JOINTS. COMPARATIVE ANATOMY OF. Among invertebrate animals there are many cases of articulation which indicate the presence of joints, but they are mostly confined to that type which is characterized by the presence of jointed ap pendages, the arthropods. Among the worms we find nothing to which the word joint can prop erly be applied. the relation of the successive segments to each other not justifying the term. It is even open to question whether the relation ship of the segments in the arms of crinoids and brittle-stars is such that the word joint ought to be used. The joints are generally very simple, consisting merely of the connecting of successive skeletal pieces, by strips of connective tissue or muscle. In crinoids we find a peculiar joint. the `syzygT,' which occurs at more or less regular intervals throughout the length of the arm. and may also occur in the stalk. A syzygy has been defined as a joint in which the two skeletal pieces "are closely and immovably fitted together, though they can be separated by alkalies." If the union is still more complete. so that alkalies will not separate the pieces, the result is called 'an kylosis.' In sea-urchins we find excellent ex amples of true joints, in the attachment of the spines to the test by means of ball-and-socket joints, the base of the spine being hollowed out to fit smoothly over a polished tubercle, and held in position by a band of muscular and connective tissue. Although the spines of many starfishes
show considerable mobility. there is seldom any well-marked joint. The only example of joints among mollusks would seem to be in the con nection between the eight pieces of the chiton's shell or in the hinge of a bivalve shell, but these are hardly worthy the designation. Among ar thropods (insects, arachnoids, and crustaceans) joints abound, for not only are the antennae and feet jointed appendages. hut even the month-parts and the wings work upon joints. In all these cases. however, there are no skeletal ossides, the segments being merely sections of the uniformly inclosing exoskeleton, with the muscle or &in flective tissue strands within themselves. The freedom of movement is rendered possible by the marked thinning of the exoskeleton at the joints into a delicate, flexible membrane, while muscles pass from each joint into the next one.
Among vertebrates the various joints are of the same general structure and plan as in •man. varying with the degree of ossification and the complexity of the organs concerned.