i 7 6 



SYSTEMATIC ZOOLOGY 



consist of a single row of segments. There is very great varia- 

 tion in the shape and structure of the abdomen, as we shall note 

 under the different orders. The sexes are always separate. 



The organs of respiration are of two kinds. In the one case 

 they consist of chambers, the wall of which is thrown into a 

 large number of parallel folds resembling the leaves of a book. 

 In the leaves themselves are the blood vessels, and between the 

 leaves the air circulates freely. The chamber opens to the out- 

 side by a slitlike opening, the stigma. Such respiratory organs 



FlG. 170. Longitudinal section through the 

 lung book lit a spider, x, external opening or 

 stigma; 2, free edge of the pulmonary leaves; 

 3, space in which the air circulates ; 4, space in 

 which the blood circulates From Shipley 

 and MacBride's Zoology.) 



Fig. 171. Portion of a trachea of a cater- 

 pillar. A, main tube ; />,' ,Z?,bian< 



r; b, nuclei. (F"rom Parker 

 and Haswell's Text-book.) 



are called lung books (Fig. 170). They arc arranged svmmetri- 

 cally, and there are never more than four pairs. In the other case 

 the organs of respiration consist of richly ramifying tubes called 

 tracheae, closed at their inner ends and opening on the surface 

 of the body by symmetrically arranged stigmata, of which there 

 are never more than four pairs. These tubes (Fig. 171) are kept 

 distended by a spirally wound, chitinous band, resemblin_ 

 spring. This subtype is sometimes divided into two classes the 



