TERTIARY SPECTACLES IN FISHES 461 



epithelium on the surface presented to the water. Those clupeoids which 

 have them are next-of-kin to the herrings and their relatives, in which the 

 vertical lids are well-nigh universal (see p. 383, Fig. 132). Functionally, 

 tertiary spectacles are streamlining devices par excellence, no doubt a bit 

 superior to the independent fore-and-aft lids from which they arose. 

 Like the vertical lids themselves, tertiary spectacles have been separately 

 evolved by both malacopterygian (clupeoid) and acanthopterygian fishes 

 — always in adaptation to the habit of swift swimming, in contrast to 

 the secondary spectacles which so much resemble them, but are charac- 

 teristic of sluggish benthonic species. 



