The reason why I bought this book was that my relatively new tropical fish tank got invaded by 90 strange critters (Yep I counted then as I gradually extracted them). The web was useless for trying to track down flatworm beasties- it's either too specialised or not specialised enough. The library had a copy of this book and a few pamphlets with line drawings. This book actually had a picture of a relative of the beast in question- it turned out to be Helobdella- a freshwater leech which eats snails. The pictures are rather garishly coloured drawings- but it did the trick and set my mind at rest. Also contains marine invertebrates. Definitely useful as an initial spot the beast guide and something I will be keeping for future reference.