Most transparent

 George Douglas Campbell ArgyllGeorge Douglas Eighth Duke of Argyll, 1823-1900, Autobiography and Memoirs, New York, Dutton & Company,Argyll 1906, pp. 80-81.

There was then none of those charming aquarium tanks which have since become so common. In stooping down to drink out of the most crystal streams, or in chasing water-beetles on stiller ditches, I had often seem mere bits of sand suddenly appear to put out legs and move or bits of mud as suddenly reveal themselves to be living creatures. We took a capacious tub for our aquarium, and soon gathered together a goodly collection of everything we could catch alive. Their structures were various and wonderful but we could always see their purpose. If sometimes we wondered how a  caddis worm could glue to its own sides bits of rotten stick and bark, we always saw in a moment the reason why. By means of this power they often deceived ourselves, and by means of it they could obviously deceive even the most watchful trout. How sometimes they could encase themselves wholly in the loveliest and most transparent grains of sand, selecting these from all coarser material, was a perpetual wonder to us.