Arthropod

“ 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. ”

“ The argument, then, from geology, like that from comparative anatomy and from the consideration of the importance of the central nervous system in the upward development of the animal race, not only points directly to the arthropod group as the ancestor of the {33}vertebrate, but also to a distinct ancient type of arthropod, the Palæostracan, the only living example of which is the King-Crab or Limulus; while the nearest approach to the trilobite group among living arthropods are Branchipus and Apus. ”

“ Of water animals only the elongated forms sent representatives to people the land—the worm, mollusk, arthropod and vertebrate. And of these, the latter two alone seemed well adapted to their new habitat, the arthropod developing into an extraordinary multitude of species, though its inferiority of organization removed it, at the start, from any competition with the vertebrate as a basis for the higher evolution. We find in the bee and the ant the ultimate development of the insect intellect, and the insect form is decidedly restricted by its characteristic condition. ”

“ As a result, from the very nature of the chitinous shell and the method of renewal which its structure entails, the growth of an arthropod, from infancy to an adult condition, takes place by a series of well-marked steps or stages, each one characterized by the formation of a new cuticle and by a sudden increase in size. In most arthropods the newly born young are very different in structure from the adults, and growth is accompanied by metamorphosis. ”

“ We must endeavour to dismiss from our imagination such forms as the salmon and shark as representatives of the fish-tribe, and the lobster and spider of the arthropods, and try to picture the kind of animals living in the seas in the early Devonian and Upper Silurian times, and then we find, to our surprise, that instead of the contrast between fishes and arthropods being so striking as to make any comparison between the two seem an absurdity, the difficulty in the last century, and even now, is to decide in many cases whether a fossil is an arthropod or a fish. ”

“ Of the creeping, wingless creatures, which can ever be found beneath rocks, rails, chunks, and especially beneath those old decaying logs which are half buried in the rich vegetable mould, the myriapods, or “thousand-legs,” deserve more than a passing notice. They are typical examples of that great branch of the animal kingdom known as arthropods, which comprises all insects and crustaceans. Each arthropod has the body composed of rings placed end to end and bearing jointed appendages, and in the myriapods each ring and its appendages can be plainly seen ”

“ 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. ”

“ Each separate vertebrate organ, one after the other, as shown in the last chapter, indicates in its development the manner in which it arose from the corresponding organ of the arthropod. There is no failure in the evidence of embryology, the failure is in the interpretation thereof.
So, too, my theory vindicates the geological method. There is no failure here; on the contrary, the record of the rocks proclaims with startling clearness not only the sequence of evolution in the vertebrate kingdom itself, but the origin of the vertebrate from the most highly-developed invertebrate race. ”

“ This remarkable discovery of the branchial cartilaginous bars of Limulus, together with that of the internal prosomatic plastron, causes the original difficulty of deriving an animal such as the vertebrate from an animal resembling an arthropod to vanish into thin air, for it shows that in the past ages when the vertebrates first appeared on the earth, the dominant arthropod race at that time, the members of which resembled Limulus, had solved the question ”

“ 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. ”

“ As regards the evolution of limbs of terrestrial arthropods, we have the following stages: 1, the soft unjointed limbs of Tardigrades, ending in two claws, and those of Peripatus, and the pseudo- or prop-legs of certain dipterous larvæ; 2, finally the evolution of the long, solid, jointed limbs of Pauropus and other primitive myriopods, the legs forming solid, firm supports elevating the body, and enabling the insect to drag itself over the ground or to walk or run. When the body is elongated and many-segmented, the legs are necessarily numerous ”

“ 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. ”

“ It is true enough that there is every reason to believe that the arthropod group has been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be looked on as forming one big division, the Appendiculata, all the members of which will some day be arranged in orderly sequence, but the same feeling of certainty does not exist in other cases. ”

“ Arthropod prey included: 11 orthopterans (4 undetermined, 3 unspecified grasshoppers, 2 gryllids, 1 blattid, 1 acridid) ; 10 coleopterans (7 undetermined, 1 each of rhynchophoran, cerambycid, carabid, staphylinid larva, elaterid adult and larva) ; 8 spiders; 5 pulmonate snails; 5 flies; 3 undetermined; and one each of lepidopteran larva and adult, ant, dragonfly, thysanuran, and sow bug. ”

“ The animals belonging to this large and important collection, which compose the arthropod phylum, have certain common characteristic features. We find a body made up of a series of more or less completely similar segments placed one behind the other. ”

