616 CAUDAL GILLS OF ZYGOPTERID LARV^., 



number of joints, points the way b}^ which the cerci of the 

 Z3'gopterid larva arrived at its present one- or two-jointed state. 



In transverse section, these organs are absolutely circular. 

 The cuticle and hypodermis are thick, the nuclei of the latter 

 numerous and closely-placed. The interior of the organ is a 

 hollow blood-space, or extension of the hjemocoele, showing the 

 presence of both plasma and corpuscles in almost ever}' section. 

 Laterally, on the outer side, close to the inner border of the 

 hypodermis, there is a single main trachea. f>atero-ventrally, 

 on either side, seated upon the inner border of the hypodermis, 

 there are two lari^e longitudinal nerves, whose cross-section is 

 greater than that of the trachea. As the caudal filan)ents of 

 the Perlaria are organs of touch rather than gills, the greater 

 size of the nerves, and the smallness of the trachea, in compari- 

 son with those of Zygopterid gills, is only to be expected. 



The most interesting structure in these caudal filaments is the 

 very small but definite dorsal blood-vessel, situated mid-dorsally 

 below the inner border of the hypodermis. This structure has 

 a well-defined wall, and might be mistaken for a trachea, were 

 it not always quite full of blood-plasma and corpuscles. Tt con- 

 tinues far along the filament, opening distally into its main 

 interioi-, i.e., the hpemocoele. 



The comparison between these organs and the lateral caudal 

 gills of Zygoptera may be summarised as follows : — 



(1) Only the afferent or primary blood-canal is marked off in 

 the Perlaria; but it is much smaller and more definitely separated 

 out than in the Zygoptera; moreover, it is placed mid-dorsally, 

 as in the median gill of Zygoptera; whereas, in the lateral gills 

 of the latter, the afferent canal is ventral. 



(2) Only one main trachea is developed, on the inner side of 

 the hsemocoele. Thus the tracheation remains at the same stage 

 as that shown in the Second Tnstar of Zygopterid larvae (Text- 

 fig. 34). 



(3) Two main longitudinal nerves are developed, as in the 

 lateral caudal gills of Zygoptera; but their position is not quite 

 the same, both being ventrally placed, whereas those of the 

 Zygoptera lie both on the outer side, one ventral and one dorsal, 



