The Encyclopedia Americana (1920)/Arthropoda

ARTHROPODA, a phylum comprising those articulated animals which have jointed appendages such as antennæ, jaws, maxillæ (or accessory jaws), palpi and legs arranged in pairs, the two halves of the body thus being more markedly symmetrical than in the lower animals. It is by far the most numerous in species of any in the animal kingdom, the insects alone probably numbering upward of a million species; other representative or typical forms are the trilobites, king crabs, scorpions, spiders and myriopods. The skin is usually hardened by the deposition of salts (carbonate and phosphate of lime) and of a pecoliar organic substance called chitine. The segments (somites or arthromeres) composing the body are usually limited in number, 20 (or 21) in the crustaceans and insects; while each arthromere is primarily divided into an upper (tergum), lower (sternum) and lateral portion (pleurum). These divisions, however, cannot be traced in the head of either the crustaceans or the insects. Moreover, the head is well marked with one or two pairs of feelers or antennæ and from two to four pairs of biting mouth-parts or jaws and two compound eyes; besides the compound eyes there are simple eyes in the insects. The germ is three-layered and there is usually in the more specialized forms a well-marked metamorphosis. The Arthropoda are most nearly related to the worms, certain annelides, with their soft-jointed appendages (tentacles as well as lateral cirri) and more or less definite head, anticipating or foreshadowing the arthropods. On the other hand, certain low parasitic arthropods, as linguatula, have been mistaken for genuine parasitic worms.

Segmentation of the Body. — The segments (somites metameres) are merely thickenings of the skin connected by a thin intersegmental membrane, so that the segments can telescope into each other or extend, thus lending the greatest freedom of motion to the trunk as well as to the appendages; otherwise a rigid chitinous skin would not permit of any movement. As in the annelid worms, this segmentation of the integument is correlated with the serial repetition of the ganglia of the nervous system, of the ostia of the dorsal vessel, the primitive disposition of the segmental and reproductive organs, of the soft, muscular dissepiments which correspond to the suture between the segments and with the metameric arrangement of the muscles controlling the movements of the segments on each other; and this internal segmentation or metamerism is indicated very early in embryonic life by the mesoblastic somites.

While we look upon the dermal tube of worms as a single but flexible lever, the body of the arthropods, as Graber states, is a linear system of stiff levers. We have here a series of stiff, solid rings or hoops united by the intersegmental membrane into a whole. When the muscles extending from one ring to the next behind contract, and so on through the entire series, the rings approximate each other.

The origin of the joints or segments in the limbs of arthropods was probably due to the mechanical strains to which what were at first soft fleshy outgrowths along the sides of the body became subjected. Indeed, certain annelid worms of the family Syllidæ have segmented tentacles and parapodia, as in Dujardinia. We do not know enough about the habits of these worms to understand how this metamerism may have arisen, but it is possibly due to the act of pushing or repeated efforts to support the body while creeping over the bottom among broken shells, over coarse gravel or among sea-weeds. It is obvious, however that the jointed structure of the limbs of arthropods, if we are to attempt any explanation at all, was primarily due mainly to lateral strains and impacts 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. By adaptation or as the result of parasitism and consequent lack of active motion the original number of segments may by disuse be diminished. Thus in adult wasps and bees the last three or four abdominal segments may be nearly lost, though the larval number is 10. Dunng metamorphosis the body is made over and the number, shape and structure of the segments are greatly modified.

History and Present Classification. — The group or sub-kingdom (phylum) of Arthropoda was founded in 1848 by Siebold. It has been supposed until recently to be a natural group. In 1893 Kingsley and also Kennel first suggested doubts as to the homogeneity of the group and in the same year Packard published the view that there are four independent lines of development in the Arthropoda, and in 1894 Kingsley divided the group into three subphyla, Laurie and Pocock also considering that the group is polyphyletic. In 1898 Packard stated: “It is becoming evident, however, that there was no common ancestor of the Arthropoda as a whole, and that the group is a polyphyletic one. Hence, though a convenient group, it is a somewhat artificial one, and may eventually be dismembered into at least three or four phyla or branches.”

The five phyla as afterward proposed by Packard are, beginning with the most primitive: (1) Palæostraca, embracing the classes of Trilobita; (2) Merostomata (Limulus), and Arachnida; (3) Poncarida (Crustacea); (4) Prosogoneata, including three classes: Pauropoda, Diplopoda and Lymphyla (Scolopendrella); and (5) Entomoptera, comprising the Chilopoda and Insecta; the great majority of the group being winged insects. Each of these phyla represent independent lines of development, judging by their structure and what we know of their development, and have no genetic connection beyond the theory that they each have descended from one or more annelid worms.