The apertures of the tracheal system are placed in the depressions between the papillae or ridges of the skin. Each of them leads into a tube, which may be called the tracheal pit (Fig. 10), the walls of which are formed of epithelial cells bounded towards the lumen of the pit by a very delicate cuticular membrane continuous with the cuticle covering the surface of the body. The pits vary somewhat in depth; the pit figured was about 0.09 mm. It perforates the dermis and terminates in the subjacent muscular layer. Internally it expands in the transverse plane and from the expanded portion the tracheal tubes arise in diverging bundles. Nuclei similar in character to those in the walls of the tracheal {15}pit are placed between the tracheae, and similar but slightly more elongated nuclei are found along the bundles. The tracheae are minute tubes exhibiting a faint transverse striation which is probably the indication of a spiral fibre. They appear to branch, but only exceptionally. The tracheal apertures are diffused over the surface of the body, but are especially developed in certain regions. [Illustration: FIG. 10.—Section through a tracheal pit and diverging bundles of tracheal tubes taken transversely to the long axis of the body. (After Balfour.) _tr_, Tracheae, showing rudimentary spiral fibre; _tr.c_, cells resembling those lining the tracheal pits, which occur at intervals along the course of the tracheae; _tr.o_, tracheal stigma; _tr.p_, tracheal pit.] THE MUSCULAR SYSTEM. The general muscular system consists of—(1) the general wall of the body; (2) the muscles connected with the mouth, pharynx, and jaws; (3) the muscles of the feet; (4) the muscles of the alimentary tract. The muscular wall of the body is formed of—(1) an external layer of circular fibres; (2) an internal layer of longitudinal muscles. The main muscles of the body are unstriated and divided into fibres, each invested by a delicate membrane. The muscles of the jaws alone are transversely striated. THE VASCULAR SYSTEM. The vascular system consists of a dorsal tubular heart with paired ostia leading into it from the pericardium, of the pericardium, and the various other divisions of the perivisceral cavity (Fig. 14, D). As in all Arthropoda, the perivisceral cavity is a haemocoele; I.E. it contains blood and forms part of the vascular system. The heart extends from close to the hind end of the body to the head. {16}THE BODY CAVITY.