References
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Gillis , Rick and Haro, Roger J.. 2010. Zoo lab: a website for animal biology. Accessed 21 April 2010. <http://www.bioweb.uwlax.edu/zoolab/index.htm>
Mordan, Peter B. and Fitzgerald, Kevin F.. 2004. Pulmonata (lung-bearing snails and slugs). Pages 411-417. Michael Hutchins, editor. Grzimek’s animal life encyclopedia volume 2: protostomes. Gale Group Inc. Farmington Hills, MI.
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