will be seen, the tables embrace a number of groups of species which are not injurious. ”

“ The animals thus associated, the Rotifera, Chaetopoda and Arthropoda, are composed of a larger or smaller number of hollow rings, each ring possessing typically a pair of hollow lateral appendages, moved by intrinsic muscles and penetrated by blood-spaces. ”

“ The whole history of Flies, and, moreover, the history of the whole tribe of Arthropoda, essentially confirms the great laws of differentiation and perfecting which, according to Darwin’s theory of selection, must be 190 considered as the necessary results of Natural Selection. The whole tribe, so rich in forms, begins in the Archilithic period with the class of Crabs breathing by gills, and with the lowest Primæval Crabs, or Archicaridæ. ”

“ In reading these pages one cannot but appreciate the loss to bionomics that accrues by the neglect of observations on many lower forms of life. If we except the Phylum Arthropoda and the higher vertebrates, we shall find this volume describing animals which seem totally neglected by field naturalists. ”

“ A text-book for the educational public with suggestions for laboratory work. The volume is intended for high-school or elementary college classes and the inductive method is used with each class and phylum of invertebrate animals. In the first chapter after the remainder of the Arthropoda are described the other invertebrate phyla follow in a descending series, ending with Protozoa. Then, beginning with the fishes, the order ascends to the mammals and closes with man. There are 233 illustrations. ”

“ But it has its limitations, and it goes without saying that the more complete the fossil skeleton of a vertebrate, or the remains of an arthropod, the more complete will be our conception of the form of the extinct organism. It may be misleading in the numerous cases of convergence and of generalized forms which now abound in our palæontological collections. We can well understand how guarded one must be in working out the restorations of dinosaurs and fossil birds, of the Permian and Triassic theromorphs, and the Tertiary creodonts as compared with existing carnivora. ”

“ This fact is one of many pointing to a general law deducible from the fossil vertebrate and invertebrate animals of the eras immediately antecedent to the human, namely, that the present geographical distribution of organic forms dates back to a period anterior to the creation of existing species; in other words, the limitation of particular genera or families of quadrupeds, mollusca, &c., to certain existing provinces of land and sea, began before the species now contemporary with man had been introduced into the earth. ”

“ It would be a great error to suppose that these organic remains were fragmentary relics. Our museums exhibit fossil shells of immeasurable antiquity, as perfect as the day they were formed, whole skeletons without a limb disturbed—nay, the changed flesh, the developing embryos, and even the very footsteps of primeval organisms. Thus the naturalist finds in the bowels of the earth species as well defined as, and in some groups of animals more numerous than, those that breathe the upper air. ”

“ It is a pity that the theory of our evolution from the primordial protoplasm in an inclusive line through every living species should now be considered old-fashioned. I like to imagine that among my remote ancestors every living thing is represented—it gives them a family interest. And if, further, I can persuade myself that I have been everything, at one time or another, from a bluebottle to a giraffe—why, then I can brown-study for ever. ”

“ This, too, without taking into account at all the continually increasing number of facts Palæontology is bringing to light, placing before our eyes the forms which the Evolution theory postulates as the ancestors of the organic world of to-day: birds with teeth in their bills, reptile-like forms clothed with feathers, and numerous other long-extinct forms of life, which, covered up by the mud of earlier waters, and preserved as 'fossils' in the later sedimentary rocks, tell us plainly how the earlier world of animals and plants was constituted. ”

“ Let them gather up the fossil fragments of a lost Fauna, reproducing the ancient forms which inhabited the land or the seas, bringing them together, bone to his bone, till Leviathan and Behemoth stand before us in bodily presence and in their full proportions, and we almost tremble lest these dry bones should live again! ”

“ The faunas of the more ancient formations he holds to be far less imperfect than has been often supposed, where the vertebrate type is represented by the fishes, and whose structure is as complicated and finished as the most recent of their kind; while the invertebrate again furnishes numerous examples of fossilized gasteropoda and cephalopoda, the most perfect orders of the molluscous class. ”

“ The modifications of living beings accord with these oscillations of the inorganic world. The principal animal species seem to predominate in turn; the human races appear in succession, increase and decline.
During the deposition of the lower alluvium (bas niveaux) of our valleys, the mammoth, rhinoceros, and great carnivora, seem to have played the principal part. Man disputed the [290] ground with them, and fed upon their flesh. The struggle against the conditions of life, and the wild beasts of the ancient world was terrible. ”

“ They afford the first positive proof as yet obtained of the co-existence of a varied fauna of the highest class of vertebrata with that ample development of reptile life which marks all the periods from the Trias to the Lower Cretaceous inclusive, and with a gymnospermous flora, or that state of the vegetable kingdom when cycads and conifers predominated over all kinds of plants, except the ferns, so far, at least, as our present imperfect knowledge of fossil botany entitles us to speak. ”

“ They have become successfully adapted to a wide variety of environments and live on land, in water, and in the air. The typical arthropod has a segmented body which is usually covered by a chitinous exoskeleton which, in some forms, contains additions of calcium carbonate. They are highly specialized and well-developed animals in which locomotion is by means of paired jointed appendages.
Although the arthropods are of great importance in nature today, only a few groups are of importance to the paleontologist. Only two of these, the trilobites and the ostracodes, are discussed herein. ”

“ If the extinct species of plants and animals of the later geological periods were the progenitors of the living species, and gave origin to them by variation and natural selection, where are all the intermediate forms, fossil and living, through which the lost types must have passed during their conversion into the living ones? And why do we not find almost everywhere passages between the nearest allied species and genera, instead of such strong lines of demarcation, and often wide intervening gaps? ”

“ Supposing a number of some species of Arthropod or Fish to be swept into a cavern or to be carried from less to greater depths in the sea, those individuals with perfect eyes would follow the glimmer of light and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. A natural selection would thus be effected" in successive generations. ”

“ The large preponderance of extinct species among the animals we find in caves, and in superficial deposites of diluvium, and the non-discovery of human bones along with them, afford other strong reasons for referring these species to a period anterior to the creation of man. ”

“ Will these veins and arteries, I marvel, form, in their turn, the fossils of another period, when a higher platform than that into which they have been laid will be occupied to the full by plants and animals specifically different from those of the present scene of things,—the existences of a happier and more finished creation? My business to-day, however, was with the fossils which the deposit now contains,—not with those which it may ultimately form. ”

“ The anatomical, morphological, and, so far as we can judge, the physiological and biological relationship of the fossil with living organisms, is so great and comprehensive that in the present state of science a systematic botany or zoölogy, that should only treat of the fossils or of living organisms alone, would be imperfect. ”

“ It may have happened that the entire organism has decayed, and as the animal matter when it perished was not rebuilt with mineral deposit, only a cast or natural lithograph of the former living thing remains in the rock. Third: The animal or plant may have been preserved unchanged for hundreds or thousands of years, so that the very creature, with its bones, flesh, hair, even the soft portions of the body, as the brain and eye-ball, may remain complete. ”