324 HISTOLOGY 



they rest is only clearly to be seen when a happy chance shows the epithe- 

 lium torn partly away from the walls of these vessels. This condition 

 is shown in Figure 289, and it can here be seen that the respiratory cells 

 form a rather even layer, and do not have any portion of their cytoplasm 

 extending down between the blood vessels to separate them and form 

 "tunnels" for them, as was the case in the salamander. Some few pro- 

 cesses of the cytoplasm do dip in far enough to secure an anchorage, 

 but this is of small extent and occurs rather seldom. 



The blood vessels with their thin walls form a very close and small- 

 meshed plexus. On this account, and also because the vessels are large 

 as compared with the meshes, all sections of them appear to be trans- 

 verse sections or slightly oblique. The walls of the vessels are formed of 

 a single layer of large thin cells, whose rather widely spaced nuclei appear 

 but seldom in the section. Coagulated blood, however, containing a few 

 typical mollusk blood corpuscles, fills the blood channels. Sections 

 of the inter-vascular spaces or islands show merely a few connective- 

 tissue cells. 



The blood vessels rest on a longitudinal and a transverse layer of 

 muscle fibers that lie between them and the shell epithelium of the outer 

 integument of the animal. The blood must derive some oxygen through 

 these latter thin outer layers. 



Respiratory tissues that operate without the intermediate use of 

 blood. The above caption is not strictly true, as will be seen from the 

 following account, but it will serve to materialize the principle involved. 

 The tissue in consideration is the tracheal respiratory system of the 

 insects. This structure consists, from an histological point of view, of 

 an imagination of the same tissues that were evaginated in the lobster 

 and other Crustacea to form gills. 



These respiratory tubes, which have arisen by such invagination of 

 the surface epithelium, branch and rebranch to ultimately form minute 

 ramifications. It is the source of much controversy as to whether these 

 ultimate branches anastomose or end blindly with a terminal cell. Be 

 that as it may, this invaginated epithelium is carried as fine tubes to all 

 parts of the body, and so generally distributed that all tissues can be 

 supplied with oxygen directly from the tracheoles. Oxygen is thus 

 distributed, and carbon dioxide collected without the direct intervention 

 of the blood. In regions of great blood supply, however, the plexus 

 of trachea becomes greatest, and by means of such plexuses the blood 

 is most probably charged with a certain amount of oxygen for distribu- 

 tion. 



All of these tracheas are composed primarily of a layer of epithelium, 

 which is derived from and continuous with the hypodermis of the body. 

 The epithelium is composed of flattened, six-sided cells with large 



