MUSCULAR AND VASCULAR SYSTEMS. 



561 



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 

 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 exception- 

 ally. The tracheal 

 apertures are diffused 

 over the surface of 

 the body, but are 

 especially developed in 

 certain regions. 



The general muscu- 

 lar system consists of t.r.c\ 

 (1) the general wall 

 of the body ; (2) the 

 muscles connected 

 with the m o u t h, 

 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 longitu- 

 dinal muscles. The main muscles of the body are unstriated 

 and divided into fibres, each invested by a delicate membrane. 

 The muscles of the jaws ^done are transversely striated. 



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. 345 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. There do not appear to be any definite 

 blood-vessels, other than the heart. 



The body cavity is formed of four compartments one central, 



z in o o 



Fiu. 341. Section through a tracheal pit and diverging 

 Imndles of tracheal tubes taken transversely to the 

 long axis of the body (after Balfour). t.r Tracheae 

 showing rudimentary spiral fibre ; t.r.c cells resembling 

 those lining the tracheal pits, which occur at intervals 

 along the course of the tracheae ; t.r.o tracheal stigma ; 

 t.r.p tracheal pit. 



