TRACHEAL, MUSCULAR AND VASCULAR SYSTEM 



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 prob- 

 ably the indication of a spiral fibre. They appear to branch, but 



Fig. 10. Section through a tracheal 

 pit and diverging bundles of 

 tracheal tubes taken transversely 

 to the long axis of the body. 

 (After Balfour.) ir. 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.jp, tracheal pit. 



only exceptionally. The tracheal apertures are diffused over the 

 surface of the body, but are especially developed in certain regions. 



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 pericar- 

 dium, 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. 



