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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



plates (compare LB of fig. 34 A with LB of fig. i A). The lateral 

 and ventral surfaces of each lobe are often separated by a sharp fold 

 or ridge (fig. 34 B, LB), and the ventral part of the lobe may be 

 more or less united with the sternum of its segment {Stn). The gill 

 (Brn) arises from the posterior end of the lateral surface of the 

 supporting lobe, and is usually provided with four muscles inserted 

 on its base (C). The branchial muscles, as above noted, take their 

 origins on the ventral plate of the supporting lobe (B), and therefore 



Fig. 34.— The abdominal gill-bearing appendages of ephemerid larvae. 



A, diagrammatic cross section of abdominal segment, showing limb base lobes 

 (LB) bearing gill appendages (Brn), which are evidently the styli, each mov- 

 able by muscles (hnicls) arising in the bases; the lateral body muscles (0 

 extend from tergum to sternum. 



B, external view of a gill and its supporting lobe (LB), in which arise the 

 gill muscles (bvicis). 



C, musculature and tracheation of a gill, lateral view. 



not on the sternum, as stated by Diirken (1907), who did not dis- 

 tinguish the ventral plate of the gill basis from the true segmental 

 sternum. 



The old idea that the ephemerid larval gills are appendages of the 

 dorsum is no longer tenable. The organs are very evidently rudiments 

 of segmental appendages, as claimed by Heymons (1896a, 1896^?) 

 and by Borner (1909), each being composed of a basis (LB), and 

 of a terminal appendicular part (Brn) movable on the basis by mus- 