“ RELATIONS OF INSECTS TO OTHER ARTHROPODA The insects by general consent stand at the head of the Arthropoda. Their bodies are quite as much complicated or specialized, and indeed, when we consider the winged forms, more so, than any other class of the branch, and besides this they have wings, fitting them for an aërial life. It is with little doubt that to their power of flight, and thus of escaping the attacks of their creeping arthropod enemies, insects owe, so to speak, their success in life; i.e. their numerical superiority in individuals, species, and genera. ”

“ It is obvious, however, that the jointed structure of the limbs of arthropods, if we are to attempt any explanation at all of the origin of such structure, was primarily due mainly to lateral strains and 35impacts resulting from the primitive endeavors of the ancestral arthropods to raise and to support the body while thus raised, and then to push or drag it forward by means of the soft, partially jointed, lateral limbs which were armed with bristles, hooks, or finally claws. ”

“ All the Arthropods agree in having the terminal portion of the intestinal canal derived from the outer, the middle portion from the inner embryonic layer. Lastly, the Arthropods, in common with the Molluscs and Annelids, have their body-cavity, or perivisceral space, formed by the splitting of the mesoblast, or derivative layer formed between the outer and inner layer (epiblast and hypoblast, and ectoderm and endoderm) . In Insects, in the higher Arachnids, and Crustaceans, yelk segmentation is partial; in Myriapods, and the lower Arachnids, and Crustaceans, it is total ”

“ These are the ‘Insects’ of the Linnaean classification, and, for the matter of that, of popular phraseology; for though few people would now venture to call a Lobster an ‘insect,’ we still style some of its near relatives Water ‘Fleas,’ as Swammerdam did two hundred years ago.
The Arthropods form a phylum, or main division of the Animal Kingdom. Above this phylum comes that of the Molluscs, or soft-bodied animals, such as the Oyster, the Snail, and the Cuttlefish. ”

“ In fact, the three divisions into which the body of an arthropod is normally divided—head, thorax, and abdomen—are indistinguishable in mites, the body forming an unconstricted whole. As a rule, these little creatures breathe, as do insects, by tracheae, or, if these be absent, by the general surface of the body. They live for the most part on vegetable and animal juices, [90] and their mouth-parts are, as a rule, piercing and suctorial; but in some species the appendages of the mouth are capable of biting as well as piercing. The adults have typically eight legs. ”

“ 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. ”

“ But embryological evidence enables us to postulate with confidence still more remote ancestors in which the head carried well developed compound eyes and five pairs of appendages, while it may be supposed that all the abdominal segments, except the anal, bore limbs. In these very ancient arthropods, all the limbs, except the feelers, had ambulatory and branchial branches; and one important feature in the evolution of the Arachnida must have been the division of labour between the anterior and posterior limbs, the former becoming specialized for locomotion, the latter for breathing. ”

“ 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. ”

“ Arthropods And Insects: all Arthropods have jointed limbs but external, shell-like skeletons from which the internal organs hang. Insects are one group of arthropods; they have one pair of antennae, three pairs of legs and three body sections, and often also have wings. ”

“ 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. ”

“ The muscles of a man clothe his bones. In the arthropods, on the contrary, the hard parts clothe the muscles. Arthropods are therefore said to have exoskeletons, “outside skeletons.” The body, the legs, and other organs of an insect or a crab consist of a series of hollow tubes held together by flexible skin at the points of union, and controlled in their movements by muscles which pull from the inside. The meat of a lobster is inside of the shell, or exoskeleton, as everybody who has eaten a lobster knows. The arrangement is exactly the reverse of that which we find in the vertebrates. ”

“ Thus, in the Silurian age a very rich fauna has already developed, and representatives are found of all the main Invertebrate groups—sponges, corals, hydroid colonies, five types of Echinoderms, Bryozoa, Brachiopods, Worms, many types of Mollusca and Arthropoda. Of Vertebrates, at least two types of fish are present—Ganoids and Elasmobranchs. In the very earliest fossiliferous rocks of all, the Precambrian formation, there are remains of Molluscs, Trilobites and Gigantostraca, similar to those which flourished in Cambrian and Silurian times. ”

“ There can be no doubt that many of the Arthropods, a division of animals more ancient and even now more prevalent than the Vertebrata, have undergone more phylogenetic modification"—a beautiful phrase—"than even the most modified of vertebrated animals. ”

“ It is apparent that the jointed or metameric structure of the bodies of insects and other arthropods is an inheritance from the segmented worms. In the worms the body is a continuous dermo-muscular tube, while in arthropods this tube is divided into regions, and the cuticle is thicker and more resistant. To go back to the incipient stages in the process of segmentation of the body, we conceive that the worms probably arose from a creeping gastrula-like form, the gastræa. ”