THE ORGANS OF CIRCULATION AND RESPIRATION. 151 



areas, each of which is occupied by a reticulation of very fine 

 threads. This structure may be traced for a short distance 

 between the turns of the spiral thread. 



The chitinogenous layer of the tracheal tubes is single, and 

 consists of polygonal, nucleated cells, forming a mosaic pattern, 

 but becoming irregular and even branched in the finest 

 branches. The cell walls are hardly to be made out without 

 staining. Externally, the chitinogenous cells rest upon a 

 delicate basement membrane. 



Where a number of branches are given off together, the 

 tracheal tube may be dilated. Fine branches, such as accom- 

 pany nerves, are often sinuous. In the very finest branches 

 the tube loses its thread, the chitinogenous cells become 

 irregular, and the intima is lost in the nucleated protoplasmic 

 mass which replaces the regular epithelium of the wider 

 tubes.* 



The Spiracles. 



The spiracles of the Cockroach are by no means of compli- 

 cated structure, but their small size, and the differences between 

 one spiracle and another, are difficulties which cost some pains 

 to overcome. 



The first thoracic spiracle (fig. 85) is the largest in the body. 

 It lies in front of the mesothorax, between the bases of the first 

 and second legs. It is placed obliquely, the slit being inclined 

 downwards and backwards, and is closed externally by a large, 

 slightly two-lobed valve, attached by its lower border. The 

 aperture immediately within the valve divides into two nearly 

 equal cavities, each of which leads to a separate tracheal trunk ; 

 and between these cavities is a septum, thickened on its free 

 edge, against which the margin of the valve appears to close. 

 A special occlusor muscle arises from the integument below the 

 spiracle, and is inserted into a chitinous process which projects 

 inwardly from the centre of the valve. A second muscle, whose 

 connections and mode of action we have not been able to make 

 out satisfactorily, lies beneath the first, and is inserted into the 

 thickened edge of the septum. 



* It has been supposed that these irregular cells of the tracheal endings pass into 

 those of the fat-body, but the latter can always be distinguished by their larger and 

 more spherical nuclei. 



