I read with a pen and find plenty I read in need of improvement in one form or another. I reworked the "Soil" article and am moving from there to subjects related to soil. This article needs a bit more. Here is my reference...Cleveland P. Hickman Jr., Larry S. Roberts, Frances M Hickman (1984, 7th ed.). Integrated Principles of Zoology. Times Mirror/Mosby College Publishing. p 45. ISBN 0-8016-2173-9.   It is a college level reference.