PL.~TE 1. (a) Stoiiy shore typical of those used to obtain data on triclad distribution and
abundaiire. (b) An upland unproductive lake (LIyn Idwal. ralciurn 1.5 iiig/litre) in t,he
Siiowdonia area of Nort,h Wales whirh did not support a trirlud population.
((b) and (r) reproduced with permission of the Commit,tee for Aerial Pliot,ography,
University of Cambridge.)
[to face pl-ge 6