Extremities

(1) In animals, the organs that serve, as a rule, for locomotion. In different groups of animals the extremities may differ in origin and structure but perform similar functions (analogous organs). The simplest extremities, the parapodia of polychaete annelid worms, are paired (one pajir on each body segment), short, muscular, mobile appendages, each consisting of a common base and two branches—dorsal and ventral—and often equipped with special bristles. Because of the monotonous rowing movements of the parapodia, the animal can swim or move along the bottom by catching hold with its bristles.

The extremities of arthropods—a further development of the parapodia—are connected to the trunk by joints and form multimembered levers that are considerably more mobile. Originally each body segment of arthropods had a pair of extremities, but, because of differentiation of sections of the trunk and the growing complexity of the functions of the extremities, they disappeared from some segments and on others partially or completely lost their locomotor functions. Thus, the extremities of the cephalic segment were converted to tactile appendages and jaws, some extremities of the thoracic segment were converted into what are called maxillipeds, and some extremities of the abdominal segment were converted into copulatory organs (in the male) or ovipositors (in the female).

The extremities of crustaceans, being originally two-branched and consisting of a base (the protopodite) and two (the outer, exopodite, and the inner, endopodite) branches, often lose one of the branches (or it is drastically reduced). The walking extremities of arachnids (four pairs), insects (three pairs), and myriopods usually consist of one row of segments. Various appendages on the trunks of other invertebrates, which often perform a motor function as well, are not usually called extremities—for example, the tentacle arms of cephalopod mollusks and the ray arms of echinoderms.

In chordate animals a distinction is made between unpaired and paired extremities. In lower chordates (the larvae of tunicates and the lancelet) unpaired extremities are represented by a skin fold in which it is possible to distinguish dorsal, ventral, and caudal parts. Unpaired extremities are also formed in the larvae of cyclostomes, fish, and amphibians in the form of a common fold. In adult lower vertebrates, owing to differentiation of functions, the single fold is divided into separate fins, which are supported by cartilaginous or bony rays and have their own musculature. The fold is preserved only in aquatic caudate amphibians. No terrestrial vertebrates have unpaired extremities, but they may arise secondarily upon return to aquatic life (for example, in ichthyosaurs, manatees, and whales). Unpaired fins provide stability for the body in water, promote the animal’s forward movement, and serve chiefly as rudders.

Paired extremities appear in fish; in some they serve as depth rudders and as organs of equilibrium. Paired extremities evidently also originated in the form of continuous lateral skin folds, of which only the functionally most important—the anterior and posterior parts—were subsequently preserved.

What is called the girdle of the extremity serves as the support of each pair of extremities. The skeleton, which consists of cartilaginous or (more frequently) bony formations that are articulated with one another and are set in motion by muscles, makes up the base of each extremity. Among the paired extremities (fins) of fish a distinction is made between the thoracic fins, located behind the head, and the ventral fins, which usually lie anterior to the anal opening; extremities are called thoracic, brachial, or pelvic, according to their girdles. The skeleton of the extremities is but poorly developed in fish; the fins are basically attached by means of rays of cutaneous origin. Only in lungfishes and dipnoans is the skeleton of the extremities better developed and more differentiated. The transformation of the paired extremities of some fossil lungfishes led to the appearance of the fundamentally pentadactyl extremities of terrestrial vertebrates, which became the principal organs of locomotion on dry land.

The extremities of terrestrial vertebrates consist of three sections: the brachium (of anterior extremities) or femur (of posterior ones), which articulates with the girdle; the antebrachium (anterior) or crus (posterior), with two bones each (respectively, the ulna and radius, fibula and tibia); and the carpals (anterior) and tarsals (posterior), which consist of a large number of small bones that are grouped in the anterior extremities in the carpus, metacarpus, and phalanges of the digits and in the posterior extremities in the tarsus, metatarsus, and phalanges of the digits.

Paired extremities have undergone considerable transformations in the course of evolution. The development of flight in flying reptiles, birds, and bats gave rise to the conversion of the anterior extremities into wings. The extremities of sea reptiles, cetaceans, and pinnipeds became flippers, which externally resemble the fins of fish. Adaptation to rapid running led to a curtailment of the number of digits (to one in horses) and to a decrease in the area of support of the extremities by means of substituting digitigrade locomotion for plantigrade and in ungulates even unguligrade locomotion, with support only on the terminal phalanx.

The extremities of terrestrial vertebrates often perform a number of supplemental functions; for example, the anterior extremities of moles were converted into digging organs; and in arboreal animals, such as apes, they became grasping organs. In a number of instances paired extremities have disappeared with loss of functional significance—for example, the ventral fins of eels, the posterior extremities of cetaceans and manatees, and both pairs of extremities in legless amphibians, some lizards, and all snakes.

(2) In man a distinction is made between the upper and lower extremities, which articulate with the body by means of the brachial and pelvic girdles. Owing to the conversion of man’s ancestors to erect posture, that is, to locomotion only on the posterior extremities, the anterior extremities were freed and, under the influence of labor, were transformed into complete all-purpose organs—the hands.