BY R. J. TILLYARb. 



161 



the larva often raises these gills considerably, 

 together with the terminal part of the abdo- 

 men. When resting on a rock-surface, the 

 larva, if alarmed, at once depresses the gills 

 closely against the rock. A curious point 

 about these gills is that they are slightly 

 concave on their upper surfaces. This is well 

 seen in the cross-section in Text-fig. 28. It 

 is due to the almost complete suppression of 

 the dorsal mid-rib in all three gills, the 

 ventral mid-ribs being strongly developed. 

 The median gill is symmetrical, and lies 

 quite horizontally. The laterals are asym- 

 metrical, — the outer portion of the blade 

 being wider than the inner, — and are held 

 at a slight angle to the horizontal, slanting 

 ujDwards and outwards. 



The transverse section of one of these 

 gills may be described as an extremely flat- 

 tened triangle, except near the base of the 

 gill, where the cavity is wider, as shown in 

 Plate vi., figs.40-41. The vertex of the 

 triangle is represented by the ventral mid- 

 rib, the base by the wide upper surface of 

 the blade. The rachis is small, subtrian- 

 gular, and carries along its middle line the 

 blood-canals, which lie pi'actically in contact 

 with one another. The dorsal blood-canal 

 is smaller and more regularly formed than 

 the ventral. On either side of the blood - 

 canals, towards the lateral limits of the 

 rachis, run the two main longitudinal tra- 



cliet^ 



The nerve-supply is as usual, the 



Text-ficr.28.- 



* Ideal (semi-diagranimatie) T.S. across the Hori- 

 zontal Lamellar (i ill-system of Arfjio/e-'<tt.^, to show 

 the internal structure of the gills. [>ettei-ing as on 

 p. 109. 



