64 CAUDAL GILLS OF ZYGOPTERID LARViE, 



of the heart itself, and the ligaments connecting it posteriorly 

 to the wall of the tenth segment. Thus, finally, the blood from 

 all three gills enters the ostia of the ninth segment in two 

 streams, one from the right, the other from the left of the heart. 



I71 the lateral gillfi, therefore, the ventral blood-caval is affere'itt 

 and primary, the dorsal blood-canal efferent and secondary. 



In Text-fig. 4, the seven lines of arrows numbered 1-7 are 

 intended to show all possible courses for any blood-corpuscle 

 entering the eighth segment from in front. Of these, only Nos. 

 3 and 7 concern the gills. It should be noticed that portion of 

 No.^ (the ventral circulation of seg. 10) passes into and out of 

 the larval cercoids (c), in which no blood-canals are developed. 



The difference in the circulation of blood in the median and 

 lateral gills puzzled me greatly, and I could find no explanation 

 to account for it, until I thought of studying the caudal pro- 

 cesses of the May-flies (Plectoptera). Then, as I hope to explain 

 more fully in Part iv., I discovered that, in this Order, each 

 process possesses only one blood-canal, which is dorsal in the 

 median process and ventral in the lateral process. Also, in 

 these caudal processes of the May-fly, the blood passes into the 

 process along the closed canal, the return journey being made 

 along the open hjemocoele of the process-cavity. Thus it became 

 evident that the afferent canal, whether dorsally or ventrally 

 placed, must always be the primary canal, originally present in 

 the organ before it took on the form now seen in the Zyguptera; 

 whereas the efferent canal is only a secondary formation, being 

 in fact nothing more nor less than what is left of the original 

 open hsemocoele of the gill, after the elaboration of the alveolar 

 meshwork, which occupies by far the gieater volume of the 

 haemoccele. Further interesting evidence along the same lines 

 may be expected from the study of the Ontogeny of the Caudal 

 Gills themselves. 



The actual circulation of the blood-corpuscles in the gill seems 

 t<) be entirely confined to the two blood-canals. That is to sa}', 

 although blood-plasma is to be found in the nerve-canals, some 

 of the alveoli, and occasionally in the small spaces of the haemo- 

 ccele around or near the tracheae (Text-fig. 9, hi])), and although 



