FORM, GROWTH, AND CHANGE 21 



to that usual in most animals, whose blood circulates through a 

 continuous system of closed vessels. In insects the nutrient 

 and cleansing fluid escapes from the vessels into great blood- 

 spaces, so as to bathe the various organs, and of these spaces 

 the main body-cavity is the largest. Thence the blood passes 

 upwards, through the perforations in the delicate roof, into the 

 pericardial blood-space, and re-enters the heart through a series 

 of paired slits, guarded by valves, in the wall of that organ. 



oe 



FIG. II. AIR-TUBES IN SURFACE VIEW (A) AND IN CROSS- 

 SECTION (B). DIAGRAMMATIC. 



ep, cell-layer or epithelium ; ct, cuticular lining ; tl, tracheoles ; 

 oe, oenocytes. (A) x 30 ; (B) x 100. 



This last-mentioned arrangement of openings which allow 

 blood to pass inwards through the wall of the heart, is one of 

 the most remarkable of the characters that distinguish not 

 insects only but the whole great group of arthropods from 

 all other animals. It is impossible, in this book, to deal 

 adequately with physiological questions, but it may be 

 pointed out here that the living tissues of an insect are 

 everywhere directly bathed by the blood and are directly in 



