From ZooTerms (Dictionary of Invertebrate Zoology)
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cyst (noun; Greek kystis, bladder): 1. A small sac, capsule or bladderlike structure.

2. A protective covering formed about an organism during unfavorable conditions or reproduction.

3. (NEMATA: Secernentea) The tanned cuticle of certain mature female nematodes (Heterodera or Globodera) in which eggs are retained.